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Geochemical constraints on origin of hydrothermal volatiles from southern Tibet and the Himalayas: Understanding the degassing systems in the India-Asia continental subduction zone
Accepted Manuscript Geochemical constraints on origin of hydrothermal volatiles from southern Tibet and the Himalayas: Understanding the degassing systems in the India-Asia continental subduction zone
Maoliang Zhang, Zhengfu Guo, Lihong Zhang, Yutao Sun, Zhihui Cheng PII: DOI: Reference:
S0009-2541(17)30111-0 doi: 10.1016/j.chemgeo.2017.02.023 CHEMGE 18265
To appear in:
Chemical Geology
Received date: Revised date: Accepted date:
27 July 2016 16 February 2017 23 February 2017
Please cite this article as: Maoliang Zhang, Zhengfu Guo, Lihong Zhang, Yutao Sun, Zhihui Cheng , Geochemical constraints on origin of hydrothermal volatiles from southern Tibet and the Himalayas: Understanding the degassing systems in the India-Asia continental subduction zone. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Chemge(2016), doi: 10.1016/ j.chemgeo.2017.02.023
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ACCEPTED MANUSCRIPT Geochemical constraints on origin of hydrothermal volatiles from southern Tibet and the Himalayas: Understanding the degassing systems in the India-Asia continental subduction
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zone
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Maoliang Zhang, Zhengfu Guo *, Lihong Zhang, Yutao Sun, Zhihui Cheng
Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and
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Geophysics, Chinese Academy of Sciences, Beijing 100029, China
* Corresponding author.
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Tel.: +86 10 82998393;
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E-mail: [email protected] (Z. Guo).
Fax: +86 10 62010846;
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Address: No. 19, Beitucheng Western Road, Chaoyang District, Beijing, China
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ACCEPTED MANUSCRIPT Abstract Hydrothermal activities (e.g., hot springs and geysers) are extensively distributed in southern Tibet and the Himalayas, forming an east-west trending, ~2000-km-long, hydrothermal belt that represents the ongoing degassing systems in the India-Asia
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continental subduction zone (IACSZ). In this study, we report new data of chemical
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compositions and He-C-N isotopes for gas samples from representative hot springs in
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the hydrothermal belt, aimed at understanding volatile origin of the hydrothermal degassing systems in the IACSZ and their tectonic implications. According to spatial
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location, the samples are divided into western, central and eastern subgroups, which
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display spatially discernible geochemical characteristics, such as 3He/4He and CO2/3He ratios. The essential components required for volatile evolution include
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silicate rocks, carbonate rocks, sedimentary organic matter and a mantle-derived
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end-member, which are all expected in the IACSZ following a tectonic model incorporating an accretionary wedge and a magmatic front. On the basis of He-C-N
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isotope mixing calculations, together with constraints from petrogeochemical and
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geophysical studies, an enriched mantle wedge (EMW) is proposed as a potential candidate for the source of the mantle-derived components, which highlights the importance of recycled Indian continental materials compared to previous models that do not take this possibility for mantle source enrichment in account. The regional crustal rock assemblages composed of silicate rocks, carbonate rocks and sedimentary organic matter are interpreted as contaminants of the EMW-derived volatiles (as exemplified by degassing systems in the magmatic front) or as source materials of 2
ACCEPTED MANUSCRIPT hydrothermal volatiles from degassing systems in the accretionary wedge. Following tectonic framework of the IACSZ, we suggest that spatial variations in volatile geochemistry (e.g., 3He/4He) are predominantly controlled by tectonic affinities (i.e., the accretionary wedge and magmatic front) of the hydrothermal degassing systems in
He contents agree well with the high sedimentary contributions to degassing systems
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4
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southern Tibet and the Himalayas. The crustal-like 3He/4He ratios and high N2 and
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in the accretionary wedge (including the Himalayas and fore-arc basins), whereas the high mantle-derived helium emissions in southern Tibet exhibit close affinities with
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contributions from the EMW-derived melts beneath the magmatic front. Moreover,
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regional fault systems with variable scales and depths of penetration would act as an extra factor that may perturb the across-IACSZ profile of 3He/4He ratios controlled by
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tectonic settings. Our interpretation on degassing systems in southern Tibet and the
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Himalayas may have the potential to provide constraints from volatile geochemistry
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for understanding material recycling mechanism and tectonic settings of the IACSZ.
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Key words: Gas geochemistry; Enriched mantle source; Hydrothermal degassing systems; Continental subduction zone; Southern Tibet and the Himalayas
1. Introduction Subduction-zone degassing systems, characterized by outgassing of volatiles derived from the subducting plate, mantle wedge and overlying plate, play an important role in modulating contents of volatile elements (e.g., carbon) in the Earth’s 3
ACCEPTED MANUSCRIPT atmosphere. Therefore, considerable studies on volatile recycling in subduction zones have been carried out, especially for oceanic subduction zones (OSZ; e.g., Hilton et al., 2002; Sano and Fischer, 2013; Lee and Lackey, 2015; Kagoshima et al., 2015; Kelemen and Manning, 2015; Barry and Hilton, 2016). However, comprehensive
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understanding of the degassing systems remain deficient for continental subduction
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zones (CSZ) where two colliding continental plates are separated by an OSZ-like
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mantle wedge (Ducea, 2016). Following the India-Asia collision in Early Cenozoic (ca. 55–50 Ma; Tapponnier et al., 2001; Najman et al., 2010; Bouilhol et al., 2013;
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Zhu et al., 2015), northward subduction of the Indian continental lithosphere beneath
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the Asian continent defines the most significant, ongoing continental subduction zone on Earth at present, as suggested by constraints from geophysical (e.g., Nábělek et al.,
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2009; Replumaz et al., 2014; Chen et al., 2015; Tian et al., 2015; Liang et al., 2016),
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petrogeochemical (e.g., Guo et al., 2006, 2007, 2013, 2014, 2015) and geodynamic (e.g., Capitanio et al., 2010; Duretz and Gerya, 2013; Burov et al., 2014; Replumaz et
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al., 2016) studies. Located at the leading edge of the IACSZ, southern Tibet and the
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Himalayas provide an important opportunity for understanding the degassing systems related with subduction of the Indian continental lithosphere. Although a number of studies on hydrothermal volatiles from southern Tibet and the Himalayas have been carried out (e.g., Yokoyama et al., 1999; Hoke et al., 2000; Zhao et al., 2001, 2002; Newell et al., 2008; Klemperer et al., 2013; Zhang et al., 2014), most of them focus on relatively limited or individual region, and thus genetic models for volatile origin have not been fully integrated in the geodynamic setting of 4
ACCEPTED MANUSCRIPT the Indian continental subduction. Additionally, helium isotopes of hydrothermal volatiles have been taken as an effective tool for understanding lithospheric structure (e.g., Hoke et al., 2000; Newell et al., 2008; Klemperer et al., 2013), when combined with geophysically imaged lithospheric profile of the IACSZ (e.g., Zhao et al., 2010;
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Chen et al., 2015; Shi et al., 2015; Tian et al., 2015; Xie et al., 2016). Nevertheless,
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the geochemical and tectonic implications inferred from chemical compositions and
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isotopes of major volatile elements (e.g., carbon and nitrogen; Newell et al., 2008) need to be further constrained, especially considering the volatile recycling processes
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(e.g., mantle wedge enrichment) and regional tectono-magmatic evolution related
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with northward subduction of the Indian continental lithosphere. In this study, we report new data of chemical compositions and He-C-N isotopes
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for bubble gas samples from representative hot springs in southern Tibet and the
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Himalayas, in order to constrain origin and evolution of these hydrothermal volatiles and to understand tectonic implications of the hydrothermal degassing systems in the
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IACSZ. On the basis of He-C-N isotope mixing calculations, together with constraints
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from petrogeochemical and geophysical studies (e.g., Guo et al., 2013, 2015; Chen et al., 2015; Shi et al., 2015), we propose a genetic model that incorporates an enriched mantle wedge (EMW) as the potential source for mantle-derived contributions to the hydrothermal volatiles. Additionally, tectonic affinities of the degassing systems in the IACSZ are interpreted from the perspective of volatile geochemistry and northward subduction of the Indian continent beneath the Asian continent.
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ACCEPTED MANUSCRIPT 2. Geological setting The southern Tibetan Plateau (i.e., the Lhasa terrane) is separated from the Himalayas by the Indus-Tsangpo suture (ITS) to the south and from the Qiangtang terrane by the Bangong-Nujiang suture (BNS) to the north (Fig. 1). Compared with
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the northern Tibetan Plateau, southern Tibet is characterized by relatively higher
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topographic relief (Feilding et al., 1994; Clark and Royden, 2000), which may suggest
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complicated orogenic and uplift history as a consequence of collision and subduction between the Indian and Asian continents. Precambrian basement rocks of the southern
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Tibetan Plateau, which are rarely outcropped, can be represented by the Amdo gneiss
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(Guynn et al., 2006, 2012) and metamorphic rocks of the Nyainqentanglha Group to the west of the Nam Lake (Hu et al., 2005). Phanerozoic cover sequences of the
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southern Tibetan Plateau are mainly composed of Paleozoic to Mesozoic sedimentary
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rocks (e.g., slate, limestone and sandstone) and Mesozoic to Cenozoic magmatic rocks (Zhu et al., 2013 and references therein). The Himalayas are composed of the
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Lesser Himalayan sequence (LHS), Higher Himalayan sequence (HHS) and Tethyan
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Himalayan sequence (THS) from south to north (Fig. 1). In particular, the THS mainly consists of mud- and serpentinite-matrix mélanges, radiolarites, pillow basalts, deep-marine sedimentary rocks and metasedimentary rocks that were accreted onto southern margin of the Asian continent as the Tethyan oceanic lithosphere was subducted northward (Cai et al., 2012; DeCelles et al., 2014). These regional crustal rock assemblages have played an important role in degassing systems in the IACSZ. Owing to convergence between the Indian and Asian continents during Cenozoic 6
ACCEPTED MANUSCRIPT times (Yin and Harrison, 2000; Styron et al., 2015), different types of fault systems were formed in southern Tibet and the Himalayas (Fig. 1), such as (1) the lithosphere extensional tectonics recorded by intrusion of the post-collisional magmas (Williams et al., 2001; Huang et al., 2016) and the N-S-trending rifts (e.g., the Gulu-Yadong rift,
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GYR; Zhang et al., 2013), (2) the north-dipping southern Tibetan detachment system
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(STDS; Burchfiel et al., 1992; Yin, 2006; La Roche et al., 2016), (3) the high-angle
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thrust fault systems (e.g., the main central thrust; Searle et al., 2008) and (4) the strike-slip fault systems represented by the NW-SE-trending Karakoram fault (KKF;
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Murphy et al., 2000; Valli et al., 2008). Modern hydrothermal activities in southern
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Tibet and the Himalayas are mainly distributed within and/or around these fault systems (Fig. 1; e.g., Yokoyama et al., 1999; Hoke et al., 2000; Newell et al., 2008;
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Klemperer et al., 2013), which can provide variable scales of pathways for the
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uprising hydrothermal fluids in the degassing systems. The representative types of the hydrothermal activities in the IACSZ include boiling hot spring (Fig. 2a) and geyser
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(Fig. 2b), where carbonate and/or silica sinters (or travertine) are commonly
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outcropped (Fig. 2c, d). Additionally, geophysical studies have recognized several low-velocity–high-conductivity zones (LV–HCZs) beneath southern Tibet and the Himalayas (e.g., Brown et al., 1996; Wei et al., 2001; He et al., 2016; Xie et al., 2016), suggesting the presence of partial melts in crustal depth and their relationship with the shallow hydrothermal degassing systems. Our study areas consist of the Namuru, Baer, Tirthapuri, Qiwusi, Darong, Dagejia, Liudaoban, Xiqin and Mianjiu hot springs from west to east (Fig. 1), which are 7
ACCEPTED MANUSCRIPT generally distributed along the southern boundary of the Tibetan Plateau (i.e., the ITS). According to field observations, the temperatures of hot springs vary from 37 oC to 82 o
C (Table 1), and the highest fluid temperature occurs in the Dagejia geysers (Figs. 1
and 2). Combined with literature records, there are no location-dependent features of
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fluid temperatures between hot springs in the Himalayas and those in southern Tibet.
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It may be a general agreement that the hydrothermal fluids that feed hot springs are
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uprising through fault systems with variable scales and depths of penetration. For example, the Tirthapuri (or Menshi; Klemperer et al., 2013) hot springs are related
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with the lithospheric-scale KKF, while the GYR may be important for the hot springs
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in eastern Lhasa terrane (Fig. 1). Additionally, some small-scale fault systems could provide pathways for volatiles released by hot springs in the Himalayas and fore-arc
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basins (Fig. 1). These regional fault systems can affect chemical compositions of the
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hydrothermal fluids by conditioning residence time, cooling and mixing processes (Tardani et al., 2016). Apart from fault-related influence, the differences in crustal
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lithologies and tectonic settings between southern Tibet and the Himalayas are also
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likely to affect volatile evolution. As exemplified by the Liudaoban and Xiqin hot springs in the Xigaze fore-arc basin (Fig. 1), the sedimentary features (e.g., organic sediments) are prominent for hot springs in the Himalayas and fore-arc basins. In contrast, organic sediments appear to be less important for hydrothermal degassing systems in the GYR, southern Tibet. These field characteristics of degassing systems in southern Tibet and the Himalayas could be linked to origin and evolution of the hydrothermal volatiles. 8
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3. Samples and analytical methods Following the water displacement method (Sano et al., 1994), eighteen samples were collected using glass bottles from the hot springs listed in Table 1. In particular,
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samples from the Darong hot springs were collected and studied for the first time (Fig.
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1). In order to capture bubbling hot spring gases, an inverted funnel connected with
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rubber pipes was submerged in spring water and placed on top of the uprising bubbles. Before collecting accumulated gases in the funnel, rubber pipes were completely
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flushed by the continuously outgassing bubbles to avoid air contamination. According
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to variable bubble fluxes of the hot springs, a time interval of ca. 5–15 minutes is empirically enough to rule out atmospheric contributions to the accumulated gases in
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the inverted funnel and rubber pipes. After the pipe flushing step, the bubble gases
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were forced into glass bottles that were filled with spring water of the sampling location and then were prepared for laboratory analyses.
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Bulk chemical compositions and He-C-N isotope compositions of the selected
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samples were determined within one week after fieldtrip in Lanzhou Center for Oil and Gas Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences. Bulk chemical compositions were determined by a MAT 271 mass spectrometer (Cao et al., 2011, 2016; Zhou et al., 2015), as summarized in Table 1. Helium isotopes were analyzed by the Noblesse noble gas mass spectrometer that incorporates a two-stage separation and purification system for noble gas enrichment (Cao et al., 2016). Air sample from the top of the Gaolan mountain (to the south of the 9
ACCEPTED MANUSCRIPT city of Lanzhou) was collected and analyzed to meet the laboratory standards (i.e., 3
He/4He = 1.39 × 10-6). Additionally, carbon and nitrogen stable isotope ratios were
analyzed by the GC-C-IRMS system consisting of a TRACE GC Ultra gas chromatograph (GC) coupled to a Finnigan MAT 253 mass spectrometer (Li et al.,
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2014). The results of helium isotopes, carbon and nitrogen stable isotope ratios of
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samples in this study are reported in Table 2. Additionally, all literature data (e.g.,
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Yokoyama et al., 1999; Hoke et al., 2000; Zhao et al., 2002; Walia et al., 2005; Newell et al., 2008; Klemperer et al., 2013) compiled in this study are available in the
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Supplementary Table S1.
4. Results
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To understand spatial variations in geochemical characteristics of the studied
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hydrothermal volatiles, samples from southern Tibet and the Himalayas, including those in literature and this study, are divided into three subgroups based on their
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locations (Fig. 1), which are the western subgroup (to the west of longitude 82oE), the
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central subgroup (between longitude 82oE and 90oE), and the eastern subgroup (to the east of longitude 90oE), respectively. 4.1. Chemical compositions Hydrothermal volatiles from southern Tibet and the Himalayas are predominated by either CO2 or N2, or both of them (Table 1), as indicated by the well-correlated CO2/N2 and CO2 in Fig. 3a. Compared with the CO2-rich samples (CO2/N2 = 4.18– 52.2) of the eastern subgroup, the N2-rich hydrothermal volatiles tend to be more 10
ACCEPTED MANUSCRIPT common for hot springs from the western (CO2/N2 = 0.03–87.3) and central (CO2/N2 = 0.02–722) segments of southern Tibet and the Himalayas, although some CO2-rich samples can also be observed (Fig. 3a). Another prominent feature in compositions of these hydrothermal volatiles is the
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different range of radiogenic helium contents between the eastern subgroup samples
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and those of the western and central subgroups (Fig. 3b). Most of the eastern
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subgroup samples (except for two N2-rich samples) have relatively lower 4He contents, whereas most of the western and central subgroup samples are characterized by
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relatively higher 4He contents as reported in previous studies (e.g., Hoke et al., 2000;
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Zhao et al., 2002; Newell et al., 2008). Moreover, high 4He contents appear to be coupled with high N2 contents, especially for samples of the western and central
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subgroups (Fig. 3b). The coupled high N2 and 4He contents can be linked to their
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affinities with sedimentary environment, because (1) high contents of organic-sourced nitrogen can be provided by the biologic activities during sedimentation processes
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(Bebout et al., 2013), and (2) radiogenic helium tend to be accumulated in
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sedimentary basins and crustal aquifers (Torgersen, 2010; Lowenstern et al., 2014). With respect to N2/Ar and He/Ar ratios (Fig. 4), most of the eastern subgroup samples have low He/Ar ratios and N2/Ar ratios that range from the ASW value (air saturated water, N2/Ar = 38; Hilton, 1996) to the air value (N2/Ar = 84). In contrast, the western and central subgroup samples are characterized by relatively higher He/Ar and N2/Ar ratios (Fig. 4). Moreover, irrespective of sample locations, all the samples display large variations in CO2/3He ratios (e.g., CO2/3He = 3.04 × 107 – 1.31 × 1010 11
ACCEPTED MANUSCRIPT for samples in this study; Table 2), which may result from multi-component mixing (Barry et al., 2013) and/or elemental fractionation induced by different solubilities of CO2 and helium in melts/fluids (e.g., calcite precipitation; Hilton et al., 1998; Ray et al., 2009; Iacono-Marziano et al., 2010; Güleç and Hilton, 2016).
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4.2. Helium isotopes
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The measured 3He/4He ratios of samples in this study range from 0.014 RA to
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0.572 RA (where RA denotes the atmospheric 3He/4He = 1.382 × 10-6; Mabry et al., 2013), which are in good agreement with previously reported helium isotope data (Fig.
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5a; e.g., Hoke et al., 2000; Newell et al., 2008). Assuming that all 20Ne of the
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measured samples originates from the air dissolved in water, the air-derived helium contributions were corrected by the ASW method (Craig et al., 1978; Rison and Craig,
He/4He ratios (0.015−0.373 RA for samples in this study; Table 2) exhibit clear
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3
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1983), as summarized in Table 2. Combined with literature data, the air-corrected
E-W-trending variations for hydrothermal volatiles from southern Tibet and the
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Himalayas (Fig. 5b).
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Following the conventional 3He/4He ratio of continental crust (3He/4He = 0.02 RA with a range of 0.01−0.1 RA; Lupton, 1983), typical crustal helium isotopes are more common for samples of the western and central subgroups (Fig. 5b), in contrast to the eastern subgroup samples that are dominated by excess mantle 3He contributions. For example, none of the central subgroup samples have 3He/4He ratios that are higher than 0.1 RA (Fig. 5b). Notably, some western subgroup samples exhibit high mantle helium contributions as shown by the abnormally high 3He/4He ratio of one sample 12
ACCEPTED MANUSCRIPT along the KKF (2.24 RA; Klemperer et al., 2013). The spatial variations in 3He/4He ratios are generally consistent with previously recognized crustal helium domain (<0.05 RA) and mantle helium domain (>0.1 RA) in Hoke et al. (2000). However, there might be no clear compositional gap (0.05−0.1 RA; Klemperer et al., 2013)
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between the crustal and mantle helium domains (Fig. 5b). Taking 3He/4He ratios of 8
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RA and 0.02 RA for the mantle and crustal helium end-members, respectively, the
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proportions of mantle helium generally range from 1−10% for most high-3He/4He (>0.1 RA) samples of the western and eastern subgroups. In particular, the KKF
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sample exhibits mantle helium contribution of over 20% (Klemperer et al., 2013). On
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the contrary, the proportions of mantle helium in central subgroup samples are less than 1% (Fig. 5a).
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4.3. Carbon and nitrogen stable isotope ratios
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As reported in Table 2, the δ13C (for CO2, versus PBD) and δ15N (for N2, versus air; corrected for air contamination) values of the samples in this study range from
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−12.9‰ to −5.0‰ and from −0.8‰ to 5.0‰, respectively. In combination of the
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literature data, the hydrothermal volatiles from southern Tibet and the Himalayas define (1) positive correlation between CO2/N2 and δ13C (Fig. 6a), and (2) negative correlation between CO2/N2 and δ15N (Fig. 6b), respectively. The variations in δ13C values among samples of the three subgroups are not very clear compared with their spatially discernible 3He/4He ratios (Figs. 5 and 6a). However, negative δ15N values appear to be more common for the eastern subgroup samples, in contrast to the positive δ15N values observed in most samples of the western and central subgroups 13
ACCEPTED MANUSCRIPT (Fig. 6b). This may suggest variable contributions from source components involved in evolution of the He-C-N isotope systematics of the hydrothermal volatiles from southern Tibet and the Himalayas.
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5. Discussion
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5.1. Essential components involved in evolution of the hydrothermal volatiles
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The chemical compositions of the studied hydrothermal volatiles indicate that essential end-members required for volatile evolution can be reasonably attributed to
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helium-, nitrogen- and carbon-rich components. In this section, we will discuss the
5.1.1. Helium-rich components
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possible origin of these essential components and their roles in the degassing systems.
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As shown in Figs. 3b and 5, the helium-rich signatures of the hydrothermal
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volatiles can be divided into two types of helium enrichment, which are the 4He enrichment (for samples within crustal helium domain) and the 3He enrichment (for
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samples within mantle helium domain), respectively. The former scenario is best
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exemplified by some samples with extremely high 4He contents from the western and central subgroups (Fig. 3b), while the latter scenario can be observed in the samples with high 3He/4He ratios (>0.1 RA; Fig. 5). It is straightforward that the crustal rocks (e.g., silicate rocks represented by granites; Hoke et al., 2000) in southern Tibet and the Himalayas are reasonable origin of the radiogenic helium-rich components. Notably, the eastern subgroup samples that are close to the Nyainqentanglha granites (Kapp et al., 2005; Weller et al., 2016) have 14
ACCEPTED MANUSCRIPT relatively lower radiogenic helium contents (e.g., the Yangbajing samples; Yokoyama et al., 1999; Zhao et al., 2001, 2002; Zhang et al., 2014), unlike the western and central subgroup samples (Fig. 3b). The contrasting variations in radiogenic helium contents of the hydrothermal volatiles from degassing systems along the IACSZ may
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suggest either different degrees of fluid-rock interaction and/or additional origin of
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radiogenic helium other than granites. The latter possibility can be linked to the
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volatile affinities with sedimentary environment, because the contents of radiogenic He that has accumulated in sedimentary basins and crustal aquifers over geological
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time can be extremely high (Lowenstern et al., 2014 and references therein), and thus
the high radiogenic 4He contents.
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hydrothermal fluids with long residence time in such degassing systems would inherit
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With respect to the mantle-derived 3He, the hydrothermal volatiles released from
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hot springs within lithospheric-scale fault systems are characterized high 3He/4He ratios (>0.1 RA; Fig. 5), such as the KKF (Klemperer et al., 2013). Given that the
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samples close to the ITS have crustal-like 3He/4He ratios, mantle relics represented by
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the ITS ophiolites were considered to have negligible contributions to the excess mantle 3He emissions (Klemperer et al., 2013). Additionally, Hoke et al. (2000) also ruled out the possibility of mafic magmatic rocks with variable crystallization ages (ranging from Late Cretaceous to Miocene) as reasonable source of the mantle helium, because these mantle-derived rocks are too old to maintain their initial high 3He/4He ratios. Therefore, a mantle end-member related with recent (e.g., Quaternary; Hoke et al., 2000) melting event might be required for the high mantle 3He emissions. 15
ACCEPTED MANUSCRIPT 5.1.2. Nitrogen-rich components Following different nitrogen reservoirs of the Earth, the end-members involved in nitrogen isotope systematics are composed of the mantle (δ15N = −5 ± 2‰; Sano et al., 2001; Cartigny and Marty, 2013), crust (δ15N = 5 ± 2‰; Sano et al., 1998; Li and
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Bebout, 2005; Busigny and Bebout, 2013; Cartigny and Marty, 2013; Halama et al.,
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2014) and atmosphere (δ15N = 0‰ by definition; Cartigny and Marty, 2013). In
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particular, the crustal end-member is mainly represented by organic-sourced nitrogen that has been sequestered in sedimentary and crystalline rocks through biologic
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activities, sedimentation and diagenesis processes (Bebout et al., 2013).
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For the studied hydrothermal volatiles, the nitrogen-rich components have close affinities with organic matter, as demonstrated by (1) the positive δ15N values (e.g., up
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to 5‰; Fig. 6b) that point to the involvement of organic matter, and (2) the higher
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N2/Ar ratios relative to the atmospheric value (Kita et al., 1993; Taran, 2011). Thermogenic breakdown of sedimentary organic matter can account for contributions
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from the nitrogen-rich components, and an alternative explanation is the interaction
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between hydrothermal fluids and silicate rocks that have incorporated organic-sourced nitrogen (as NH4+ stored in minerals such as beryls and micas; Bebout et al., 2013; Busigny and Bebout, 2013). The air-like δ15N values of a few samples suggest the contributions from atmospheric nitrogen, which can be interpreted as nitrogen from the air and/or the ASW (Fig. 6b). Moreover, the negative δ15N values of some samples may result from the involvement of mantle-derived nitrogen and/or the near surface biogenic-sourced nitrogen (Newell et al., 2008). In this study, we prefer the potential 16
ACCEPTED MANUSCRIPT involvement of mantle-derived nitrogen in volatile evolution, because some samples with excess mantle 3He emissions also have negative δ15N values. 5.1.3. Carbon-rich components Irrespective of sample locations, the hydrothermal volatiles from southern Tibet
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and the Himalayas have pervasively received contributions from inorganic and
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organic carbon end-members, i.e., the carbon-rich components. The CO2-rich samples
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with high δ13C values and CO2/3He ratios (Fig. 7a) may result from the involvement of carbonate rocks [δ13C = 0 ± 2‰ (Hoefs, 2009); CO2/3He = 1013 (O’Nions and
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Oxburgh, 1988; Sano and Marty, 1995)] in volatile evolution. The three-component
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mixing model of Sano and Marty (1995) can well explain the contributions from the inorganic carbon end-member (i.e., CAR; Fig. 7a), especially for the CO2-rich
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hydrothermal volatiles. In particular, calcite precipitation may be not important for the
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samples inside the mixing envelope, as suggested by the samples with have high δ13C values and CO2/3He ratios from the Sanglai hot springs (where carbonate sinters are
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common; Fig. 7a). Thus, these samples can be considered for further geochemical
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interpretation (e.g., the multi-component mixing model in Fig. 8). As discussed in Newell et al. (2008), decarbonation of marine limestones, which are widespread in southern Tibet and the Himalayas (Kapp et al., 2003; DeCelles et al., 2011, 2014), can provide the inorganic carbon-rich components in these degassing systems. In contrast, the samples outside the Sano & Marty mixing envelope have low δ13C values and CO2/3He ratios (i.e., <2×109; Fig. 7a), which are commonly attributed to the potential effects of significant calcite precipitation (e.g., Hilton et al., 1998; Ray 17
ACCEPTED MANUSCRIPT et al., 2009; Güleç and Hilton, 2016). Following this possibility, the low δ13C values and CO2/3He ratios of these samples appear to be consistent with the expected results of calcite precipitation at temperatures between 25 oC and 192 oC, respectively (Fig. 7a). However, compared with the CO2-rich hydrothermal volatiles, the samples
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outside the mixing envelope have excess nitrogen (CO2/N2 <1; Fig. 7b) and positive
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δ15N values (Fig. 6b), suggesting that contributions from the nitrogen-rich organic
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matter should also be taken into account.
It may be a general agreement that organic matter contains both carbon-rich
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organic components (δ13C = −30 ± 10‰; Hoefs, 2009) and nitrogen-rich organic
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components (δ15N = 5 ± 2‰; Sano et al., 1998; Cartigny and Marty, 2013). Owing to the contributions from both carbon-rich and nitrogen-rich organic components, these
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samples would have excess nitrogen contents (i.e., higher N2 and lower CO2), low
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CO2/3He ratios and low δ13C values. For example, low CO2/3He ratios of the N2-dominated samples NMR1501 and NMR1502 are likely to reflect the predominant
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contributions from nitrogen-rich organic matter, which agrees well with their positive
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δ15N values and low δ13C values that point to organic affinities (Table 2). Therefore, aside from the potential effects of calcite precipitation, we suggest that the N2-rich samples outside the mixing envelope may result from involvement of organic matter in volatile evolution, especially the nitrogen-rich organic components. Considering their crustal-derived signatures (see details in Section 5.3.2) and the potential effects of significant calcite precipitation, these N2-rich samples would not be considered for further discussion in a multi-component mixing model (Figs. 8 and 9). 18
ACCEPTED MANUSCRIPT 5.2. Enriched mantle wedge (EMW) as the source for the mantle-derived components On the basis of bulk volatile compositions (Figs. 3 and 4), together with helium isotopes (Fig. 5), carbon and nitrogen stable isotope ratios (Figs. 6 and 7), different types of source materials have been identified for the origin of hydrothermal volatiles
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from southern Tibet and the Himalayas, which are the silicate rocks, carbonate rocks,
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sedimentary organic matter and a mantle-derived end-member, respectively. In the
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case of the mantle-derived components, Hoke et al. (2000) suggested that recent melts of the Tibetan lithospheric mantle and/or the asthenosphere can provide the excess
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mantle 3He emissions. Additionally, high 3He/4He ratios of some western subgroup
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samples were attributed to mantle-derived melts related with rapid slip rate of the lithospheric-scale KKF (Hoke et al., 2000) or mantle-derived fluids that penetrate
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upward along the KKF (Klemperer et al., 2013). However, little attention has been
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paid to an enriched mantle source that is reasonably expected for the geodynamic setting of the Indian continental subduction. In this section, we will discuss the
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scenario that the enriched mantle wedge (EMW) acts as the source of the
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mantle-derived components. 5.2.1. The feasibility of an EMW source in the IACSZ Recycled slab-derived materials have been widely interpreted as the agents for source enrichment of mantle-derived magmas that are genetically related not only with oceanic lithosphere subduction (e.g., Spandler and Pirard, 2013; Kimura et al., 2016; Zhang and Guo, 2016) but also with continental lithosphere subduction (e.g., Ding et al., 2003; Guo et al., 2013, 2015). For the IACSZ, the Sr-Nd-Pb isotopes of 19
ACCEPTED MANUSCRIPT post-collisional (25–8 Ma) mafic volcanic rocks in southern Tibet point to an EMW source fluxed by continental materials (melts and/or fluids) from the northward subducting Indian plate (e.g., Guo et al., 2013, 2015). Moreover, the hydrothermal volatiles from Tengchong volcanic field, SE Tibet, have been suggested to be released
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by partial melts derived from an EMW source related with recycling of the Indian
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continental materials (Zhang et al., 2016).
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Notably, geophysical studies have revealed the presence of a narrow gap (i.e., the asthenosphere) between the subducting Indian lithosphere and the overriding Asian
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lithosphere (Zhou and Murphy, 2005; Chen et al., 2015; Shi et al., 2015), which is
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consistent with the geodynamic modeling of continental lithosphere subduction in a context of intra-continental collision (Replumaz et al., 2016). We suggest that the
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geophysically detected narrow gap can be interpreted as mantle wedge of the IACSZ.
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Taking carbon recycling model as an example, experimental studies have revealed that sedimentary carbon and carbonate mineral-bearing rocks can be recycled into
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mantle wedge by the subducting plates, which would release carbon-bearing melts
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and/or fluids under P-T conditions of mantle wedge (e.g., Dasgupta and Hirschmann, 2010; Frezzotti et al., 2011; Ague and Nicolescu, 2014; Sverjensky et al., 2014; Thomson et al., 2016). In particular, noble gases could also be recycled into the sub-arc mantle depth during continental lithosphere subduction; and these noble gases would be transferred to the post-collisional magmas at the slab-mantle interface (Dai et al., 2016), which may shed light on formation of the EMW source in the IACSZ. Therefore, we suggest that an EMW source generated by mixing between the Indian 20
ACCEPTED MANUSCRIPT slab-derived materials and the asthenosphere can be expected for volatile recycling mechanism in the IACSZ. 5.2.2. Formation of the EMW source: slab-mantle interaction Following the He-C isotope coupling model in Zhang et al. (2015, 2016), together
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with proportions of the Indian components in mantle wedge enrichment constrained
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by genesis of the EMW-derived mafic magmas (Guo et al., 2013), we calculated the
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possible range of helium and carbon isotope compositions of the EMW source (Table 3 and Supplementary Table S2). In a scenario of the silicate-dominated mantle source
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enrichment (Guo et al., 2013, 2015), the calculated 3He/4He ratios of the EMW source
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vary in a range of 2.28–5.23 RA as proportion of the silicate melts (represented by the SED end-member; Fig. 8a and Table 3) increases from 5% to 20%. Given that the
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roles of silicate- and carbonate-related mantle metasomatism remain controversial for
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origin of the mantle-derived mafic magmas (e.g., Guo et al., 2013, 2015; Liu et al., 2015), the potential influence of subducted carbonate (e.g., dehydration fluids; Ague
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and Nicolescu, 2014) on formation of the EMW is denoted by the question mark in
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Fig. 8a, suggesting that the CAR contributions to the He-C isotope systematics of the EMW source are uncertain. For the He-N isotope systematics during slab-mantle interaction (Fig. 8b), the EMW source is assumed to have 3He/4He ratios of 2.28–5.23 RA following results of the He-C isotope coupling model, while δ15N values of the EMW source may vary from −4.5‰ to −3‰ according to simple mixing between the SED components (with proportions ranging in 5–20%) and the mantle (with proportions ranging in 80–95%). 21
ACCEPTED MANUSCRIPT As suggested by the He-C-N isotope calculations (Fig. 8), it is clear that the EMW can be considered as a reasonable candidate for the mantle-derived end-member, which can provide the discernible mantle contributions (e.g., excess 3He, recycled C and N) for the hydrothermal degassing systems in the IACSZ. It should be noted that
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only samples with high 3He/4He ratios (>0.1 RA) exhibit clear contributions from the
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EMW-derived melts, whereas the low 3He/4He (<0.1 RA) samples are inferred to have
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negligible mantle contributions.
Combined with constraints from petrogeochemical (Guo et al., 2013, 2015) and
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geophysical (e.g., Brown et al., 1996; Wei et al., 2001; Unsworth et al., 2005; Xie et
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al., 2016) studies, we suggest that recent partial melts from the EMW source in crustal depth beneath southern Tibet, which can act as the origin of mantle-derived materials
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and as heat source of the shallow hydrothermal degassing systems. Mantle upwelling
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and the LV–HCZs in southern Tibet inferred by geophysical studies (e.g., Wei et al., 2001; Unsworth et al., 2005; Chen et al., 2015; Tian et al., 2015) are likely to reflect
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emplacement and storage of the EMW-derived melts beneath the magmatic front.
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Alternatively, the newly-formed Tibetan lithospheric mantle in a lithospheric delamination model can also account for the formation of an enriched mantle source, if the recycled Indian continental materials are taken as metasomatic agents. Considering its equivalent role in volatile origin to the above EMW model, this scenario would not be focused in this study. 5.3. The role of regional crustal rock assemblages in volatile evolution 5.3.1. Contaminants of the EMW-derived volatiles 22
ACCEPTED MANUSCRIPT Similar to post-collisional, mantle-derived magmas that tend to be affected by magma differentiation processes during their storage and transit in continental crust (Liu et al., 2017), crustal contamination should also be taken into account for volatile evolution, especially in consideration of the thickened continental crust of southern
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Tibet (70–75 km; Zhang et al., 2011). The crustal contributions to the EMW-derived
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volatiles can be provided by (1) partial melts of crustal country rocks (dominated by
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silicate rocks) in emplacement depth of the EMW-derived melts, and (2) hydrothermal fluids that interact with crustal rock assemblages (e.g., carbonate rocks, silicate rocks
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and different types of sediments) in shallow degassing systems.
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In the crustal contamination scenario, the He-C isotope compositions of the SED and CAR components in continental crust are assumed to be identical to those of the
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recycled crustal materials from subducting Indian plate (see details in Supplementary
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Table S2). Following above discussion, the crustal contamination processes is only valid for the high 3He/4He samples with clear mantle contributions. Namely, the
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crustal rock assemblages would act as contaminants of the EMW-derived volatiles. As
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indicated by the He-C isotope coupling model (Fig. 8a), most of the eastern subgroup samples and some western subgroup samples near the KKF are consistent with the results of crustal contamination. The 3He/4He ratios and δ15N values of these samples may reflect the interaction between the EMW-derived volatiles and continental crustal materials that are enriched in organic-sourced nitrogen and radiogenic 4He (Fig. 8b). Additionally, crustal contamination processes can also be illustrated by the plot of 3
He/4He versus CO2/3He (Fig. 9), which displays mixing between the EMW-derived 23
ACCEPTED MANUSCRIPT volatiles (3He/4He = 2.28–5.23 RA, CO2/3He = 2.31×109 – 3.46×109; Table 3) and continental crustal materials with variable CO2/3He ratios. 5.3.2. Source materials of the crustal-derived volatiles Except for some samples along the KKF (e.g., the Menshi sample; Klemperer et
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al., 2013), the western and central subgroup samples can be classified as typical
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crustal-derived volatiles that generally have low 3He/4He ratios (<0.1 RA), suggesting
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negligible contributions from the EMW-derived volatiles (Figs. 8 and 9). Namely, regional crustal rocks assemblages would act as the source materials of these
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hydrothermal volatiles. This is consistent with tectonic setting of the fore-arc region
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(see details in Section 5.4.2), including the Himalayas and fore-arc basins that cover most of the western and central subgroup samples (Fig. 1).
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A prominent signature of the crustal-derived volatiles is the excess N2 contents
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(Fig. 3), which may have close affinities with sedimentary environment. For example, Jenden et al. (1988) discussed the presence of excess nitrogen in the gas and oil wells
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in the California Great Valley; and Mariner et al. (2003) found excess nitrogen in
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thermal springs in the Cascade Range, which was attributed to their origin from low to moderate-temperature sedimentary environment. These observations indicate that the crustal-derived volatiles in the IACSZ are dominated by sedimentary environment in the fore-arc region, which is also consistent with the high 4He contents (Fig. 3b) and He-C-N isotopic compositions (Figs. 5 and 6) of the most western and central subgroup samples. 5.4. Tectonic implications of degassing systems in southern Tibet and the Himalayas 24
ACCEPTED MANUSCRIPT 5.4.1. Accretionary wedge and magmatic front of the IACSZ For the tectonic framework of an active subduction zone, accretionary wedge is usually formed along convergent boundary of the overriding plate, while magmatic front represents a trench-paralleled belt where subduction-related magmas are erupted
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and/or emplaced (Stern, 2002). According to previous studies (e.g., DeCelles et al.,
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2014; Hu et al., 2016 and references therein), we suggest that the classic tectonic
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model incorporating an accretionary wedge and a magmatic front can be reasonably applied to the IACSZ.
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First, northward subduction and accretion of the Neo-Tethyan oceanic lithosphere
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and the Indian continental lithosphere (Ding et al., 2005; Cai et al., 2012) have led to formation of a near E-W-trending accretionary wedge in the IACSZ, as suggested by
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many studies (e.g., Cai et al., 2012; DeCelles et al., 2014; Kelly et al., 2016; Leary et
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al., 2016). The Himalayan sequences on northern margin of the Indian continent (Yin, 2006) and the fore-arc basins on southern margin of the Asian continent (Wu et al.,
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2010) are major tectonic units of the accretionary wedge. Second, several stages of
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subduction-related magmatism have occurred in southern Tibet during closure of the Neo-Tethys ocean and the India-Asia continental collision and subduction. As a result, a near E-W-trending magmatic front was formed in southern Tibet, as represented by the Gangdese batholith (e.g., Wen et al., 2008; Ji et al., 2014; Zhu et al., 2015), the Linzizong volcanic rocks (e.g., Mo et al., 2008; Lee et al., 2012; Niu et al., 2013) and the post-collisional magmatic rocks (e.g., Chung et al., 2003; Guo et al., 2007, 2013, 2015; Liu et al., 2014; Hou et al., 2015; Zhang et al., 2017). 25
ACCEPTED MANUSCRIPT Additionally, it should be noted that magmatic front of the IACSZ represents a series of significant subduction-related magmatic activities, which may cover the time interval ranging from Late Cretaceous (i.e., the onset of the Gangdese magmatism; Wen et al., 2008) to Late Miocene (i.e., the end of post-collisional magmatism; Guo et
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al., 2015). In particular, the EMW model of volatile origin is based on the origin of
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post-collisional, ultrapotassic volcanic rocks in southern Tibet (25–8 Ma; Guo et al.,
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2013), suggesting that the EMW-derived melts (i.e., the magma chamber) may have formed since Early Miocene. This is consistent with slow-downs of the India-Asia
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convergence in Miocene times [e.g., 25–8 Ma (Lee and Lawver, 1995) and 20–10 Ma
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(Molnar and Stock, 2009; Iaffaldano et al., 2013)], which could lead to rollback of the subducting Indian continental lithosphere and partial melting of the EMW source
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generated by recycling of the Indian slab-derived materials (Guo et al., 2013, 2015).
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Although active volcanoes are absent in southern Tibet and the Himalayas at present, low resistivity anomalies based on magnetotelluric data have been imaged in
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crustal depth beneath typical hydrothermal systems in southern Tibet (e.g., Unsworth
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et al., 2005; Xie et al., 2016). This may provide evidence for the present-day residual melts beneath magmatic front of the IACSZ. In addition, recent mantle-derived melts are required for the high mantle helium emissions from hydrothermal degassing systems in southern Tibet. Therefore, the geochemical and geophysical constraints jointly support the presence of recent [e.g., Quaternary following assumption in Hoke et al. (2000)] EMW-derived melts beneath magmatic front of the IACSZ. 5.4.2. Tectonic affinities of the degassing systems revealed by volatile geochemistry 26
ACCEPTED MANUSCRIPT Unlike the degassing systems controlled by an identical tectonic setting (e.g., those in southern volcanic zone of Chile; Tardani et al., 2016), the hydrothermal belt in the IACSZ covers both accretionary wedge and magmatic front, making it an ideal laboratory for understanding tectonic implications of hydrothermal degassing systems
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in subduction zones. In this section, we will discuss the relationship between spatial
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variations in gas geochemical characteristics and tectonic settings of the degassing
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systems. On the plot of air-corrected 3He/4He ratios versus distance to the ITS (Fig. 10), the samples generally display a northward increasing trend of 3He/4He ratios
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from the Himalayas to southern Tibet, suggesting their potential tectonic affinities
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with the accretionary wedge in the south and the magmatic front in the north. For hydrothermal degassing systems in the accretionary wedge, regional crustal
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rock assemblages are considered as the source materials of the hydrothermal volatiles
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and contributions from the EMW-derived melts may be absent or negligible. This is consistent with the crustal-like 3He/4He ratios (<0.1 RA), as exemplified by the central
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subgroup samples (Figs. 5b and 10). In addition, the sediment (including both organic
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and inorganic types) contributions are important for hydrothermal volatiles from the accretionary wedge, which can well explain their geochemical characteristics such as high N2 and radiogenic 4He (Fig. 3) and positive δ15N values (Fig. 6b). We suggest that the widespread sediments in the Tethyan Himalayan sequence (Gehrels et al., 2011) and the fore-arc basins (Gehrels et al., 2011; Cai et al., 2012) may be the most plausible origin of sediment contributions to these degassing systems. For example, serpentinization and weathering of ultramafic rocks (Sleep et al., 2004; Kelley et al., 27
ACCEPTED MANUSCRIPT 2005; Morrill et al., 2013) is likely to generate the required organic matter. The significant role of sediments in accretionary wedge of the IACSZ agrees well with a series of geodynamic processes (e.g., subduction accretion, tectonic erosion and sedimentation) that are expected for tectonically accretionary margins (Clift and
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Vannucchi, 2004; Clift, 2016). For example, the crustal-like 3He/4He ratios of samples
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between the ITS and the India-Asia junction belt (JIA; Fig. 10) may represent volatile
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emissions from degassing systems in fore-arc basins, where sediments are inferred to be the first-order control on volatile geochemistry. Towards further north, the excess
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mantle 3He emissions from the hot springs in southern Tibet may point to their close
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affinities with mantle-derived melts beneath the magmatic front (Fig. 10). Therefore, we suggest that the EMW contributions are important for hydrothermal degassing
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systems in magmatic front of the IACSZ, in contrast to those in accretionary wedge
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that are dominated by sediment contributions. 5.4.3. The role of regional fault systems in the degassing systems
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Aside from the control of tectonic settings, regional fault systems (i.e., pathways
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of uprising fluids and/or volatiles) should also be taken as critical parameters for the spatially discernible geochemical characteristics, because their scales and depths of penetration would exert significant influence on outgassing rate of the EMW-derived volatiles and thus affect the degree of crustal contamination. As a result, uncertainties caused by the properties of regional fault systems may obscure the interpretation on tectonic affinities of hydrothermal degassing systems (i.e., accretionary wedge and magmatic front). As shown in the across-IACSZ profile of 3He/4He ratios (Fig. 10), 28
ACCEPTED MANUSCRIPT the abnormally high mantle helium emissions from the KKF can be attributed to the lithospheric-scale fault systems with deep depths of penetration and rapid slip rate (Klemperer et al., 2013). Therefore, regional fault systems such as the KKF can explain the samples with high 3He/4He ratios (>0.1 RA; Fig. 10) in fore-arc region (i.e.,
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south of the magmatic front). Additionally, similar fault-related mantle degassing can
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be exemplified by recent work of Xu et al. (2017), who recognized mantle-derived
lithosphere (200–250 km; Xu et al., 2002).
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helium emissions in foreland basins around the Tarim craton with extremely thick
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The across-IACSZ profile of 3He/4He ratios suggests that spatial variations in
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volatile geochemistry could shed light on tectonic framework and lithospheric structure of the IACSZ (e.g., the location of the junction belt where the subducting
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Indian plate steepens), especially when combined with constraints from geophysical
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studies. It is noted that the southern limit of mantle helium emissions in the magmatic front (i.e., the JIA revealed by 3He/4He ratios that is about 50–100 km north of the ITS;
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Hoke et al., 2000) displays a southward offset from the geophysically inferred
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northern extent of the subducted Indian crust (Figs.1 and 11; Nábělek et al., 2009). We suggest that the offset may result from southward uprising of hydrothermal fluids and the EMW-derived volatiles controlled by geometry of regional fault systems and location of residual melts beneath the magmatic front (Fig. 11). Therefore, as stated in Tardani et al. (2016), the effects of regional fault systems should be considered for understanding tectonic implications of hydrothermal degassing systems using volatile geochemistry. 29
ACCEPTED MANUSCRIPT 6. Conclusions On the basis of chemical compositions and He-C-N isotopes, the hydrothermal degassing systems in southern Tibet and the Himalayas are interpreted from the perspective of the Indian continental subduction, as summarized below:
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(1) The essential components required for volatile evolution are attributed to
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silicate rocks, carbonate rocks, sedimentary organic matter and mantle-derived
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components, which can be expected in a tectonic model incorporating accretionary wedge and magmatic front of the IACSZ.
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(2) Considering the critical role of the Indian slab-derived materials in mantle
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enrichment, an EMW source is proposed as the candidate for the required mantle-derived end-member, which is supported by the He-C-N isotope mixing
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calculations and constraints from petrogeochemical and geophysical studies.
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(3) Spatially discernible geochemical characteristics of the hydrothermal volatiles may have close affinities with tectonic settings of the degassing systems, as suggested
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by the significant sediment contributions to hydrothermal volatiles from accretionary
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wedge and the significant EMW-derived contributions to those from magmatic front. Additionally, regional fault systems would act as an extra factor that can affect geochemical characteristics of the hydrothermal volatiles.
Acknowledgements This study was financially supported by the Strategic Priority Research Program (B) of Chinese Academy of Sciences (Grant No. XDB03010600) and the National 30
ACCEPTED MANUSCRIPT Natural Science Foundation of China (Grant Nos. 41572321 and 41602341). We are grateful to editorial work of Prof. David Hilton and Prof. Yunpeng Wang. Two anonymous reviewers are appreciated for constructive comments that have greatly improved quality of this manuscript. Drs. Zhongping Li, Liwu Li, Li Du, Xiangxian
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Ma and Chunhui Cao are appreciated for sample analyses.
31
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ACCEPTED MANUSCRIPT Acta Petrologica Sinica 18, 539–550 (in Chinese with English abstract). Zheng, Y.-F., Fu, B., Gong, B., Li, L., 2003. Stable isotope geochemistry of ultrahigh pressure metamorphic rocks from the Dabie–Sulu orogen in China: implications for geodynamics and fluid regime. Earth-Science Reviews 62, 105-161.
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Zheng, Y.-F., Gong, B., Li, Y., Wang, Z., Fu, B., 2000. Carbon concentrations and
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isotopic ratios of eclogites from the Dabie and Sulu terranes in China. Chemical
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Geology 168, 291-305.
Zhou, H., Murphy, M., 2005. Tomographic evidence for wholesale underthrusting of
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India beneath the entire Tibetan plateau. Journal of Asian Earth Sciences 25,
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445-457.
Zhou, X., Wang, W., Chen, Z., Yi, L., Liu, L., Xie, C., Cui, Y., Du, J., Cheng, J., Yang,
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L., 2015. Hot Spring Gas Geochemistry in Western Sichuan Province, China
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After the Wenchuan Ms 8.0 Earthquake. Terrestrial, Atmospheric & Oceanic Sciences 26, 361-373.
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Zhu, D.-C., Wang, Q., Zhao, Z.-D., Chung, S.-L., Cawood, P.A., Niu, Y., Liu, S.-A.,
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Wu, F.-Y., Mo, X.-X., 2015. Magmatic record of India-Asia collision. Scientific Reports 5, 14289. Zhu, D.-C., Zhao, Z.-D., Niu, Y., Dilek, Y., Hou, Z.-Q., Mo, X.-X., 2013. The origin and pre-Cenozoic evolution of the Tibetan Plateau. Gondwana Research 23, 1429-1454.
52
ACCEPTED MANUSCRIPT Figure Captions
Fig. 1. Simplified map showing tectonic framework, post-collisional volcanic rocks and representative hot springs in southern Tibet and the Himalayas. Abbreviations:
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MBT, main boundary thrust; MCT, main central thrust; STDS, southern Tibet
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detachment system; ITS, Indus-Tsangpo suture; BNS, Bangong-Nujiang suture; LHS,
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Lesser Himalayan sequence; HHS, Higher Himalayan sequence; THS, Tethyan Himalayan sequence. Sample locations in previous studies (Yokoyama et al., 1999;
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Hoke et al., 2000; Zhao et al., 2002; Newell et al., 2008; Klemperer et al., 2013) are
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represented by red filled circles, and those in this study are shown as yellow stars with numbers as follows: 1, Namuru; 2, Baer; 3, Tirthapuri; 4, Qiwusi; 5, Darong; 6,
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Dagejia; 7, Liudaoban; 8, Xiqin; 9, Mianjiu. In particular, the yellow stars with red
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dots represent hot springs studied by both previous studies and this study, while the yellow star without red dot represents the Darong hot springs that are studied for the
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first time. Profile AA’ denotes the geophysically imaged lithospheric structure of the
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IACSZ (Zhou and Murphy, 2005), as shown in Fig. 11. The red dashed line represents the geophysically inferred northern extent of the Indian crust (Nábělek et al., 2009), and the blue dashed line represents the India-Asia junction belt revealed by 3He/4He ratios (Hoke et al., 2000).
Fig. 2. Field photos showing hot springs (a, Namuru), geysers (b, Dagejia) and sinters (c, Dagejia; d, Darong) in southern Tibet. During fieldwork in the Dagejia region (i.e., 53
ACCEPTED MANUSCRIPT the Sample location 6 in Fig. 1), soil CO2 emissions were measured but were under detection limit of the portable soil CO2 flux meter (GXH-3010E; Zhang et al., 2016), which may suggest significant influence of thick sinters on soil microseepage of the
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hydrothermal degassing systems.
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Fig. 3. (a) CO2/N2 versus CO2 (%) and (b) N2 (%) versus 4He (ppm) for hydrothermal
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volatiles from southern Tibet and the Himalayas. Filled and open symbols represent, respectively, data in this study and published data in Yokoyama et al. (1999), Hoke et
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al. (2000), Zhao et al. (2002), Walia et al. (2005), Newell et al. (2008), Klemperer et
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al. (2013) and unpublished data of Z. Guo. The compiled data in previous studies are
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available in Supplementary Table S1.
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Fig. 4. Triangle plot of N2-He -Ar for hydrothermal volatiles from southern Tibet and the Himalayas (modified from Giggenbach and Poreda, 1993). The ASW represents
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air saturated water. Data source and symbols are as in Fig. 3.
Fig. 5. (a) 3He/4He (Rm/RA) versus (He/Ne)m/ (He/Ne)ASW and (b) 3He/4He (RC/RA) versus Longitude (E) for hydrothermal volatiles from southern Tibet and the Himalayas. The calculated binary mixing curves between the ASW and crust-mantle mixtures with variable proportions of mantle helium (i.e., 100%, 20%, 10%, 5%, 1% and 0%) are shown. (He/Ne)m/ (He/Ne)ASW is calculated based on air-normalized He/Ne ratio multiplied by the ratio of their Bunsen coefficients (βNe/βHe = 1.25 at 54
ACCEPTED MANUSCRIPT groundwater recharge temperature of 10 °C; Weiss, 1971; Hilton, 1996). Reference values for end-members are as follows: ASW, 3He/4He = 0.987 RA (Klemperer et al., 2013), (He/Ne)m/(He/Ne)ASW = 1.12; Crust, 3He/4He = 0.02 RA (Lupton, 1983); Mantle, 3He/4He = 8 ± 1.5 RA (Sano and Fischer, 2013). For the crust, mantle and
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crust-mantle mixture end-members, 4He/20Ne ratio of 10000 is assumed (Morikawa et
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al., 2008). Data source and symbols are as in Fig. 3.
Fig. 6. (a) δ13C (‰) versus CO2/N2 and (b) δ15N (‰) versus CO2/N2 for hydrothermal
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volatiles from southern Tibet and the Himalayas. Data source and symbols are as in
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Fig. 3.
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Fig. 7. (a) δ13C (‰) versus CO2/3He for hydrothermal volatiles from southern Tibet
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and the Himalayas. Data source and symbols are as in Fig. 3. Abbreviations: DMM, depleted MORB-source mantle; CAR, carbonate rocks; SED, organic sediments
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(Sano and Marty, 1995; Zhang et al., 2016). In the three-component mixing model of
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Sano and Marty (1995), the SED end-member is solely based on carbon-rich organic matter. The effects of nitrogen-rich organic matter (and/or calcite precipitation) on the samples outside the mixing envelope are denoted by the yellow arrow. Considering the potential effects of calcite precipitation and their crustal origin, these samples would not be considered for further discussion of mixing between the EMW-derived components and crustal contaminants (e.g., Figs. 8 and 9). The solid grey lines with arrows represent calcite precipitation at temperatures of 25 oC and 192 oC, 55
ACCEPTED MANUSCRIPT respectively (Ray et al., 2009), and their starting point is represented by the sample DR1501. The rectangle represents carbonate rocks with δ13C (‰) values of 0 ± 2‰ (Hoefs, 2009) and CO2/3He ratios ranging in 1012−1014 (O’Nions and Oxburgh, 1988). (b) CO2/N2 versus CO2/3He for hydrothermal volatiles from southern Tibet and the
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Himalayas.
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Fig. 8. He-C isotope coupling model (a) and He-N isotope plot (b) showing origin of hydrothermal volatiles from southern Tibet and the Himalayas. Data source and
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symbols are as in Fig. 3. Abbreviations: DMM, depleted MORB-source mantle; EMW,
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enriched mantle wedge; CAR, carbonate rocks; SED, sediments enriched in radiogenic helium and organic-sourced carbon and nitrogen. Reference values for
D
end-members are listed in Supplementary Table S2. K value [= (He/C)A/(He/C)B]
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represents the curvature of mixing curve between end-member A and end-member B. Numbers (5%, 10%, 15% and 20%) on the K = 0.29 curve represent the proportions
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of subducted SED components in formation of the EMW source. The yellow
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parallelogram represents possible He-C isotope range of the EMW. Potential influence of crustal melts on the EMW-derived melts is marked. The dashed curves represent mantle source enrichment and/or crustal contamination by the CAR components. The question mark represents uncertainty of contributions from subducted carbonate to the He-C isotope systematics of the EMW. Helium isotopes of the EMW-derived melts or volatiles in (b) are as those in (a), and the potential influence of crustal melts on the EMW-derived melts is marked. In particular, typical crustal-derived volatiles are 56
ACCEPTED MANUSCRIPT shown for comparison.
Fig. 9. 3He/4He (RC/RA) versus CO2/3He for hydrothermal volatiles from southern Tibet and the Himalayas. Data source and symbols are as in Fig. 3. The solid curves
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represent mixing between the EMW-derived volatiles and continental crustal
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materials. The purple diamonds with proportions (i.e., 5%, 10%, 15% and 20%)
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represent the EMW-derived melts following results of the He-C isotope coupling
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model (Table 3).
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Fig. 10. Across-IACSZ profile of 3He/4He ratios in southern Tibet and the Himalayas showing transition of tectonic setting from accretionary wedge to magmatic front.
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Data sources and symbols are as in Fig. 3. Abbreviations: ITS, Indus-Tsangpo suture;
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Karakoram fault.
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JIA, junction between the Indian and Asian plates (Hoke et al., 2000); KKF,
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Fig. 11. Schematic map showing tectonic settings of the hydrothermal degassing systems in the IACSZ. Abbreviations: MBT, main boundary thrust; MCT, main central thrust; STDS, southern Tibet detachment system; ITS, Indus-Tsangpo suture; BNS, Bangong-Nujiang suture. Location of the profile AA’ is given in Fig. 1. The LV–HCZs denote geophysically recognized low-velocity–high-conductivity zones in crustal depth beneath southern Tibet (e.g., Brown et al., 1996; Wei et al., 2001; Xie et al., 2016). The red dashed line (31oN) represents the geophysically inferred northern 57
ACCEPTED MANUSCRIPT extent of the Indian crust (Nábělek et al., 2009), which suggests steepening and subducting of the Indian continental lithosphere beneath regions north of the latitude 31oN. Similarly, the blue dashed line (JIA) represents the India-Asia junction belt
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revealed by 3He/4He ratios (Hoke et al., 2000).
58
ACCEPTED MANUSCRIPT Supplementary Table Captions
Table S1. Compiled literature data used in this study.
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Table S2. Reference values for end-member parameters used in the He-C isotope
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coupling model
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Fig. 2
61
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Fig. 3
62
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Fig. 4
63
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Fig. 5
64
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Fig. 6
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Fig. 7
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Fig. 8
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Fig. 9
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Fig. 11
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ACCEPTED MANUSCRIPT Table 1 Chemical compositions of hydrothermal volatiles from southern Tibet and the Himalayas.
31°55'2
80°09'5
501
locatio Namur n u
2.6"
8.9"
NMR1
Namur
31°55'2
80°09'5
502
u
2.6"
8.9"
31°26'4
80°24'1
4.7"
8.5"
31°26'4
80°24'1
4.7"
8.5"
BE150 2
Baer
Temper
ude (E)
tion
le
ature
(m)
subgr
(oC)
4389
oup WS
77
95.1
0.03
4389
WS
75
89.4
4779
WS
69
4779
WS
4402
Tirtha
31°07'3
80°45'0
01
puri
8.8"
7.2"
MS15
Tirtha
31°07'3
80°45'0
02
puri
8.8"
7.2"
QW15
Qiwus
30°46'0
81°21'4
01
i
1.4"
6.1"
DR15
Daron
30°25'3
83°33'5
01
g
7.5"
2.6"
DR15
Daron
30°25'3
83°33'5
02
g
7.5"
2.6"
DGJ1
Dageji
29°36'1
85°44'4
501
a
9.2"
4.8"
DGJ1
Dageji
29°36'1
85°44'4
503
a
5.0"
LDB1
Liuda
29°19'2
502
oban
5.6"
02
Xiqin
29°04'1
(%)
(ppm)
2.87
0.22
% 0. ) 90
8623
0.32
8.24
0.27
43.9
11.9
43.7
−
51
9.67
1.80
87.5
0.76
WS
47
26.5
4402
WS
60
4400
WS
4990
CS
(%)
(%)
67.1
−
2.07
−
6.10
96.0
38
34.5
8.28
56.6
0.14
37
19.5
2.89
76.1
0.98
11.7
48.6
5.81
42.2
1.78
5090
CS
48
62.1
6.24
29.1
0.50
5074
CS
82
56.3
4.94
35.7
0.71
4046
CS
46
75.1
0.49
0.11
23.2
4066
CS
53
93.9
0.07
4.48
0.16
3734
ES
59
18.5
2.57
77.3
1.14
3734
ES
65
15.1
1.62
81.4
1.20
8.1"
87°05'4 7.4"
87°44'2 8.7"
29°20'2
90°00'0
0.5"
2.3"
MJ150
Mianji
1
u
MJ150
Mianji
29°20'2
90°00'0
2
u
0.5"
2.3"
5015
(%)
CO2
60
7.5"
AC
He
(
O2
CS
CE
XQ15
Ar
N2
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MS15
CH4
D
1
Baer
Samp
PT
no. NMR1
BE150
Eleva
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e
e (N)
Longit
SC
e
Latitud
NU
Sampl
MA
Sampl
1. 08 0. 56 0. 18 0. 36 0. 25 0. 41 0. 34 0. 66 0. 62 0. 71 0. 95 1. 41 .3 3 .3 1
6760
1835
767 − −
242
2260
9445
13956
16859
1365
110
1199
1117
Abbreviations are as follows: WS, western subgroup; CS, central subgroup; ES, eastern subgroup.
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ACCEPTED MANUSCRIPT Table 2 Helium isotopes, carbon and nitrogen stable isotope ratios of hydrothermal volatiles from southern Tibet and the Himalayas. Sampl
Samp
no.
e
le
locati
subgr WS oup
4
NMR1501
He/ Ne
Rm/RA
or
RC/RA
01
ru
1688
0.016
ru
1355
0.016
0.020
0.020 0.0
0.068
WS
0.041 0.0 01
MS1502
WS
puri Tirtha
0.046
0.572
WS 1.50
0.350
2.90 WS
D
i
−
i
g
1889
CE
DR1501
AC
CS
1076
0.015
1793
0.014
2.06
−
1010
-0.14
-5.35
−
−
0.85
1.07
0.02
× 0.023
-5.86
1010
0.85 1.50
0.0
0.0
-6.03
109
1.50
0.015
2.02
−
−
109
2.02
0.014
0.78
−
-11.3
109
0.78 −
−
0.46
3.04
0.55
×
03
CS
0.020
−
−
− 72
1.09
×
0.0
204
1.01
×
−
CS
2.00
× 0.016
-6.34
CS
Xiqin
-0.25
02
Liuda
XQ1501
−
-5.19
-5.05
oban
oban
-0.45
×
a
LDB1502
−
02 0.016
Dageji
LDB1501
-0.21
-5.14
a
Liuda
-0.80 -0.23
0.0
1478
7.45 109
-0.02
02
CS
DGJ1503
109
×
−
0.023
3.80
×
-0.22
01
Dageji
DGJ1501
0.218
108
4.94
0.0
Daron g
-5.00
4.47
×
-0.02
0.090
−
CS
DR1502
20
0.169
CS Daron
0.373
He
108
-6.73
-6.76
CO2/
×
01
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QW1502
23
0.0
Qiwus
Qiwus
1.52
-0.25
0.0
WS
QW1501
(‰) 4.99
0.0
0.80
puri
-11.6
0.046
NU
MS1501
1182
MA
Tirtha
3
4.99
SC
11.0
Baer
(‰)
N2)C
4.95
24 Baer
(δ15N-
-12.9
01
WS
BE1502
(‰)
δ15N-N2
0.0
Namu
BE1501
δ13C-CO2
0.0
WS
NMR1502
Err
RI
on Namu
20
PT
Sample
0.019
-11.0
0.46
−
-12.3
0.76
107 −
−
ACCEPTED MANUSCRIPT CS
0.0
0.59
×
12 XQ1502
Xiqin
11.0
0.057
ES
MJ1501
u
1304
0.036
0.035
0.045
1010
1.18 1.05
1.17
×
-7.75 0.045
1.31
×
-7.72
03 1477
109
0.60
0.0
Mianji u
-12.3
1.18
04
ES
MJ1502
0.035 0.0
Mianji
8.63
1.05
1010
SC
RI
PT
Abbreviations are as follows: WS, western subgroup; CS, central subgroup; ES, eastern subgroup. Measured 3He/4He ratios (Rm/RA) are corrected for air contamination using air-normalized He/Ne ratio multiplied by the ratio of their Bunsen coefficients (Weiss, 1971), assuming a groundwater recharge temperature of 10 °C (see details in Hilton, 1996). Corrected 3He/4He ratio is reported in RC/RA notation, where RC denotes air-corrected 3He/4He ratio and RA represents 3He/4He ratio of air, i.e. 1.382 × 10-6 (Mabry et al., 2013). δ15N-N2 (‰) values are also corrected following method for helium isotope correction (Halldórsson et al., 2013). The CO2/3He ratios are calculated based on air
AC
CE
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D
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corrected 3He contents.
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ACCEPTED MANUSCRIPT Table 3 Calculated 3He/4He, δ13C-CO2, CO2/3He, CO2/4He and carbon contents of the EMW source. 3
He/4He
Proportion of a
δ13C-CO2 b
CO2/3He
CO2/4He
Carbon content
(×10 )
(×104)
(ppm)
9
(RA)
(‰)
5%
5.23
−8.11
2.31
1.67
2108
10%
3.77
−9.45
2.65
1.38
2295
15%
2.88
−10.59
3.03
1.21
2483
20%
2.28
−11.58
3.46
1.09
2670
silicate melts
Silicate melts involved in mantle wedge enrichment are generated by partial melting of subducted Indian
PT
a
slab-derived silicate sediments (SED) that incorporate organic-sourced carbon, as commonly discovered in ultra-high pressure metamorphic rocks (Zheng et al., 2000, 2003). Proportions (5−20%) of the silicate melts are b
RI
from Guo et al. (2013).
Calculated δ13C values are based on the scenario of silicate-dominated mantle enrichment, which is consistent
SC
with petrogenesis of the post-collisional K-rich volcanic rocks (see details in text; Guo et al., 2013, 2015). Helium contents of the SED end-member are calculated based on U-Th decay of silicate rocks (U = 1.3 ppm, Th = 5.6 ppm; Rudnick and Gao, 2003) with age of 500 Ma (Kundig, 1989); Carbon contents of the SED end-member
NU
are calculated by carbon contents and proportions of continental crustal materials (except for carbonate; see details in Wedepohl, 1995). According to genesis of the EMW-derived K-rich magmas (Guo et al., 2013), degree of partial melting of the SED end-member is assumed at 10%, following batch melting model and partition
MA
coefficient (D) of helium (DHe = 1.8 × 10-4; Graham et al., 2016) and carbon (DHe = 3.3 × 10-4; Rosenthal et al., 2015). Partial melts of the SED components are mixed with asthenosphere during slab-mantle interaction to
AC
CE
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D
generate the EMW source.
74
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Graphical abstract
75
ACCEPTED MANUSCRIPT Highlights The degassing systems are studied in context of the Indian continental subduction Enriched mantle wedge (EMW) is proposed as source of the mantle contributions The EMW-derived volatiles are contaminated by regional crustal rock assemblages
CE
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D
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SC
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PT
Tectonic affinities of degassing systems are revealed by volatile geochemistry
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Maoliang Zhang, Zhengfu Guo, Lihong Zhang, Yutao Sun, Zhihui Cheng PII: DOI: Reference:
S0009-2541(17)30111-0 doi: 10.1016/j.chemgeo.2017.02.023 CHEMGE 18265
To appear in:
Chemical Geology
Received date: Revised date: Accepted date:
27 July 2016 16 February 2017 23 February 2017
Please cite this article as: Maoliang Zhang, Zhengfu Guo, Lihong Zhang, Yutao Sun, Zhihui Cheng , Geochemical constraints on origin of hydrothermal volatiles from southern Tibet and the Himalayas: Understanding the degassing systems in the India-Asia continental subduction zone. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Chemge(2016), doi: 10.1016/ j.chemgeo.2017.02.023
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Geochemical constraints on origin of hydrothermal volatiles from southern Tibet and the Himalayas: Understanding the degassing systems in the India-Asia continental subduction
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zone
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Maoliang Zhang, Zhengfu Guo *, Lihong Zhang, Yutao Sun, Zhihui Cheng
Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and
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Geophysics, Chinese Academy of Sciences, Beijing 100029, China
* Corresponding author.
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Tel.: +86 10 82998393;
D
E-mail: [email protected] (Z. Guo).
Fax: +86 10 62010846;
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Address: No. 19, Beitucheng Western Road, Chaoyang District, Beijing, China
1
ACCEPTED MANUSCRIPT Abstract Hydrothermal activities (e.g., hot springs and geysers) are extensively distributed in southern Tibet and the Himalayas, forming an east-west trending, ~2000-km-long, hydrothermal belt that represents the ongoing degassing systems in the India-Asia
PT
continental subduction zone (IACSZ). In this study, we report new data of chemical
RI
compositions and He-C-N isotopes for gas samples from representative hot springs in
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the hydrothermal belt, aimed at understanding volatile origin of the hydrothermal degassing systems in the IACSZ and their tectonic implications. According to spatial
NU
location, the samples are divided into western, central and eastern subgroups, which
MA
display spatially discernible geochemical characteristics, such as 3He/4He and CO2/3He ratios. The essential components required for volatile evolution include
D
silicate rocks, carbonate rocks, sedimentary organic matter and a mantle-derived
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end-member, which are all expected in the IACSZ following a tectonic model incorporating an accretionary wedge and a magmatic front. On the basis of He-C-N
CE
isotope mixing calculations, together with constraints from petrogeochemical and
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geophysical studies, an enriched mantle wedge (EMW) is proposed as a potential candidate for the source of the mantle-derived components, which highlights the importance of recycled Indian continental materials compared to previous models that do not take this possibility for mantle source enrichment in account. The regional crustal rock assemblages composed of silicate rocks, carbonate rocks and sedimentary organic matter are interpreted as contaminants of the EMW-derived volatiles (as exemplified by degassing systems in the magmatic front) or as source materials of 2
ACCEPTED MANUSCRIPT hydrothermal volatiles from degassing systems in the accretionary wedge. Following tectonic framework of the IACSZ, we suggest that spatial variations in volatile geochemistry (e.g., 3He/4He) are predominantly controlled by tectonic affinities (i.e., the accretionary wedge and magmatic front) of the hydrothermal degassing systems in
He contents agree well with the high sedimentary contributions to degassing systems
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4
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southern Tibet and the Himalayas. The crustal-like 3He/4He ratios and high N2 and
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in the accretionary wedge (including the Himalayas and fore-arc basins), whereas the high mantle-derived helium emissions in southern Tibet exhibit close affinities with
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contributions from the EMW-derived melts beneath the magmatic front. Moreover,
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regional fault systems with variable scales and depths of penetration would act as an extra factor that may perturb the across-IACSZ profile of 3He/4He ratios controlled by
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tectonic settings. Our interpretation on degassing systems in southern Tibet and the
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Himalayas may have the potential to provide constraints from volatile geochemistry
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for understanding material recycling mechanism and tectonic settings of the IACSZ.
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Key words: Gas geochemistry; Enriched mantle source; Hydrothermal degassing systems; Continental subduction zone; Southern Tibet and the Himalayas
1. Introduction Subduction-zone degassing systems, characterized by outgassing of volatiles derived from the subducting plate, mantle wedge and overlying plate, play an important role in modulating contents of volatile elements (e.g., carbon) in the Earth’s 3
ACCEPTED MANUSCRIPT atmosphere. Therefore, considerable studies on volatile recycling in subduction zones have been carried out, especially for oceanic subduction zones (OSZ; e.g., Hilton et al., 2002; Sano and Fischer, 2013; Lee and Lackey, 2015; Kagoshima et al., 2015; Kelemen and Manning, 2015; Barry and Hilton, 2016). However, comprehensive
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understanding of the degassing systems remain deficient for continental subduction
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zones (CSZ) where two colliding continental plates are separated by an OSZ-like
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mantle wedge (Ducea, 2016). Following the India-Asia collision in Early Cenozoic (ca. 55–50 Ma; Tapponnier et al., 2001; Najman et al., 2010; Bouilhol et al., 2013;
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Zhu et al., 2015), northward subduction of the Indian continental lithosphere beneath
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the Asian continent defines the most significant, ongoing continental subduction zone on Earth at present, as suggested by constraints from geophysical (e.g., Nábělek et al.,
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2009; Replumaz et al., 2014; Chen et al., 2015; Tian et al., 2015; Liang et al., 2016),
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petrogeochemical (e.g., Guo et al., 2006, 2007, 2013, 2014, 2015) and geodynamic (e.g., Capitanio et al., 2010; Duretz and Gerya, 2013; Burov et al., 2014; Replumaz et
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al., 2016) studies. Located at the leading edge of the IACSZ, southern Tibet and the
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Himalayas provide an important opportunity for understanding the degassing systems related with subduction of the Indian continental lithosphere. Although a number of studies on hydrothermal volatiles from southern Tibet and the Himalayas have been carried out (e.g., Yokoyama et al., 1999; Hoke et al., 2000; Zhao et al., 2001, 2002; Newell et al., 2008; Klemperer et al., 2013; Zhang et al., 2014), most of them focus on relatively limited or individual region, and thus genetic models for volatile origin have not been fully integrated in the geodynamic setting of 4
ACCEPTED MANUSCRIPT the Indian continental subduction. Additionally, helium isotopes of hydrothermal volatiles have been taken as an effective tool for understanding lithospheric structure (e.g., Hoke et al., 2000; Newell et al., 2008; Klemperer et al., 2013), when combined with geophysically imaged lithospheric profile of the IACSZ (e.g., Zhao et al., 2010;
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Chen et al., 2015; Shi et al., 2015; Tian et al., 2015; Xie et al., 2016). Nevertheless,
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the geochemical and tectonic implications inferred from chemical compositions and
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isotopes of major volatile elements (e.g., carbon and nitrogen; Newell et al., 2008) need to be further constrained, especially considering the volatile recycling processes
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(e.g., mantle wedge enrichment) and regional tectono-magmatic evolution related
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with northward subduction of the Indian continental lithosphere. In this study, we report new data of chemical compositions and He-C-N isotopes
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for bubble gas samples from representative hot springs in southern Tibet and the
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Himalayas, in order to constrain origin and evolution of these hydrothermal volatiles and to understand tectonic implications of the hydrothermal degassing systems in the
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IACSZ. On the basis of He-C-N isotope mixing calculations, together with constraints
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from petrogeochemical and geophysical studies (e.g., Guo et al., 2013, 2015; Chen et al., 2015; Shi et al., 2015), we propose a genetic model that incorporates an enriched mantle wedge (EMW) as the potential source for mantle-derived contributions to the hydrothermal volatiles. Additionally, tectonic affinities of the degassing systems in the IACSZ are interpreted from the perspective of volatile geochemistry and northward subduction of the Indian continent beneath the Asian continent.
5
ACCEPTED MANUSCRIPT 2. Geological setting The southern Tibetan Plateau (i.e., the Lhasa terrane) is separated from the Himalayas by the Indus-Tsangpo suture (ITS) to the south and from the Qiangtang terrane by the Bangong-Nujiang suture (BNS) to the north (Fig. 1). Compared with
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the northern Tibetan Plateau, southern Tibet is characterized by relatively higher
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topographic relief (Feilding et al., 1994; Clark and Royden, 2000), which may suggest
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complicated orogenic and uplift history as a consequence of collision and subduction between the Indian and Asian continents. Precambrian basement rocks of the southern
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Tibetan Plateau, which are rarely outcropped, can be represented by the Amdo gneiss
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(Guynn et al., 2006, 2012) and metamorphic rocks of the Nyainqentanglha Group to the west of the Nam Lake (Hu et al., 2005). Phanerozoic cover sequences of the
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southern Tibetan Plateau are mainly composed of Paleozoic to Mesozoic sedimentary
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rocks (e.g., slate, limestone and sandstone) and Mesozoic to Cenozoic magmatic rocks (Zhu et al., 2013 and references therein). The Himalayas are composed of the
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Lesser Himalayan sequence (LHS), Higher Himalayan sequence (HHS) and Tethyan
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Himalayan sequence (THS) from south to north (Fig. 1). In particular, the THS mainly consists of mud- and serpentinite-matrix mélanges, radiolarites, pillow basalts, deep-marine sedimentary rocks and metasedimentary rocks that were accreted onto southern margin of the Asian continent as the Tethyan oceanic lithosphere was subducted northward (Cai et al., 2012; DeCelles et al., 2014). These regional crustal rock assemblages have played an important role in degassing systems in the IACSZ. Owing to convergence between the Indian and Asian continents during Cenozoic 6
ACCEPTED MANUSCRIPT times (Yin and Harrison, 2000; Styron et al., 2015), different types of fault systems were formed in southern Tibet and the Himalayas (Fig. 1), such as (1) the lithosphere extensional tectonics recorded by intrusion of the post-collisional magmas (Williams et al., 2001; Huang et al., 2016) and the N-S-trending rifts (e.g., the Gulu-Yadong rift,
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GYR; Zhang et al., 2013), (2) the north-dipping southern Tibetan detachment system
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(STDS; Burchfiel et al., 1992; Yin, 2006; La Roche et al., 2016), (3) the high-angle
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thrust fault systems (e.g., the main central thrust; Searle et al., 2008) and (4) the strike-slip fault systems represented by the NW-SE-trending Karakoram fault (KKF;
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Murphy et al., 2000; Valli et al., 2008). Modern hydrothermal activities in southern
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Tibet and the Himalayas are mainly distributed within and/or around these fault systems (Fig. 1; e.g., Yokoyama et al., 1999; Hoke et al., 2000; Newell et al., 2008;
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Klemperer et al., 2013), which can provide variable scales of pathways for the
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uprising hydrothermal fluids in the degassing systems. The representative types of the hydrothermal activities in the IACSZ include boiling hot spring (Fig. 2a) and geyser
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(Fig. 2b), where carbonate and/or silica sinters (or travertine) are commonly
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outcropped (Fig. 2c, d). Additionally, geophysical studies have recognized several low-velocity–high-conductivity zones (LV–HCZs) beneath southern Tibet and the Himalayas (e.g., Brown et al., 1996; Wei et al., 2001; He et al., 2016; Xie et al., 2016), suggesting the presence of partial melts in crustal depth and their relationship with the shallow hydrothermal degassing systems. Our study areas consist of the Namuru, Baer, Tirthapuri, Qiwusi, Darong, Dagejia, Liudaoban, Xiqin and Mianjiu hot springs from west to east (Fig. 1), which are 7
ACCEPTED MANUSCRIPT generally distributed along the southern boundary of the Tibetan Plateau (i.e., the ITS). According to field observations, the temperatures of hot springs vary from 37 oC to 82 o
C (Table 1), and the highest fluid temperature occurs in the Dagejia geysers (Figs. 1
and 2). Combined with literature records, there are no location-dependent features of
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fluid temperatures between hot springs in the Himalayas and those in southern Tibet.
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It may be a general agreement that the hydrothermal fluids that feed hot springs are
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uprising through fault systems with variable scales and depths of penetration. For example, the Tirthapuri (or Menshi; Klemperer et al., 2013) hot springs are related
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with the lithospheric-scale KKF, while the GYR may be important for the hot springs
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in eastern Lhasa terrane (Fig. 1). Additionally, some small-scale fault systems could provide pathways for volatiles released by hot springs in the Himalayas and fore-arc
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basins (Fig. 1). These regional fault systems can affect chemical compositions of the
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hydrothermal fluids by conditioning residence time, cooling and mixing processes (Tardani et al., 2016). Apart from fault-related influence, the differences in crustal
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lithologies and tectonic settings between southern Tibet and the Himalayas are also
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likely to affect volatile evolution. As exemplified by the Liudaoban and Xiqin hot springs in the Xigaze fore-arc basin (Fig. 1), the sedimentary features (e.g., organic sediments) are prominent for hot springs in the Himalayas and fore-arc basins. In contrast, organic sediments appear to be less important for hydrothermal degassing systems in the GYR, southern Tibet. These field characteristics of degassing systems in southern Tibet and the Himalayas could be linked to origin and evolution of the hydrothermal volatiles. 8
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3. Samples and analytical methods Following the water displacement method (Sano et al., 1994), eighteen samples were collected using glass bottles from the hot springs listed in Table 1. In particular,
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samples from the Darong hot springs were collected and studied for the first time (Fig.
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1). In order to capture bubbling hot spring gases, an inverted funnel connected with
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rubber pipes was submerged in spring water and placed on top of the uprising bubbles. Before collecting accumulated gases in the funnel, rubber pipes were completely
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flushed by the continuously outgassing bubbles to avoid air contamination. According
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to variable bubble fluxes of the hot springs, a time interval of ca. 5–15 minutes is empirically enough to rule out atmospheric contributions to the accumulated gases in
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the inverted funnel and rubber pipes. After the pipe flushing step, the bubble gases
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were forced into glass bottles that were filled with spring water of the sampling location and then were prepared for laboratory analyses.
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Bulk chemical compositions and He-C-N isotope compositions of the selected
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samples were determined within one week after fieldtrip in Lanzhou Center for Oil and Gas Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences. Bulk chemical compositions were determined by a MAT 271 mass spectrometer (Cao et al., 2011, 2016; Zhou et al., 2015), as summarized in Table 1. Helium isotopes were analyzed by the Noblesse noble gas mass spectrometer that incorporates a two-stage separation and purification system for noble gas enrichment (Cao et al., 2016). Air sample from the top of the Gaolan mountain (to the south of the 9
ACCEPTED MANUSCRIPT city of Lanzhou) was collected and analyzed to meet the laboratory standards (i.e., 3
He/4He = 1.39 × 10-6). Additionally, carbon and nitrogen stable isotope ratios were
analyzed by the GC-C-IRMS system consisting of a TRACE GC Ultra gas chromatograph (GC) coupled to a Finnigan MAT 253 mass spectrometer (Li et al.,
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2014). The results of helium isotopes, carbon and nitrogen stable isotope ratios of
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samples in this study are reported in Table 2. Additionally, all literature data (e.g.,
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Yokoyama et al., 1999; Hoke et al., 2000; Zhao et al., 2002; Walia et al., 2005; Newell et al., 2008; Klemperer et al., 2013) compiled in this study are available in the
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Supplementary Table S1.
4. Results
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To understand spatial variations in geochemical characteristics of the studied
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hydrothermal volatiles, samples from southern Tibet and the Himalayas, including those in literature and this study, are divided into three subgroups based on their
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locations (Fig. 1), which are the western subgroup (to the west of longitude 82oE), the
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central subgroup (between longitude 82oE and 90oE), and the eastern subgroup (to the east of longitude 90oE), respectively. 4.1. Chemical compositions Hydrothermal volatiles from southern Tibet and the Himalayas are predominated by either CO2 or N2, or both of them (Table 1), as indicated by the well-correlated CO2/N2 and CO2 in Fig. 3a. Compared with the CO2-rich samples (CO2/N2 = 4.18– 52.2) of the eastern subgroup, the N2-rich hydrothermal volatiles tend to be more 10
ACCEPTED MANUSCRIPT common for hot springs from the western (CO2/N2 = 0.03–87.3) and central (CO2/N2 = 0.02–722) segments of southern Tibet and the Himalayas, although some CO2-rich samples can also be observed (Fig. 3a). Another prominent feature in compositions of these hydrothermal volatiles is the
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different range of radiogenic helium contents between the eastern subgroup samples
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and those of the western and central subgroups (Fig. 3b). Most of the eastern
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subgroup samples (except for two N2-rich samples) have relatively lower 4He contents, whereas most of the western and central subgroup samples are characterized by
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relatively higher 4He contents as reported in previous studies (e.g., Hoke et al., 2000;
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Zhao et al., 2002; Newell et al., 2008). Moreover, high 4He contents appear to be coupled with high N2 contents, especially for samples of the western and central
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subgroups (Fig. 3b). The coupled high N2 and 4He contents can be linked to their
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affinities with sedimentary environment, because (1) high contents of organic-sourced nitrogen can be provided by the biologic activities during sedimentation processes
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(Bebout et al., 2013), and (2) radiogenic helium tend to be accumulated in
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sedimentary basins and crustal aquifers (Torgersen, 2010; Lowenstern et al., 2014). With respect to N2/Ar and He/Ar ratios (Fig. 4), most of the eastern subgroup samples have low He/Ar ratios and N2/Ar ratios that range from the ASW value (air saturated water, N2/Ar = 38; Hilton, 1996) to the air value (N2/Ar = 84). In contrast, the western and central subgroup samples are characterized by relatively higher He/Ar and N2/Ar ratios (Fig. 4). Moreover, irrespective of sample locations, all the samples display large variations in CO2/3He ratios (e.g., CO2/3He = 3.04 × 107 – 1.31 × 1010 11
ACCEPTED MANUSCRIPT for samples in this study; Table 2), which may result from multi-component mixing (Barry et al., 2013) and/or elemental fractionation induced by different solubilities of CO2 and helium in melts/fluids (e.g., calcite precipitation; Hilton et al., 1998; Ray et al., 2009; Iacono-Marziano et al., 2010; Güleç and Hilton, 2016).
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4.2. Helium isotopes
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The measured 3He/4He ratios of samples in this study range from 0.014 RA to
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0.572 RA (where RA denotes the atmospheric 3He/4He = 1.382 × 10-6; Mabry et al., 2013), which are in good agreement with previously reported helium isotope data (Fig.
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5a; e.g., Hoke et al., 2000; Newell et al., 2008). Assuming that all 20Ne of the
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measured samples originates from the air dissolved in water, the air-derived helium contributions were corrected by the ASW method (Craig et al., 1978; Rison and Craig,
He/4He ratios (0.015−0.373 RA for samples in this study; Table 2) exhibit clear
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3
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1983), as summarized in Table 2. Combined with literature data, the air-corrected
E-W-trending variations for hydrothermal volatiles from southern Tibet and the
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Himalayas (Fig. 5b).
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Following the conventional 3He/4He ratio of continental crust (3He/4He = 0.02 RA with a range of 0.01−0.1 RA; Lupton, 1983), typical crustal helium isotopes are more common for samples of the western and central subgroups (Fig. 5b), in contrast to the eastern subgroup samples that are dominated by excess mantle 3He contributions. For example, none of the central subgroup samples have 3He/4He ratios that are higher than 0.1 RA (Fig. 5b). Notably, some western subgroup samples exhibit high mantle helium contributions as shown by the abnormally high 3He/4He ratio of one sample 12
ACCEPTED MANUSCRIPT along the KKF (2.24 RA; Klemperer et al., 2013). The spatial variations in 3He/4He ratios are generally consistent with previously recognized crustal helium domain (<0.05 RA) and mantle helium domain (>0.1 RA) in Hoke et al. (2000). However, there might be no clear compositional gap (0.05−0.1 RA; Klemperer et al., 2013)
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between the crustal and mantle helium domains (Fig. 5b). Taking 3He/4He ratios of 8
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RA and 0.02 RA for the mantle and crustal helium end-members, respectively, the
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proportions of mantle helium generally range from 1−10% for most high-3He/4He (>0.1 RA) samples of the western and eastern subgroups. In particular, the KKF
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sample exhibits mantle helium contribution of over 20% (Klemperer et al., 2013). On
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the contrary, the proportions of mantle helium in central subgroup samples are less than 1% (Fig. 5a).
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4.3. Carbon and nitrogen stable isotope ratios
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As reported in Table 2, the δ13C (for CO2, versus PBD) and δ15N (for N2, versus air; corrected for air contamination) values of the samples in this study range from
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−12.9‰ to −5.0‰ and from −0.8‰ to 5.0‰, respectively. In combination of the
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literature data, the hydrothermal volatiles from southern Tibet and the Himalayas define (1) positive correlation between CO2/N2 and δ13C (Fig. 6a), and (2) negative correlation between CO2/N2 and δ15N (Fig. 6b), respectively. The variations in δ13C values among samples of the three subgroups are not very clear compared with their spatially discernible 3He/4He ratios (Figs. 5 and 6a). However, negative δ15N values appear to be more common for the eastern subgroup samples, in contrast to the positive δ15N values observed in most samples of the western and central subgroups 13
ACCEPTED MANUSCRIPT (Fig. 6b). This may suggest variable contributions from source components involved in evolution of the He-C-N isotope systematics of the hydrothermal volatiles from southern Tibet and the Himalayas.
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5. Discussion
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5.1. Essential components involved in evolution of the hydrothermal volatiles
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The chemical compositions of the studied hydrothermal volatiles indicate that essential end-members required for volatile evolution can be reasonably attributed to
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helium-, nitrogen- and carbon-rich components. In this section, we will discuss the
5.1.1. Helium-rich components
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possible origin of these essential components and their roles in the degassing systems.
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As shown in Figs. 3b and 5, the helium-rich signatures of the hydrothermal
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volatiles can be divided into two types of helium enrichment, which are the 4He enrichment (for samples within crustal helium domain) and the 3He enrichment (for
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samples within mantle helium domain), respectively. The former scenario is best
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exemplified by some samples with extremely high 4He contents from the western and central subgroups (Fig. 3b), while the latter scenario can be observed in the samples with high 3He/4He ratios (>0.1 RA; Fig. 5). It is straightforward that the crustal rocks (e.g., silicate rocks represented by granites; Hoke et al., 2000) in southern Tibet and the Himalayas are reasonable origin of the radiogenic helium-rich components. Notably, the eastern subgroup samples that are close to the Nyainqentanglha granites (Kapp et al., 2005; Weller et al., 2016) have 14
ACCEPTED MANUSCRIPT relatively lower radiogenic helium contents (e.g., the Yangbajing samples; Yokoyama et al., 1999; Zhao et al., 2001, 2002; Zhang et al., 2014), unlike the western and central subgroup samples (Fig. 3b). The contrasting variations in radiogenic helium contents of the hydrothermal volatiles from degassing systems along the IACSZ may
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suggest either different degrees of fluid-rock interaction and/or additional origin of
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radiogenic helium other than granites. The latter possibility can be linked to the
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volatile affinities with sedimentary environment, because the contents of radiogenic He that has accumulated in sedimentary basins and crustal aquifers over geological
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time can be extremely high (Lowenstern et al., 2014 and references therein), and thus
the high radiogenic 4He contents.
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hydrothermal fluids with long residence time in such degassing systems would inherit
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With respect to the mantle-derived 3He, the hydrothermal volatiles released from
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hot springs within lithospheric-scale fault systems are characterized high 3He/4He ratios (>0.1 RA; Fig. 5), such as the KKF (Klemperer et al., 2013). Given that the
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samples close to the ITS have crustal-like 3He/4He ratios, mantle relics represented by
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the ITS ophiolites were considered to have negligible contributions to the excess mantle 3He emissions (Klemperer et al., 2013). Additionally, Hoke et al. (2000) also ruled out the possibility of mafic magmatic rocks with variable crystallization ages (ranging from Late Cretaceous to Miocene) as reasonable source of the mantle helium, because these mantle-derived rocks are too old to maintain their initial high 3He/4He ratios. Therefore, a mantle end-member related with recent (e.g., Quaternary; Hoke et al., 2000) melting event might be required for the high mantle 3He emissions. 15
ACCEPTED MANUSCRIPT 5.1.2. Nitrogen-rich components Following different nitrogen reservoirs of the Earth, the end-members involved in nitrogen isotope systematics are composed of the mantle (δ15N = −5 ± 2‰; Sano et al., 2001; Cartigny and Marty, 2013), crust (δ15N = 5 ± 2‰; Sano et al., 1998; Li and
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Bebout, 2005; Busigny and Bebout, 2013; Cartigny and Marty, 2013; Halama et al.,
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2014) and atmosphere (δ15N = 0‰ by definition; Cartigny and Marty, 2013). In
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particular, the crustal end-member is mainly represented by organic-sourced nitrogen that has been sequestered in sedimentary and crystalline rocks through biologic
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activities, sedimentation and diagenesis processes (Bebout et al., 2013).
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For the studied hydrothermal volatiles, the nitrogen-rich components have close affinities with organic matter, as demonstrated by (1) the positive δ15N values (e.g., up
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to 5‰; Fig. 6b) that point to the involvement of organic matter, and (2) the higher
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N2/Ar ratios relative to the atmospheric value (Kita et al., 1993; Taran, 2011). Thermogenic breakdown of sedimentary organic matter can account for contributions
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from the nitrogen-rich components, and an alternative explanation is the interaction
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between hydrothermal fluids and silicate rocks that have incorporated organic-sourced nitrogen (as NH4+ stored in minerals such as beryls and micas; Bebout et al., 2013; Busigny and Bebout, 2013). The air-like δ15N values of a few samples suggest the contributions from atmospheric nitrogen, which can be interpreted as nitrogen from the air and/or the ASW (Fig. 6b). Moreover, the negative δ15N values of some samples may result from the involvement of mantle-derived nitrogen and/or the near surface biogenic-sourced nitrogen (Newell et al., 2008). In this study, we prefer the potential 16
ACCEPTED MANUSCRIPT involvement of mantle-derived nitrogen in volatile evolution, because some samples with excess mantle 3He emissions also have negative δ15N values. 5.1.3. Carbon-rich components Irrespective of sample locations, the hydrothermal volatiles from southern Tibet
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and the Himalayas have pervasively received contributions from inorganic and
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organic carbon end-members, i.e., the carbon-rich components. The CO2-rich samples
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with high δ13C values and CO2/3He ratios (Fig. 7a) may result from the involvement of carbonate rocks [δ13C = 0 ± 2‰ (Hoefs, 2009); CO2/3He = 1013 (O’Nions and
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Oxburgh, 1988; Sano and Marty, 1995)] in volatile evolution. The three-component
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mixing model of Sano and Marty (1995) can well explain the contributions from the inorganic carbon end-member (i.e., CAR; Fig. 7a), especially for the CO2-rich
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hydrothermal volatiles. In particular, calcite precipitation may be not important for the
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samples inside the mixing envelope, as suggested by the samples with have high δ13C values and CO2/3He ratios from the Sanglai hot springs (where carbonate sinters are
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common; Fig. 7a). Thus, these samples can be considered for further geochemical
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interpretation (e.g., the multi-component mixing model in Fig. 8). As discussed in Newell et al. (2008), decarbonation of marine limestones, which are widespread in southern Tibet and the Himalayas (Kapp et al., 2003; DeCelles et al., 2011, 2014), can provide the inorganic carbon-rich components in these degassing systems. In contrast, the samples outside the Sano & Marty mixing envelope have low δ13C values and CO2/3He ratios (i.e., <2×109; Fig. 7a), which are commonly attributed to the potential effects of significant calcite precipitation (e.g., Hilton et al., 1998; Ray 17
ACCEPTED MANUSCRIPT et al., 2009; Güleç and Hilton, 2016). Following this possibility, the low δ13C values and CO2/3He ratios of these samples appear to be consistent with the expected results of calcite precipitation at temperatures between 25 oC and 192 oC, respectively (Fig. 7a). However, compared with the CO2-rich hydrothermal volatiles, the samples
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outside the mixing envelope have excess nitrogen (CO2/N2 <1; Fig. 7b) and positive
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δ15N values (Fig. 6b), suggesting that contributions from the nitrogen-rich organic
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matter should also be taken into account.
It may be a general agreement that organic matter contains both carbon-rich
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organic components (δ13C = −30 ± 10‰; Hoefs, 2009) and nitrogen-rich organic
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components (δ15N = 5 ± 2‰; Sano et al., 1998; Cartigny and Marty, 2013). Owing to the contributions from both carbon-rich and nitrogen-rich organic components, these
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samples would have excess nitrogen contents (i.e., higher N2 and lower CO2), low
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CO2/3He ratios and low δ13C values. For example, low CO2/3He ratios of the N2-dominated samples NMR1501 and NMR1502 are likely to reflect the predominant
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contributions from nitrogen-rich organic matter, which agrees well with their positive
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δ15N values and low δ13C values that point to organic affinities (Table 2). Therefore, aside from the potential effects of calcite precipitation, we suggest that the N2-rich samples outside the mixing envelope may result from involvement of organic matter in volatile evolution, especially the nitrogen-rich organic components. Considering their crustal-derived signatures (see details in Section 5.3.2) and the potential effects of significant calcite precipitation, these N2-rich samples would not be considered for further discussion in a multi-component mixing model (Figs. 8 and 9). 18
ACCEPTED MANUSCRIPT 5.2. Enriched mantle wedge (EMW) as the source for the mantle-derived components On the basis of bulk volatile compositions (Figs. 3 and 4), together with helium isotopes (Fig. 5), carbon and nitrogen stable isotope ratios (Figs. 6 and 7), different types of source materials have been identified for the origin of hydrothermal volatiles
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from southern Tibet and the Himalayas, which are the silicate rocks, carbonate rocks,
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sedimentary organic matter and a mantle-derived end-member, respectively. In the
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case of the mantle-derived components, Hoke et al. (2000) suggested that recent melts of the Tibetan lithospheric mantle and/or the asthenosphere can provide the excess
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mantle 3He emissions. Additionally, high 3He/4He ratios of some western subgroup
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samples were attributed to mantle-derived melts related with rapid slip rate of the lithospheric-scale KKF (Hoke et al., 2000) or mantle-derived fluids that penetrate
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upward along the KKF (Klemperer et al., 2013). However, little attention has been
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paid to an enriched mantle source that is reasonably expected for the geodynamic setting of the Indian continental subduction. In this section, we will discuss the
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scenario that the enriched mantle wedge (EMW) acts as the source of the
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mantle-derived components. 5.2.1. The feasibility of an EMW source in the IACSZ Recycled slab-derived materials have been widely interpreted as the agents for source enrichment of mantle-derived magmas that are genetically related not only with oceanic lithosphere subduction (e.g., Spandler and Pirard, 2013; Kimura et al., 2016; Zhang and Guo, 2016) but also with continental lithosphere subduction (e.g., Ding et al., 2003; Guo et al., 2013, 2015). For the IACSZ, the Sr-Nd-Pb isotopes of 19
ACCEPTED MANUSCRIPT post-collisional (25–8 Ma) mafic volcanic rocks in southern Tibet point to an EMW source fluxed by continental materials (melts and/or fluids) from the northward subducting Indian plate (e.g., Guo et al., 2013, 2015). Moreover, the hydrothermal volatiles from Tengchong volcanic field, SE Tibet, have been suggested to be released
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by partial melts derived from an EMW source related with recycling of the Indian
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continental materials (Zhang et al., 2016).
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Notably, geophysical studies have revealed the presence of a narrow gap (i.e., the asthenosphere) between the subducting Indian lithosphere and the overriding Asian
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lithosphere (Zhou and Murphy, 2005; Chen et al., 2015; Shi et al., 2015), which is
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consistent with the geodynamic modeling of continental lithosphere subduction in a context of intra-continental collision (Replumaz et al., 2016). We suggest that the
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geophysically detected narrow gap can be interpreted as mantle wedge of the IACSZ.
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Taking carbon recycling model as an example, experimental studies have revealed that sedimentary carbon and carbonate mineral-bearing rocks can be recycled into
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mantle wedge by the subducting plates, which would release carbon-bearing melts
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and/or fluids under P-T conditions of mantle wedge (e.g., Dasgupta and Hirschmann, 2010; Frezzotti et al., 2011; Ague and Nicolescu, 2014; Sverjensky et al., 2014; Thomson et al., 2016). In particular, noble gases could also be recycled into the sub-arc mantle depth during continental lithosphere subduction; and these noble gases would be transferred to the post-collisional magmas at the slab-mantle interface (Dai et al., 2016), which may shed light on formation of the EMW source in the IACSZ. Therefore, we suggest that an EMW source generated by mixing between the Indian 20
ACCEPTED MANUSCRIPT slab-derived materials and the asthenosphere can be expected for volatile recycling mechanism in the IACSZ. 5.2.2. Formation of the EMW source: slab-mantle interaction Following the He-C isotope coupling model in Zhang et al. (2015, 2016), together
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with proportions of the Indian components in mantle wedge enrichment constrained
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by genesis of the EMW-derived mafic magmas (Guo et al., 2013), we calculated the
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possible range of helium and carbon isotope compositions of the EMW source (Table 3 and Supplementary Table S2). In a scenario of the silicate-dominated mantle source
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enrichment (Guo et al., 2013, 2015), the calculated 3He/4He ratios of the EMW source
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vary in a range of 2.28–5.23 RA as proportion of the silicate melts (represented by the SED end-member; Fig. 8a and Table 3) increases from 5% to 20%. Given that the
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roles of silicate- and carbonate-related mantle metasomatism remain controversial for
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origin of the mantle-derived mafic magmas (e.g., Guo et al., 2013, 2015; Liu et al., 2015), the potential influence of subducted carbonate (e.g., dehydration fluids; Ague
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and Nicolescu, 2014) on formation of the EMW is denoted by the question mark in
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Fig. 8a, suggesting that the CAR contributions to the He-C isotope systematics of the EMW source are uncertain. For the He-N isotope systematics during slab-mantle interaction (Fig. 8b), the EMW source is assumed to have 3He/4He ratios of 2.28–5.23 RA following results of the He-C isotope coupling model, while δ15N values of the EMW source may vary from −4.5‰ to −3‰ according to simple mixing between the SED components (with proportions ranging in 5–20%) and the mantle (with proportions ranging in 80–95%). 21
ACCEPTED MANUSCRIPT As suggested by the He-C-N isotope calculations (Fig. 8), it is clear that the EMW can be considered as a reasonable candidate for the mantle-derived end-member, which can provide the discernible mantle contributions (e.g., excess 3He, recycled C and N) for the hydrothermal degassing systems in the IACSZ. It should be noted that
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only samples with high 3He/4He ratios (>0.1 RA) exhibit clear contributions from the
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EMW-derived melts, whereas the low 3He/4He (<0.1 RA) samples are inferred to have
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negligible mantle contributions.
Combined with constraints from petrogeochemical (Guo et al., 2013, 2015) and
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geophysical (e.g., Brown et al., 1996; Wei et al., 2001; Unsworth et al., 2005; Xie et
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al., 2016) studies, we suggest that recent partial melts from the EMW source in crustal depth beneath southern Tibet, which can act as the origin of mantle-derived materials
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and as heat source of the shallow hydrothermal degassing systems. Mantle upwelling
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and the LV–HCZs in southern Tibet inferred by geophysical studies (e.g., Wei et al., 2001; Unsworth et al., 2005; Chen et al., 2015; Tian et al., 2015) are likely to reflect
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emplacement and storage of the EMW-derived melts beneath the magmatic front.
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Alternatively, the newly-formed Tibetan lithospheric mantle in a lithospheric delamination model can also account for the formation of an enriched mantle source, if the recycled Indian continental materials are taken as metasomatic agents. Considering its equivalent role in volatile origin to the above EMW model, this scenario would not be focused in this study. 5.3. The role of regional crustal rock assemblages in volatile evolution 5.3.1. Contaminants of the EMW-derived volatiles 22
ACCEPTED MANUSCRIPT Similar to post-collisional, mantle-derived magmas that tend to be affected by magma differentiation processes during their storage and transit in continental crust (Liu et al., 2017), crustal contamination should also be taken into account for volatile evolution, especially in consideration of the thickened continental crust of southern
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Tibet (70–75 km; Zhang et al., 2011). The crustal contributions to the EMW-derived
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volatiles can be provided by (1) partial melts of crustal country rocks (dominated by
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silicate rocks) in emplacement depth of the EMW-derived melts, and (2) hydrothermal fluids that interact with crustal rock assemblages (e.g., carbonate rocks, silicate rocks
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and different types of sediments) in shallow degassing systems.
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In the crustal contamination scenario, the He-C isotope compositions of the SED and CAR components in continental crust are assumed to be identical to those of the
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recycled crustal materials from subducting Indian plate (see details in Supplementary
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Table S2). Following above discussion, the crustal contamination processes is only valid for the high 3He/4He samples with clear mantle contributions. Namely, the
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crustal rock assemblages would act as contaminants of the EMW-derived volatiles. As
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indicated by the He-C isotope coupling model (Fig. 8a), most of the eastern subgroup samples and some western subgroup samples near the KKF are consistent with the results of crustal contamination. The 3He/4He ratios and δ15N values of these samples may reflect the interaction between the EMW-derived volatiles and continental crustal materials that are enriched in organic-sourced nitrogen and radiogenic 4He (Fig. 8b). Additionally, crustal contamination processes can also be illustrated by the plot of 3
He/4He versus CO2/3He (Fig. 9), which displays mixing between the EMW-derived 23
ACCEPTED MANUSCRIPT volatiles (3He/4He = 2.28–5.23 RA, CO2/3He = 2.31×109 – 3.46×109; Table 3) and continental crustal materials with variable CO2/3He ratios. 5.3.2. Source materials of the crustal-derived volatiles Except for some samples along the KKF (e.g., the Menshi sample; Klemperer et
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al., 2013), the western and central subgroup samples can be classified as typical
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crustal-derived volatiles that generally have low 3He/4He ratios (<0.1 RA), suggesting
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negligible contributions from the EMW-derived volatiles (Figs. 8 and 9). Namely, regional crustal rocks assemblages would act as the source materials of these
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hydrothermal volatiles. This is consistent with tectonic setting of the fore-arc region
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(see details in Section 5.4.2), including the Himalayas and fore-arc basins that cover most of the western and central subgroup samples (Fig. 1).
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A prominent signature of the crustal-derived volatiles is the excess N2 contents
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(Fig. 3), which may have close affinities with sedimentary environment. For example, Jenden et al. (1988) discussed the presence of excess nitrogen in the gas and oil wells
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in the California Great Valley; and Mariner et al. (2003) found excess nitrogen in
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thermal springs in the Cascade Range, which was attributed to their origin from low to moderate-temperature sedimentary environment. These observations indicate that the crustal-derived volatiles in the IACSZ are dominated by sedimentary environment in the fore-arc region, which is also consistent with the high 4He contents (Fig. 3b) and He-C-N isotopic compositions (Figs. 5 and 6) of the most western and central subgroup samples. 5.4. Tectonic implications of degassing systems in southern Tibet and the Himalayas 24
ACCEPTED MANUSCRIPT 5.4.1. Accretionary wedge and magmatic front of the IACSZ For the tectonic framework of an active subduction zone, accretionary wedge is usually formed along convergent boundary of the overriding plate, while magmatic front represents a trench-paralleled belt where subduction-related magmas are erupted
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and/or emplaced (Stern, 2002). According to previous studies (e.g., DeCelles et al.,
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2014; Hu et al., 2016 and references therein), we suggest that the classic tectonic
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model incorporating an accretionary wedge and a magmatic front can be reasonably applied to the IACSZ.
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First, northward subduction and accretion of the Neo-Tethyan oceanic lithosphere
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and the Indian continental lithosphere (Ding et al., 2005; Cai et al., 2012) have led to formation of a near E-W-trending accretionary wedge in the IACSZ, as suggested by
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many studies (e.g., Cai et al., 2012; DeCelles et al., 2014; Kelly et al., 2016; Leary et
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al., 2016). The Himalayan sequences on northern margin of the Indian continent (Yin, 2006) and the fore-arc basins on southern margin of the Asian continent (Wu et al.,
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2010) are major tectonic units of the accretionary wedge. Second, several stages of
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subduction-related magmatism have occurred in southern Tibet during closure of the Neo-Tethys ocean and the India-Asia continental collision and subduction. As a result, a near E-W-trending magmatic front was formed in southern Tibet, as represented by the Gangdese batholith (e.g., Wen et al., 2008; Ji et al., 2014; Zhu et al., 2015), the Linzizong volcanic rocks (e.g., Mo et al., 2008; Lee et al., 2012; Niu et al., 2013) and the post-collisional magmatic rocks (e.g., Chung et al., 2003; Guo et al., 2007, 2013, 2015; Liu et al., 2014; Hou et al., 2015; Zhang et al., 2017). 25
ACCEPTED MANUSCRIPT Additionally, it should be noted that magmatic front of the IACSZ represents a series of significant subduction-related magmatic activities, which may cover the time interval ranging from Late Cretaceous (i.e., the onset of the Gangdese magmatism; Wen et al., 2008) to Late Miocene (i.e., the end of post-collisional magmatism; Guo et
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al., 2015). In particular, the EMW model of volatile origin is based on the origin of
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post-collisional, ultrapotassic volcanic rocks in southern Tibet (25–8 Ma; Guo et al.,
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2013), suggesting that the EMW-derived melts (i.e., the magma chamber) may have formed since Early Miocene. This is consistent with slow-downs of the India-Asia
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convergence in Miocene times [e.g., 25–8 Ma (Lee and Lawver, 1995) and 20–10 Ma
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(Molnar and Stock, 2009; Iaffaldano et al., 2013)], which could lead to rollback of the subducting Indian continental lithosphere and partial melting of the EMW source
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generated by recycling of the Indian slab-derived materials (Guo et al., 2013, 2015).
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Although active volcanoes are absent in southern Tibet and the Himalayas at present, low resistivity anomalies based on magnetotelluric data have been imaged in
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crustal depth beneath typical hydrothermal systems in southern Tibet (e.g., Unsworth
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et al., 2005; Xie et al., 2016). This may provide evidence for the present-day residual melts beneath magmatic front of the IACSZ. In addition, recent mantle-derived melts are required for the high mantle helium emissions from hydrothermal degassing systems in southern Tibet. Therefore, the geochemical and geophysical constraints jointly support the presence of recent [e.g., Quaternary following assumption in Hoke et al. (2000)] EMW-derived melts beneath magmatic front of the IACSZ. 5.4.2. Tectonic affinities of the degassing systems revealed by volatile geochemistry 26
ACCEPTED MANUSCRIPT Unlike the degassing systems controlled by an identical tectonic setting (e.g., those in southern volcanic zone of Chile; Tardani et al., 2016), the hydrothermal belt in the IACSZ covers both accretionary wedge and magmatic front, making it an ideal laboratory for understanding tectonic implications of hydrothermal degassing systems
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in subduction zones. In this section, we will discuss the relationship between spatial
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variations in gas geochemical characteristics and tectonic settings of the degassing
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systems. On the plot of air-corrected 3He/4He ratios versus distance to the ITS (Fig. 10), the samples generally display a northward increasing trend of 3He/4He ratios
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from the Himalayas to southern Tibet, suggesting their potential tectonic affinities
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with the accretionary wedge in the south and the magmatic front in the north. For hydrothermal degassing systems in the accretionary wedge, regional crustal
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rock assemblages are considered as the source materials of the hydrothermal volatiles
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and contributions from the EMW-derived melts may be absent or negligible. This is consistent with the crustal-like 3He/4He ratios (<0.1 RA), as exemplified by the central
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subgroup samples (Figs. 5b and 10). In addition, the sediment (including both organic
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and inorganic types) contributions are important for hydrothermal volatiles from the accretionary wedge, which can well explain their geochemical characteristics such as high N2 and radiogenic 4He (Fig. 3) and positive δ15N values (Fig. 6b). We suggest that the widespread sediments in the Tethyan Himalayan sequence (Gehrels et al., 2011) and the fore-arc basins (Gehrels et al., 2011; Cai et al., 2012) may be the most plausible origin of sediment contributions to these degassing systems. For example, serpentinization and weathering of ultramafic rocks (Sleep et al., 2004; Kelley et al., 27
ACCEPTED MANUSCRIPT 2005; Morrill et al., 2013) is likely to generate the required organic matter. The significant role of sediments in accretionary wedge of the IACSZ agrees well with a series of geodynamic processes (e.g., subduction accretion, tectonic erosion and sedimentation) that are expected for tectonically accretionary margins (Clift and
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Vannucchi, 2004; Clift, 2016). For example, the crustal-like 3He/4He ratios of samples
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between the ITS and the India-Asia junction belt (JIA; Fig. 10) may represent volatile
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emissions from degassing systems in fore-arc basins, where sediments are inferred to be the first-order control on volatile geochemistry. Towards further north, the excess
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mantle 3He emissions from the hot springs in southern Tibet may point to their close
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affinities with mantle-derived melts beneath the magmatic front (Fig. 10). Therefore, we suggest that the EMW contributions are important for hydrothermal degassing
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systems in magmatic front of the IACSZ, in contrast to those in accretionary wedge
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that are dominated by sediment contributions. 5.4.3. The role of regional fault systems in the degassing systems
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Aside from the control of tectonic settings, regional fault systems (i.e., pathways
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of uprising fluids and/or volatiles) should also be taken as critical parameters for the spatially discernible geochemical characteristics, because their scales and depths of penetration would exert significant influence on outgassing rate of the EMW-derived volatiles and thus affect the degree of crustal contamination. As a result, uncertainties caused by the properties of regional fault systems may obscure the interpretation on tectonic affinities of hydrothermal degassing systems (i.e., accretionary wedge and magmatic front). As shown in the across-IACSZ profile of 3He/4He ratios (Fig. 10), 28
ACCEPTED MANUSCRIPT the abnormally high mantle helium emissions from the KKF can be attributed to the lithospheric-scale fault systems with deep depths of penetration and rapid slip rate (Klemperer et al., 2013). Therefore, regional fault systems such as the KKF can explain the samples with high 3He/4He ratios (>0.1 RA; Fig. 10) in fore-arc region (i.e.,
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south of the magmatic front). Additionally, similar fault-related mantle degassing can
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be exemplified by recent work of Xu et al. (2017), who recognized mantle-derived
lithosphere (200–250 km; Xu et al., 2002).
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helium emissions in foreland basins around the Tarim craton with extremely thick
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The across-IACSZ profile of 3He/4He ratios suggests that spatial variations in
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volatile geochemistry could shed light on tectonic framework and lithospheric structure of the IACSZ (e.g., the location of the junction belt where the subducting
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Indian plate steepens), especially when combined with constraints from geophysical
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studies. It is noted that the southern limit of mantle helium emissions in the magmatic front (i.e., the JIA revealed by 3He/4He ratios that is about 50–100 km north of the ITS;
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Hoke et al., 2000) displays a southward offset from the geophysically inferred
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northern extent of the subducted Indian crust (Figs.1 and 11; Nábělek et al., 2009). We suggest that the offset may result from southward uprising of hydrothermal fluids and the EMW-derived volatiles controlled by geometry of regional fault systems and location of residual melts beneath the magmatic front (Fig. 11). Therefore, as stated in Tardani et al. (2016), the effects of regional fault systems should be considered for understanding tectonic implications of hydrothermal degassing systems using volatile geochemistry. 29
ACCEPTED MANUSCRIPT 6. Conclusions On the basis of chemical compositions and He-C-N isotopes, the hydrothermal degassing systems in southern Tibet and the Himalayas are interpreted from the perspective of the Indian continental subduction, as summarized below:
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(1) The essential components required for volatile evolution are attributed to
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silicate rocks, carbonate rocks, sedimentary organic matter and mantle-derived
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components, which can be expected in a tectonic model incorporating accretionary wedge and magmatic front of the IACSZ.
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(2) Considering the critical role of the Indian slab-derived materials in mantle
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enrichment, an EMW source is proposed as the candidate for the required mantle-derived end-member, which is supported by the He-C-N isotope mixing
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calculations and constraints from petrogeochemical and geophysical studies.
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(3) Spatially discernible geochemical characteristics of the hydrothermal volatiles may have close affinities with tectonic settings of the degassing systems, as suggested
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by the significant sediment contributions to hydrothermal volatiles from accretionary
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wedge and the significant EMW-derived contributions to those from magmatic front. Additionally, regional fault systems would act as an extra factor that can affect geochemical characteristics of the hydrothermal volatiles.
Acknowledgements This study was financially supported by the Strategic Priority Research Program (B) of Chinese Academy of Sciences (Grant No. XDB03010600) and the National 30
ACCEPTED MANUSCRIPT Natural Science Foundation of China (Grant Nos. 41572321 and 41602341). We are grateful to editorial work of Prof. David Hilton and Prof. Yunpeng Wang. Two anonymous reviewers are appreciated for constructive comments that have greatly improved quality of this manuscript. Drs. Zhongping Li, Liwu Li, Li Du, Xiangxian
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MA
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Ma and Chunhui Cao are appreciated for sample analyses.
31
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ACCEPTED MANUSCRIPT Figure Captions
Fig. 1. Simplified map showing tectonic framework, post-collisional volcanic rocks and representative hot springs in southern Tibet and the Himalayas. Abbreviations:
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MBT, main boundary thrust; MCT, main central thrust; STDS, southern Tibet
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detachment system; ITS, Indus-Tsangpo suture; BNS, Bangong-Nujiang suture; LHS,
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Lesser Himalayan sequence; HHS, Higher Himalayan sequence; THS, Tethyan Himalayan sequence. Sample locations in previous studies (Yokoyama et al., 1999;
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Hoke et al., 2000; Zhao et al., 2002; Newell et al., 2008; Klemperer et al., 2013) are
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represented by red filled circles, and those in this study are shown as yellow stars with numbers as follows: 1, Namuru; 2, Baer; 3, Tirthapuri; 4, Qiwusi; 5, Darong; 6,
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Dagejia; 7, Liudaoban; 8, Xiqin; 9, Mianjiu. In particular, the yellow stars with red
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dots represent hot springs studied by both previous studies and this study, while the yellow star without red dot represents the Darong hot springs that are studied for the
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first time. Profile AA’ denotes the geophysically imaged lithospheric structure of the
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IACSZ (Zhou and Murphy, 2005), as shown in Fig. 11. The red dashed line represents the geophysically inferred northern extent of the Indian crust (Nábělek et al., 2009), and the blue dashed line represents the India-Asia junction belt revealed by 3He/4He ratios (Hoke et al., 2000).
Fig. 2. Field photos showing hot springs (a, Namuru), geysers (b, Dagejia) and sinters (c, Dagejia; d, Darong) in southern Tibet. During fieldwork in the Dagejia region (i.e., 53
ACCEPTED MANUSCRIPT the Sample location 6 in Fig. 1), soil CO2 emissions were measured but were under detection limit of the portable soil CO2 flux meter (GXH-3010E; Zhang et al., 2016), which may suggest significant influence of thick sinters on soil microseepage of the
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hydrothermal degassing systems.
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Fig. 3. (a) CO2/N2 versus CO2 (%) and (b) N2 (%) versus 4He (ppm) for hydrothermal
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volatiles from southern Tibet and the Himalayas. Filled and open symbols represent, respectively, data in this study and published data in Yokoyama et al. (1999), Hoke et
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al. (2000), Zhao et al. (2002), Walia et al. (2005), Newell et al. (2008), Klemperer et
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al. (2013) and unpublished data of Z. Guo. The compiled data in previous studies are
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available in Supplementary Table S1.
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Fig. 4. Triangle plot of N2-He -Ar for hydrothermal volatiles from southern Tibet and the Himalayas (modified from Giggenbach and Poreda, 1993). The ASW represents
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air saturated water. Data source and symbols are as in Fig. 3.
Fig. 5. (a) 3He/4He (Rm/RA) versus (He/Ne)m/ (He/Ne)ASW and (b) 3He/4He (RC/RA) versus Longitude (E) for hydrothermal volatiles from southern Tibet and the Himalayas. The calculated binary mixing curves between the ASW and crust-mantle mixtures with variable proportions of mantle helium (i.e., 100%, 20%, 10%, 5%, 1% and 0%) are shown. (He/Ne)m/ (He/Ne)ASW is calculated based on air-normalized He/Ne ratio multiplied by the ratio of their Bunsen coefficients (βNe/βHe = 1.25 at 54
ACCEPTED MANUSCRIPT groundwater recharge temperature of 10 °C; Weiss, 1971; Hilton, 1996). Reference values for end-members are as follows: ASW, 3He/4He = 0.987 RA (Klemperer et al., 2013), (He/Ne)m/(He/Ne)ASW = 1.12; Crust, 3He/4He = 0.02 RA (Lupton, 1983); Mantle, 3He/4He = 8 ± 1.5 RA (Sano and Fischer, 2013). For the crust, mantle and
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crust-mantle mixture end-members, 4He/20Ne ratio of 10000 is assumed (Morikawa et
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al., 2008). Data source and symbols are as in Fig. 3.
Fig. 6. (a) δ13C (‰) versus CO2/N2 and (b) δ15N (‰) versus CO2/N2 for hydrothermal
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volatiles from southern Tibet and the Himalayas. Data source and symbols are as in
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Fig. 3.
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Fig. 7. (a) δ13C (‰) versus CO2/3He for hydrothermal volatiles from southern Tibet
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and the Himalayas. Data source and symbols are as in Fig. 3. Abbreviations: DMM, depleted MORB-source mantle; CAR, carbonate rocks; SED, organic sediments
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(Sano and Marty, 1995; Zhang et al., 2016). In the three-component mixing model of
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Sano and Marty (1995), the SED end-member is solely based on carbon-rich organic matter. The effects of nitrogen-rich organic matter (and/or calcite precipitation) on the samples outside the mixing envelope are denoted by the yellow arrow. Considering the potential effects of calcite precipitation and their crustal origin, these samples would not be considered for further discussion of mixing between the EMW-derived components and crustal contaminants (e.g., Figs. 8 and 9). The solid grey lines with arrows represent calcite precipitation at temperatures of 25 oC and 192 oC, 55
ACCEPTED MANUSCRIPT respectively (Ray et al., 2009), and their starting point is represented by the sample DR1501. The rectangle represents carbonate rocks with δ13C (‰) values of 0 ± 2‰ (Hoefs, 2009) and CO2/3He ratios ranging in 1012−1014 (O’Nions and Oxburgh, 1988). (b) CO2/N2 versus CO2/3He for hydrothermal volatiles from southern Tibet and the
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Himalayas.
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Fig. 8. He-C isotope coupling model (a) and He-N isotope plot (b) showing origin of hydrothermal volatiles from southern Tibet and the Himalayas. Data source and
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symbols are as in Fig. 3. Abbreviations: DMM, depleted MORB-source mantle; EMW,
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enriched mantle wedge; CAR, carbonate rocks; SED, sediments enriched in radiogenic helium and organic-sourced carbon and nitrogen. Reference values for
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end-members are listed in Supplementary Table S2. K value [= (He/C)A/(He/C)B]
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represents the curvature of mixing curve between end-member A and end-member B. Numbers (5%, 10%, 15% and 20%) on the K = 0.29 curve represent the proportions
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of subducted SED components in formation of the EMW source. The yellow
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parallelogram represents possible He-C isotope range of the EMW. Potential influence of crustal melts on the EMW-derived melts is marked. The dashed curves represent mantle source enrichment and/or crustal contamination by the CAR components. The question mark represents uncertainty of contributions from subducted carbonate to the He-C isotope systematics of the EMW. Helium isotopes of the EMW-derived melts or volatiles in (b) are as those in (a), and the potential influence of crustal melts on the EMW-derived melts is marked. In particular, typical crustal-derived volatiles are 56
ACCEPTED MANUSCRIPT shown for comparison.
Fig. 9. 3He/4He (RC/RA) versus CO2/3He for hydrothermal volatiles from southern Tibet and the Himalayas. Data source and symbols are as in Fig. 3. The solid curves
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represent mixing between the EMW-derived volatiles and continental crustal
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materials. The purple diamonds with proportions (i.e., 5%, 10%, 15% and 20%)
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represent the EMW-derived melts following results of the He-C isotope coupling
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model (Table 3).
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Fig. 10. Across-IACSZ profile of 3He/4He ratios in southern Tibet and the Himalayas showing transition of tectonic setting from accretionary wedge to magmatic front.
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Data sources and symbols are as in Fig. 3. Abbreviations: ITS, Indus-Tsangpo suture;
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Karakoram fault.
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JIA, junction between the Indian and Asian plates (Hoke et al., 2000); KKF,
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Fig. 11. Schematic map showing tectonic settings of the hydrothermal degassing systems in the IACSZ. Abbreviations: MBT, main boundary thrust; MCT, main central thrust; STDS, southern Tibet detachment system; ITS, Indus-Tsangpo suture; BNS, Bangong-Nujiang suture. Location of the profile AA’ is given in Fig. 1. The LV–HCZs denote geophysically recognized low-velocity–high-conductivity zones in crustal depth beneath southern Tibet (e.g., Brown et al., 1996; Wei et al., 2001; Xie et al., 2016). The red dashed line (31oN) represents the geophysically inferred northern 57
ACCEPTED MANUSCRIPT extent of the Indian crust (Nábělek et al., 2009), which suggests steepening and subducting of the Indian continental lithosphere beneath regions north of the latitude 31oN. Similarly, the blue dashed line (JIA) represents the India-Asia junction belt
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revealed by 3He/4He ratios (Hoke et al., 2000).
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ACCEPTED MANUSCRIPT Supplementary Table Captions
Table S1. Compiled literature data used in this study.
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Table S2. Reference values for end-member parameters used in the He-C isotope
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coupling model
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Fig. 6
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ACCEPTED MANUSCRIPT Table 1 Chemical compositions of hydrothermal volatiles from southern Tibet and the Himalayas.
31°55'2
80°09'5
501
locatio Namur n u
2.6"
8.9"
NMR1
Namur
31°55'2
80°09'5
502
u
2.6"
8.9"
31°26'4
80°24'1
4.7"
8.5"
31°26'4
80°24'1
4.7"
8.5"
BE150 2
Baer
Temper
ude (E)
tion
le
ature
(m)
subgr
(oC)
4389
oup WS
77
95.1
0.03
4389
WS
75
89.4
4779
WS
69
4779
WS
4402
Tirtha
31°07'3
80°45'0
01
puri
8.8"
7.2"
MS15
Tirtha
31°07'3
80°45'0
02
puri
8.8"
7.2"
QW15
Qiwus
30°46'0
81°21'4
01
i
1.4"
6.1"
DR15
Daron
30°25'3
83°33'5
01
g
7.5"
2.6"
DR15
Daron
30°25'3
83°33'5
02
g
7.5"
2.6"
DGJ1
Dageji
29°36'1
85°44'4
501
a
9.2"
4.8"
DGJ1
Dageji
29°36'1
85°44'4
503
a
5.0"
LDB1
Liuda
29°19'2
502
oban
5.6"
02
Xiqin
29°04'1
(%)
(ppm)
2.87
0.22
% 0. ) 90
8623
0.32
8.24
0.27
43.9
11.9
43.7
−
51
9.67
1.80
87.5
0.76
WS
47
26.5
4402
WS
60
4400
WS
4990
CS
(%)
(%)
67.1
−
2.07
−
6.10
96.0
38
34.5
8.28
56.6
0.14
37
19.5
2.89
76.1
0.98
11.7
48.6
5.81
42.2
1.78
5090
CS
48
62.1
6.24
29.1
0.50
5074
CS
82
56.3
4.94
35.7
0.71
4046
CS
46
75.1
0.49
0.11
23.2
4066
CS
53
93.9
0.07
4.48
0.16
3734
ES
59
18.5
2.57
77.3
1.14
3734
ES
65
15.1
1.62
81.4
1.20
8.1"
87°05'4 7.4"
87°44'2 8.7"
29°20'2
90°00'0
0.5"
2.3"
MJ150
Mianji
1
u
MJ150
Mianji
29°20'2
90°00'0
2
u
0.5"
2.3"
5015
(%)
CO2
60
7.5"
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He
(
O2
CS
CE
XQ15
Ar
N2
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MS15
CH4
D
1
Baer
Samp
PT
no. NMR1
BE150
Eleva
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e
e (N)
Longit
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e
Latitud
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Sampl
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Sampl
1. 08 0. 56 0. 18 0. 36 0. 25 0. 41 0. 34 0. 66 0. 62 0. 71 0. 95 1. 41 .3 3 .3 1
6760
1835
767 − −
242
2260
9445
13956
16859
1365
110
1199
1117
Abbreviations are as follows: WS, western subgroup; CS, central subgroup; ES, eastern subgroup.
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ACCEPTED MANUSCRIPT Table 2 Helium isotopes, carbon and nitrogen stable isotope ratios of hydrothermal volatiles from southern Tibet and the Himalayas. Sampl
Samp
no.
e
le
locati
subgr WS oup
4
NMR1501
He/ Ne
Rm/RA
or
RC/RA
01
ru
1688
0.016
ru
1355
0.016
0.020
0.020 0.0
0.068
WS
0.041 0.0 01
MS1502
WS
puri Tirtha
0.046
0.572
WS 1.50
0.350
2.90 WS
D
i
−
i
g
1889
CE
DR1501
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CS
1076
0.015
1793
0.014
2.06
−
1010
-0.14
-5.35
−
−
0.85
1.07
0.02
× 0.023
-5.86
1010
0.85 1.50
0.0
0.0
-6.03
109
1.50
0.015
2.02
−
−
109
2.02
0.014
0.78
−
-11.3
109
0.78 −
−
0.46
3.04
0.55
×
03
CS
0.020
−
−
− 72
1.09
×
0.0
204
1.01
×
−
CS
2.00
× 0.016
-6.34
CS
Xiqin
-0.25
02
Liuda
XQ1501
−
-5.19
-5.05
oban
oban
-0.45
×
a
LDB1502
−
02 0.016
Dageji
LDB1501
-0.21
-5.14
a
Liuda
-0.80 -0.23
0.0
1478
7.45 109
-0.02
02
CS
DGJ1503
109
×
−
0.023
3.80
×
-0.22
01
Dageji
DGJ1501
0.218
108
4.94
0.0
Daron g
-5.00
4.47
×
-0.02
0.090
−
CS
DR1502
20
0.169
CS Daron
0.373
He
108
-6.73
-6.76
CO2/
×
01
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QW1502
23
0.0
Qiwus
Qiwus
1.52
-0.25
0.0
WS
QW1501
(‰) 4.99
0.0
0.80
puri
-11.6
0.046
NU
MS1501
1182
MA
Tirtha
3
4.99
SC
11.0
Baer
(‰)
N2)C
4.95
24 Baer
(δ15N-
-12.9
01
WS
BE1502
(‰)
δ15N-N2
0.0
Namu
BE1501
δ13C-CO2
0.0
WS
NMR1502
Err
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on Namu
20
PT
Sample
0.019
-11.0
0.46
−
-12.3
0.76
107 −
−
ACCEPTED MANUSCRIPT CS
0.0
0.59
×
12 XQ1502
Xiqin
11.0
0.057
ES
MJ1501
u
1304
0.036
0.035
0.045
1010
1.18 1.05
1.17
×
-7.75 0.045
1.31
×
-7.72
03 1477
109
0.60
0.0
Mianji u
-12.3
1.18
04
ES
MJ1502
0.035 0.0
Mianji
8.63
1.05
1010
SC
RI
PT
Abbreviations are as follows: WS, western subgroup; CS, central subgroup; ES, eastern subgroup. Measured 3He/4He ratios (Rm/RA) are corrected for air contamination using air-normalized He/Ne ratio multiplied by the ratio of their Bunsen coefficients (Weiss, 1971), assuming a groundwater recharge temperature of 10 °C (see details in Hilton, 1996). Corrected 3He/4He ratio is reported in RC/RA notation, where RC denotes air-corrected 3He/4He ratio and RA represents 3He/4He ratio of air, i.e. 1.382 × 10-6 (Mabry et al., 2013). δ15N-N2 (‰) values are also corrected following method for helium isotope correction (Halldórsson et al., 2013). The CO2/3He ratios are calculated based on air
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corrected 3He contents.
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ACCEPTED MANUSCRIPT Table 3 Calculated 3He/4He, δ13C-CO2, CO2/3He, CO2/4He and carbon contents of the EMW source. 3
He/4He
Proportion of a
δ13C-CO2 b
CO2/3He
CO2/4He
Carbon content
(×10 )
(×104)
(ppm)
9
(RA)
(‰)
5%
5.23
−8.11
2.31
1.67
2108
10%
3.77
−9.45
2.65
1.38
2295
15%
2.88
−10.59
3.03
1.21
2483
20%
2.28
−11.58
3.46
1.09
2670
silicate melts
Silicate melts involved in mantle wedge enrichment are generated by partial melting of subducted Indian
PT
a
slab-derived silicate sediments (SED) that incorporate organic-sourced carbon, as commonly discovered in ultra-high pressure metamorphic rocks (Zheng et al., 2000, 2003). Proportions (5−20%) of the silicate melts are b
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from Guo et al. (2013).
Calculated δ13C values are based on the scenario of silicate-dominated mantle enrichment, which is consistent
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with petrogenesis of the post-collisional K-rich volcanic rocks (see details in text; Guo et al., 2013, 2015). Helium contents of the SED end-member are calculated based on U-Th decay of silicate rocks (U = 1.3 ppm, Th = 5.6 ppm; Rudnick and Gao, 2003) with age of 500 Ma (Kundig, 1989); Carbon contents of the SED end-member
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are calculated by carbon contents and proportions of continental crustal materials (except for carbonate; see details in Wedepohl, 1995). According to genesis of the EMW-derived K-rich magmas (Guo et al., 2013), degree of partial melting of the SED end-member is assumed at 10%, following batch melting model and partition
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coefficient (D) of helium (DHe = 1.8 × 10-4; Graham et al., 2016) and carbon (DHe = 3.3 × 10-4; Rosenthal et al., 2015). Partial melts of the SED components are mixed with asthenosphere during slab-mantle interaction to
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D
generate the EMW source.
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Graphical abstract
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ACCEPTED MANUSCRIPT Highlights The degassing systems are studied in context of the Indian continental subduction Enriched mantle wedge (EMW) is proposed as source of the mantle contributions The EMW-derived volatiles are contaminated by regional crustal rock assemblages
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Tectonic affinities of degassing systems are revealed by volatile geochemistry
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