Recycled noble gases preserved in podiform chromitites from Luobusa, Tibet

Accepted Manuscript Recycled noble gases preserved in podiform chromitites from Luobusa, Tibet

Wei Guo, Huaiyu He, David R. Hilton, Yongfei Zheng, Fei Su, Yan Liu, Rixiang Zhu PII: DOI: Reference:

S0009-2541(17)30167-5 doi: 10.1016/j.chemgeo.2017.03.026 CHEMGE 18294

To appear in:

Chemical Geology

Received date: Revised date: Accepted date:

31 August 2016 16 March 2017 23 March 2017

Please cite this article as: Wei Guo, Huaiyu He, David R. Hilton, Yongfei Zheng, Fei Su, Yan Liu, Rixiang Zhu , Recycled noble gases preserved in podiform chromitites from Luobusa, Tibet. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Chemge(2017), doi: 10.1016/j.chemgeo.2017.03.026

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ACCEPTED MANUSCRIPT Recycled noble gases preserved in podiform chromitites from Luobusa, Tibet Wei Guo a, b, Huaiyu He a, b, David R. Hilton c, Yongfei Zheng d, Fei Su a, Yan Liu e,

a

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Rixiang Zhu f

CAS Key Laboratory of Earth’s Deep Interior, Institute of Geology and Geophysics, Chinese Academy Sciences,

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College of Earth Sciences, University of Chinese Academy of Sciences, Beijing 100049, China Fluids and Volatiles Laboratory, Geosciences Research Division, Scripps Institute of Oceanography, UCSD, La

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b

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Beijing 100029, China

Jolla, CA 92093-0244, USA

CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences,

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d

University of Science and Technology of China, Hefei 230026, China

State Key Laboratory of Continental Tectonic and Dynamics, Institute of Geology, Chinese Academy of Geological

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e

Sciences, Beijing 100037, China

State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of

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Sciences, Beijing 100029, China

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f



Corresponding author.

E-mail address: [email protected]

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Abstract We report noble gas (He, Ne, Ar) signatures of chromite and olivine separates from the Luobusa chromitites in Tibet to better understand the volatile compositions trapped in the minerals, and further to trace the origin of melts responsible for formation of the chromite deposits. The

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studied samples can be divided into two groups based on petrography and distinct noble gas signatures. Group I samples are free of carbonates and have 3He/4He ratios from 0.81 to 2.36 Ra

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(where Ra is the 3He/4He ratio of air = 1.4×10-6) and air-like Ne and Ar isotopic compositions

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irrespective of chromitite structure types. Most 3He/4He ratios of Group I samples are higher than air, suggesting apparent presence of mantle volatiles. Given the 4He/20Ne and 3He/36Ar several

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orders of magnitude higher than air, negligible contributions are from the atmospheric helium. The observed He isotope compositions thus can be regarded as a two-component mixture of

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mantle-derived and radiogenic He. A broadly positive correlation between 3He and 36Ar in nodular chromitite samples indicates a source mixing between mantle and recycled noble gases but not due

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to shallow air contamination. In addition, the wide distribution range of 20Ne/36Ar also supports a

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subduction-related origin of neon and argon. Combined with major element data, the most appropriate tectonic setting to generate such noble gas signatures in Group I samples is subduction zone (probably forearc) where favorable conditions are present for the formation of the

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chromitites. In contrast, Group II samples containing carbonates have much more radiogenic 3

He/4He ratios of 0.03 to 0.3 Ra but much less radiogenic

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Ar/36Ar ratios of 344 to 420. In

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combination with the occurrence of carbonate veins it is suggested that Group II samples are predominated by supracrustal components that may be imparted during or after the emplacement stage. A comparison of these two group samples indicates that the primary noble gas signatures reflecting the characteristics of ore-forming melts can be preserved in chromite and olivine grains (Group I samples) and thus used to trace the origin of podiform chromitites.

Keywords Noble gas; Podiform chromitite; Ophiolite; Subduction Zone; Luobusa; Tibet

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1. Introduction The mineral chromite is a member of the spinel group of minerals and is the only source for the metallic element chromium. It is found in two forms: stratigraphic and podiform deposits which refer to concentrations in large mafic/ultramafic layered intrusions (e.g., Bushveld, Stillwater) or in pod-shaped bodies ranging from pea-size up to 100s of meters, respectively

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(Mosier et al., 2012). Podiform chromitites, the host rock of chromite and the subject of this study, are found in alpine-type peridotites associated with ophiolites (Thayer, 1964) and provide

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invaluable information on various mantle processes (Arai and Miura, 2016). They are usually

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classified into concordant and discordant chromitites (Cassard et al., 1981), high-Cr (metallurgical) and high-Al (refractory) sub-group ores (Thayer, 1964; Zhou et al., 2014), according to the

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structural relationships to the surrounding harzburgites and their chemical compositions, respectively. Many podiform chromitites are surrounded by dunite envelopes with variable

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thickness and have the most common textural types of nodular, massive, disseminated, brecciated and some transitional types (e.g., Arai and Miura, 2016; Zhou et al., 1996).

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Podiform chromitites have been generally considered as products of melt-peridotite reaction

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and subsequent magma mixing in upper mantle (e.g., Arai and Yurimoto, 1994; Zhou et al., 1994), but the origin and history of podiform chromitites are still controversial with tectonic settings ranging from mid-ocean ridge to supra-subduction zone (SSZ) (e.g., Arai and Matsukage, 1998;

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Arai and Miura, 2015; Rollinson and Adetunji, 2013; Zhou and Robinson, 1997). Moreover, the presence of ultrahigh-pressure (UHP) and highly-reduced minerals such as diamond, coesite,

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clinopyroxene exsolution lamellae, nitrides, native elements and so on (Dobrzhinetskaya et al., 2009; Robinson et al., 2004; Xu et al., 2015; Yamamoto et al., 2009; Yang et al., 2007) makes the genesis of podiform chromitites more confused. These ‘unusual’ minerals were originally found in the Luobusa (Tibet) chromitites, but more recently also in their host peridotites (Yang et al., 2014) and the Polar Ural (Russia) chromitites (Yang et al., 2015). In addition to the UHP and highly-reduced minerals, varieties of crustally-derived minerals represented by zircons with a wide range of age (Yamamoto et al., 2013) were also identified in the Luobusa chromitites and summarized by Robinson et al. (2015). These continuous findings require reevaluation of our understanding of the whole picture of podiform chromitite genesis (Arai and Miura, 2016), and

ACCEPTED MANUSCRIPT make the origin or evolution of the Luobusa chromitites hotly debated. Previous works were mainly concentrated on solid element analysis and mineral inclusions, few studies were conducted on volatile compositions of the enigmatic bodies. Noble gases have distinct isotope and relative elemental compositions in Earth’s mantle, crust and atmosphere (Ballentine et al., 2002; Graham, 2002; Hilton et al., 2002; Ozima and Podosek,

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2002; Porcelli and Ballentine, 2002). In combination with their rarity and inert nature, noble gases are therefore excellent natural tracers of mantle evolution, crust-mantle interaction, and source

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contribution in mineralized deposits (Burnard et al., 1999; Gonnermann and Mukhopadhyay, 2009; Holland and Ballentine, 2006; Matsumoto et al., 2001; Stuart et al., 1995). Thus, noble gases are

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eminently suitable to be used to trace the origin of melts responsible for the formation of chromite deposits when considering possible mantle processes like partial melting, melt-rock interaction

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and refertilization. In this study, we present one-step crushing noble gas data of chromite and olivine separates from the Luobusa chromitites. The results display two groups of distinct

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compositions, which are consistent with petrology of the samples. The noble gas isotope records provide a powerful tool to trace the origin of podiform chromitites.

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2. Geological background and sample description The Luobusa ophiolite is located about 200 km southeast of Lhasa, Tibet (Fig. 1a) and lies in the eastern part of the Yarlung Zangbo suture zone which extends for more than 2000 km in a nearly

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east-west direction, tectonically separating the Himalaya block to the south from the Lhasa block to the north (Allègre et al., 1984). The ophiolite is over-turned and over-thrust onto the Tertiary

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Luobusa Formation and Gangdese Batholith to the north, and separated from the Triassic flysch by steep reverse faulting to the south. It is mainly composed of mantle peridotites with minor crustal cumulates and mafic dikes which include wehrlite, pyroxenite and layered gabbro (Fig. 1b) (Zhou et al., 1996). In the mantle sequence, harzburgite occurs as the chief block overlying minor clinopyroxene-bearing harzburgite (lherzolite) and underlying lesser amounts of dunite in a reconstructed section (Zhou et al., 2005; Zhou et al., 1996). However, recent studies find the ophiolite is much different from the classical definition of an ideal ophiolite section, and suggest the mantle peridotite massifs could be ancient sub-continental lithospheric mantle (SCLM) (Griffin et al., 2016; Wu et al., 2014).

ACCEPTED MANUSCRIPT The Luobusa ophiolite is believed to have formed in two stages, based on geochronological and geochemical studies. Sm-Nd isochron dating for whole-rock and plagioclase and clinopyroxene separates from gabbro dikes of the ophiolite yield an age of 177±31 Ma, with Nd and Pb isotopic characteristics indicating an Indian MORB affinity (Zhou et al., 2002). This age is confirmed by a SHRIMP zircon U-Pb age of 163±3 Ma for diabase dikes in Luobusa (Zhong et al., 2006),

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suggesting an early stage of processes for the ophiolite. The second stage is characterized by a magmatic event at ~126 Ma (Malpas et al., 2003), which is confirmed by a SIMS zircon U-Pb age of

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128.40.9 Ma for a gabbroic dyke and 131.01.2 Ma for amphibolite (Zhang et al., 2016). This later stage of processes shows the superimposition of LREE enrichment on previously depleted REE

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patterns for some mantle peridotites (Xu et al., 2011; Zhou et al., 2005). Afterwards, the Luobusa ophiolite was postulated to be initially displaced at ca. 90-80 Ma (Malpas et al., 2003). However,

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this postulation has not been confirmed by recent studies; instead, it is suggested that the Luobusa ophiolite was formed in the Early Cretaceous and underwent the intra-oceanic emplacement

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immediately after its formation (Zhang et al., 2016).

The Luobusa podiform chromitites, being mainly hosted in mantle peridotites of the Luobusa

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ophiolite, are the largest chromite deposits in China and contain more than 5 million tons of

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ore-grade material (Zhang et al., 1996). They are found in three districts: from west to east, these are the Luobusa, Xiangkashan, and Kangjinla districts (Xu et al., 2011; Zhang et al., 1996). Most of the chromitites, occurring as lenses and tabular bodies with limited areal extent, are hosted in

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harzburgites with dunitic envelopes and concordant or subconcordant with deformation of the hosting rocks. Some of them are also distributed in diopside-harzburgites along the southern

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margin of the ophiolite. Nodular, disseminated, and massive chromitites are the most common textural types and they also show transitions between these various types. Each type of chromitite commonly transects each other, indicating a multi-stage magmatic history (Bai et al., 2000; Xu et al., 2011; Yamamoto et al., 2009; Zhou et al., 1996). The age of the chromitites is controversial and model-based. The chromitites were thought to have bearing on their formation and evolution with the ophiolite which was originated from a mid-ocean ridge spreading center at 177±31 Ma and later modified by SSZ magmatism at 120±10 Ma (Malpas et al., 2003; Robinson et al., 2004; Xu et al., 2011; Zhou et al., 2005; Zhou et al., 1996). The Osmium isotopic studies of Os-Ir alloys from the Luobusa chromitites give a model age peak at 235 Ma, suggesting a MORB-stage origin for

ACCEPTED MANUSCRIPT the chromitites (Shi et al., 2007). Thus, a comprehensive age from the Triassic to the Cretaceous (ca. 230-120 Ma) is normally accepted for the Luobusa chromitites (Yamamoto et al., 2013). However, recent studies report magmatic zircon U-Pb age of 376±7 Ma and suggest the chromitites were formed in a continental-margin supra-subduction zone in the Devonian (Griffin et al., 2016; McGonwan et al., 2015).

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The Chromitite samples studied here are from the Luobusa and Kangjinla districts. There are three textural types: nodular, massive, and transitional between the two (Fig. 2). Both nodular and

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transitional types are available from the two districts; however, massive chromitites are restricted to the Luobusa district. The samples can be further divided into two groups based on the occurrence

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of carbonates. Four massive chromitites (L6, L7, LBS13-34 and LBS13-39) contain carbonates (e.g., Fig. 2m-n), and are thus designated Group II; all other samples are carbonates-free,

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irrespective of structural type, and are thus named Group I.

In Group I samples, mostly the chromite and olivine grains are typically fresh (e.g., Fig. 2o-p),

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which is consistent with abundant observations reported previously (Bai et al., 2000; Xiong et al., 2015; Xu et al., 2011; Zhou et al., 1996). Nevertheless, serpentine veinlets surrounding or

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infiltrating olivine (e.g., Fig. 3b) are not unusual. Solid inclusions in chromite are predominated

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by primary mantle silicate minerals, mainly olivine occasionally rimmed by serpentine (Fig. 3a). Most common solid inclusions in olivine are euhedral chromite grains (Fig. 2p; Fig. 3b and Fig. 4b). Sub-euhedral, round and amorphous clinopyroxene inclusions were also found in olivine (Fig.

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3b). The orientated needle-shape inclusions in olivine are likely to be clinopyroxene (Huang et al., 2014; Yamamoto et al., 2009) (Fig. 4a). Round, tiny (generally <1 or 2 μm) and scattered primary

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micro-inclusions are pervasive in olivine (Fig. 4). In rare cases, linear inclusions (Fig. 4b) are also found. They may be secondary inclusions trapped during the healing of fluid-filled cracks (Miura et al., 2011).

The four massive chromitites of Group II samples contain carbonates, some in the form of veins, suggesting an intrusive origin (Fig. 2m and Fig. 3c). Among these samples, all silicate minerals are hydrated (Fig. 3c-d) and no fresh olivine inclusions were observed. Some of the hydrated silicates are distributed in trails with spherical or negative crystal shapes (Fig. 3d).

ACCEPTED MANUSCRIPT 3. Analytical methods Each specimen was divided into two parts, one of which was polished to thin section and examined by detailed petrographic observation using electron microscope and electron microprobe analysis. The other part was crushed and sieved to between 40 to 20 (450-850 μm) mesh fractions for the noble gas isotope analysis. In this case, fresh olivine separates were carefully picked out by

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hand and sequentially washed in an ultrasonic bath with 5% HNO3, deionized water, ethanol and acetone in order to remove any possible alteration products on the surface or in the cracks of the

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grains. The same cleaning procedure was adopted for chromite grains but without diluted HNO3 due

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to interaction between chromite and HNO3. After washing and drying, each sample was weighed and around 2 g was then loaded into the crusher chamber.

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The analysis of major elements of the minerals was performed in the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing (IGGCAS), using the wavelength dispersive

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JEOL JXA-8100 electron microprobe. The microprobe was set to operate at a voltage of 15 kV and a beam current of 10 nA with a focus beam diameter of 5 μm. Results were corrected with a program based on the ZAF procedure. Each datum presented in Table 1 is derived from the average of at

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least 3 point analysis.

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The noble gas isotope analysis was conducted using Noblesse mass spectrometer at the Institute of Geology and Geophysics, Chinese Academy of Sciences in Beijing. One-step crushing

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extractions were carried out on all samples in order to analyze the noble gases trapped in fluid inclusions and lattice defects. Before crushing, the samples were baked under vacuum at about

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120℃ for three days to reduce any adsorbed atmospheric noble gases. Gases were extracted by vacuum crushing using a hydraulic press to the piston within 2 minutes under a pressure of about 1500 psi at room temperature, and then introduced into the purifying system. After a series of purification and noble gases segregation procedures, detailed by He et al. (2011) and Su et al. (2014), helium, neon and argon were sequentially input into the mass spectrometer for concentration and isotope ratio analyses carried out in static mode. Procedural blanks were run prior to each sample, with typical values being <1.0×10-10 cm3 STP 4He, <1.0×10-12 cm3 STP 20Ne and <1.0×10-9 cm3 STP 40Ar. Although the crusher blanks were negligible in most cases (<1% for He and Ar, <10% for 20Ne and 22Ne, but about 10-30% for 21Ne),

ACCEPTED MANUSCRIPT blank corrections were conducted for all samples. We use HESJ (He Standard of Japan, 3He/4He = 20.63±0.10 Ra, Matsuda et al., 2002) and air as standards to normalize measured He, Ne, and Ar isotopic ratios. Results of He, Ne and Ar isotopes and concentrations are presented in Table 2 with all uncertainties being 1σ including uncertainties in sensitivity for concentration and correction factors for mass discrimination and procedure blank.

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4. Results

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4.1. Major element compositions of minerals

Chromite grains of different textural types of chromitite from the two districts show relatively

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uniform Cr2O3 and Al2O3 contents being 57.5 to 61.1 wt.% and 9.9 to 13.1 wt.%, respectively, thus a very narrow range of Cr# [100×Cr/(Cr+Al), atomic ratio] which varies from 75 to 80 (Table 1; Fig.

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5a). Therefore, all samples can be classified as high-Cr (Cr# >60) chromitites (Thayer, 1964; Zhou et al., 1994). In comparison to Cr#, Mg# [100×Mg/(Mg+Fe2+), atomic ratio] of the chromite has a

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wider range from 60 to 78 (Table 1; Fig. 5a), and the MgO contents are negatively correlated with FeO (total iron) for all samples (Fig. 5b). It seems that the MgO content has an increasing trend

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from nodular chromitites to transitional chromitites to massive chromitites (Fig. 5b). In nodular

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and transitional chromitites, some euhedral chromite grains (marked as Chr* in Table 1, Fig. 2p and Fig. 3b) dispersed in the matrix or included in olivine exhibit higher FeO contents but lower MgO (and correspondingly lower Mg#) compared to the nodular grains (Fig. 5b). The chromite from all

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three textural types have essentially the same TiO2 contents of around 0.2 wt.%, higher than that in harzburgite (Fig. 5c). In the plots of TiO2 versus Cr# and TiO2 versus Al2O3, all chromitite samples

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fall within boninite area and arc-related setting, respectively (Fig. 5c and Fig. 5d). Olivine is the most common gangue mineral in the matrix of the chromitites and it has uniformly high MgO contents of around 54 wt.% with Fo value [100×Mg/(Mg+Fe2+), atomic ratio] between 94 and 98 (Fig. 5e). Generally, the olivine becomes more magnesian from harzburgite to dunite to chromitite, despite some overlaps between the lithologies (Fig. 5e). Like chromite, olivine also differs in composition between the small inclusion grains in chromite (Fig. 2p and Fig. 3a) and coarse grains in the matrix. The small inclusion olivine, labeled as Ol* in Table 1, is slightly more magnesian rich than the matrix olivine (Fig. 5f). The olivine has NiO contents of 0.4 to 0.9 wt.% which show a positive correlation with Fo (Fig. 5e).

ACCEPTED MANUSCRIPT Clinopyroxene and orthopyroxene are very scarce in the chromitite samples. Only some small parts of discrete, anhedral clinopyroxene were found in two thin sections (L2 and 07y-445) with orthopyroxene observed in one sample only (L1, Fig. 2o). The clinopyroxene grains occur as fine inclusions in olivine and sometimes surround the dispersed chromite (Fig. 3b); they have extremely high Mg# (around 97, Table 1), consistent with the high MgO olivine.

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4.2. Noble gas concentrations, isotopic and elemental ratios Chromite and olivine separates of the different structure-types (nodular, massive, and

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transitional) from the Luobusa and Kangjinla district podiform chromitites were analyzed for He,

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Ne and Ar contents and isotopic compositions (Table 2). Fresh olivine grains were not available for sample L4, 07y-444 and 07y-459, and not for all the massive chromitites.

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Helium isotopic ratios of all the samples have a relatively wide distribution (Fig. 6a), with the highest value of 3He/4He being 2.36 Ra in nodular chromitite L1 and the lowest value of 0.03 Ra in

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massive chromitite LBS13-39. Within this wide range, distinct isotope compositions can be identified between the two different groups of chromitites. Group I samples (no carbonates), consisting of fresh chromite and olivine in nodular and transitional chromitites but only chromite

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in massive type, have higher 3He/4He ratios of 0.81 to 2.36 Ra, mostly >1 Ra, and a simple binary

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mixing trend (Fig. 6b). In contrast, Group II samples show much lower 3He/4He ratios of 0.03 to 0.3 Ra, and there seems a negative correlation between 3He/4He ratios and 4He concentrations (Fig.

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6a). They do not follow the mixing trajectory defined by Group I samples (Fig. 6b). In contrast to the He isotope ratios, the Ne and Ar are overwhelming atmospheric in their

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isotopic composition (Table 2). Note the term ‘atmospheric’ designates air-like noble gas isotope compositions whereas their element ratios can be very different from those in air. The Ne isotopic ratios for all the samples are identical within uncertainty to air (20Ne/22Ne = 9.8, 21Ne/22Ne = 0.029). The observed 40Ar/36Ar ratios lie between 295 and 420, which is only slightly higher than the ratio of 295.5 for air (Fig. 7a). However, Group II samples have the highest 40Ar/36Ar ratios, showing slightly more radiogenic 40Ar (Fig. 7a). In addition there is a broadly positive correlation between 3

He and

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Ar concentrations for nodular chromitites (Fig. 7b). Elemental ratios

samples have a wide range from 0.009 to 2.2 (Fig. 8).

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Ne/36Ar of the

ACCEPTED MANUSCRIPT 5. Discussion 5.1. Mineralogy and chromitite petrogenesis It is well known that chromite compositions of different lithological units provide important information for understanding the petrogenesis of podiform bodies. Cr2O3 and Al2O3 contents (thus Cr#) of chromite combined with FeO and MgO contents (thus Mg#) and TiO2 contents are

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usually used to indicate the processes, tectonic settings, and parental magmas compositions involved in the formation of podiform chromitites. The high-Cr chromitites in Luobusa, having

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higher Cr# than residual chromite in harzburgite (Fig. 5a), can be explained as products by

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melt-rock reaction in arc-related settings (Zhou and Robinson, 1997; Zhou et al., 2005; Zhou et al., 1996). In the diagrams of Cr# versus TiO2 and Al2O3 versus TiO2 (Fig. 5c and Fig. 5d), chromitite

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samples fall within boninite area and arc-related setting, respectively, suggesting that they were crystallized from exotic boninitic magmas at subduction zone (Zhou et al., 2005; Zhou et al.,

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1996).

The distinct FeO composition between the small euhedral chromite inclusions and the nodular

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ones (similar situation to olivine) (Fig. 5b and Fig. 5f) is attributed to intense subsolidus

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equilibration between olivine and chromite (Xiao et al., 2016). The olivine in chromitites (especially small inclusions) has impressive higher Fo value than the one in mantle peridotites (Fig. 5e). Although the olivine could be magmatic origin, it is generally interpreted as result of

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subsolidus equilibration with chromite (Xiao et al., 2016; Xiong et al., 2015). Major and trace elements of the Luobusa chromitites were extensively studied (e.g.

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McGowan et al., 2015; Xiong et al., 2015; Xu et al., 2011; Zhou et al., 2005; Zhou et al., 1996; Zhou et al., 2014). The reader is suggested to dig more detailed information and implication from literatures elsewhere. It should be noted that the two groups of samples classified by presence or absence carbonates and distinct noble gas signatures are not differentiated from each other based on major element compositions. We suggest that Group II samples may have experienced some processes during which the noble gas compositions were changed but without effects on the major elements, as discussed in section 5.3.2.

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5.2. The possible origin and modification of noble gases in the crushed minerals Noble gas isotope sites in minerals are mainly inclusions and matrix which refer to trapped and radiogenic/cosmogenic components (Tolstikhin et al., 2010). The trapped component is usually considered as being sequestered during mineral formation under high-pressure conditions (Tolstikhin et al., 2010). It is widely accepted that the vacuum crushing is the most appropriate

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method to discriminate ‘primary’ component trapped in fluid inclusions from the in situ produced radiogenic and cosmogenic component, especially in initial crushing steps (Hilton et al., 1993;

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Scarsi, 2000).

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In this study, the noble gases released by crushing the fresh chromite and olivine are mainly from fluid inclusions and lattice defects. In spite of the presence of secondary micro-inclusion

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trails (Fig. 4b), they are negligible in volume (<1%, roughly estimated under microscope) when compared to the abundant primary micro-inclusions. Thus, we suggest that the noble gases

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released in Group I samples are mainly primary trapped component when the chromitites were formed.

However, before using the noble gas data to trace origin or evolutionary processes of the

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chromitites, we have to evaluate possible interferences to the ‘primary’ noble gas compositions

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which can be derived from air contamination, post-entrapment loss (especially helium, because helium is more mobile than other noble gases) and post-entrapment modification by radiogenic

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and/or cosmogenic isotopes. As the studied minerals (olivine and chromite) are excellent natural samplers of trapped volatile species (including helium) under general conditions (Blard et al.,

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2008; Tolstikhin et al., 2010), we assume that there is no post-entrapment loss for all samples. However, the other two modes of interference are discussed below.

5.2.1. Air contamination

Modern air contamination, occurred during sample processing and/or during the laboratory measurement, could be an interference for noble gas signatures of fluid inclusion bearing samples especially when the samples have low noble gas abundances and contain ‘empty’ fluid inclusions filled by air (Ballentine and Barfod, 2000; Kendrick and Burnard, 2013). However, this can be ruled out by careful sample preparation; and analysis of multiple noble gas isotopes enables the

ACCEPTED MANUSCRIPT presence of modern air contaminants to be rigorously tested (Kendrick and Burnard, 2013). However, the samples studied here were carefully prepared. Pure olivine and chromite grains were hand-picked out, and repeatedly washed (see analytical method section) to remove adhered groundmass and alteration parts (serpentine veinlet) which may contain atmospheric component. Size of the grains (450-850 μm) is significantly larger than the threshold value (<125 μm) of a

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“lobster pot” adsorption even for helium (Protin et al., 2016). Sufficient baking time (3 days) after sample loading was achieved to get ultrahigh vacuum level. The blank amount obtained before

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each sample analysis was negligible compared to gas abundance of the sample.

The elemental ratios of different noble gas isotopes do not support shallow air contamination.

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The ratios 4He/20Ne span a range from 180 to 4566 (Table 2), which are several orders of magnitude higher than air value (4He/20Ne = 0.318), indicating that there is negligible contributions from the

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atmospheric helium (Hilton et al., 1992). In addition, 3He/36Ar (from 2×10-5 to 2.3×10-3) are higher than the air (2×10-7) also ruling out atmospheric contamination as the source of the He

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(Stuart et al., 1995). There is a broadly positive correlation between 3He and

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Ar in nodular

chromitite samples (Fig. 7b). As given here the low 40Ar/36Ar ratios close to air in these samples,

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all the 36Ar should be an atmospheric origin. However, all the 3He is of mantle origin in contrast.

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If this correlation was due to shallow air contamination samples with higher 3He concentrations would each require equal degrees of air contamination to maintain the observed correlation, which seems unlikely (Broadley et al., 2016; Matsumoto et al., 2001). Thus, despite the air-like Ne and

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Ar isotopic ratios, they could be intrinsic in origin.

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5.2.2. Post-entrapment modification

All samples used in this study were collected from mines, indicating that they were far removed from the Earth’s surface and have avoided prolonged exposure to cosmogenic spallation reactions in the most upper few meters under the ground (Kurz, 1986). The extreme low concentrations of Li (Su et al., 2016) and U and Th (Xu et al., 2011) in Luobusa mantle peridotites make the thermal-neutron 3He production through 6Li(n,α)3H(β)→3He negligible. Thus, 3He of all samples can be safely regarded as mantle-derived. Radiogenic 4He is mainly produced by alpha decay of the

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U and

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Th decay chains,

ACCEPTED MANUSCRIPT which is ultimately responsible for the widely distributed 3He/4He ratios observed in Earth materials (Ballentine and Burnard, 2002; Day et al., 2015). It is extremely challenging to quantitatively distinguish the ‘primary’ He composition in fluid inclusions from the subsequent addition of radiogenic 4He generated by in situ ingrowth within surrounding matrix when crushing old xenoliths and samples with prolonged emplacement time (Day et al., 2015). Nevertheless, repeat

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crushing experiments and step-crushing may be helpful to evaluate this effect. The repeat crushing of most samples show no discernible systematic changes in He isotopic ratio and insignificant

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varieties in concentration (Table 2), suggesting homogenous distribution of He on a specimen scale, at least in these samples. Step-crushing was conducted on sample L7 whose 3He/4He is

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dominated by radiogenic component. Subsequent crushing with higher pressure and longer duration (5 minutes under 2000 psi for the second crush and 10 minutes under 2000 psi for the

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third crush, respectively) released decreased gas amounts but consistent 3He/4He ratios, which implies limited incorporation of radiogenic 4He from matrix (Table 2).

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Moreover, the amount of radiogenic 4He* (4He cumulated within the matrix) can be estimated using the equation given by Graham et al. (1987): He* = 2.80×10-8 (4.35 + Th/U)[U]·T (cm3 STP g-1)

(1)

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4

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where Th/U is the atomic ratio, [U] is the uranium concentration in ppm, and T is time in Myr. Because of intimate relationship between podiform chromitites and their hosting peridotites, we assume that the studied chromitite samples have the same Th and U concentrations with the

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hosting mantle peridotites. Xu et al. (2011) measured trace element of freshest whole-rock peridotites (including lherzolite, harzburgite, dunite) from Kangjinla district using ICP-MS

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(inductively coupled plasma mass spectrometry). The Th and U concentrations of the three lithologies are nearly the same within error. Thus, we use an average of 0.027 ppm Th and 0.009 ppm U (Xu et al., 2011) for our calculation. These values could be upper limits, because the fresh olivine and chromite separates studied here should have lower U and Th contents than the whole rocks. We choose 375 Ma (McGowan et al., 2015) as formation age of Luobusa chromitite, which is the maximum documented age reported so far, and assume no 4He* loss since formation. Note that the assumptions give maximum 4He* estimation. Then the samples generate 6.9×10-7 cm3 STP g-1 4He* based on equation (1). In order to reproduce the effect of estimated 4He* incorporation into released gases from crushing, 0.1%, 1% and up to 10% addition are assumed.

ACCEPTED MANUSCRIPT The 10% end-member may represent an extreme situation which rarely happens in one-step crushing but highlights this effect on 3He/4He in very old xenoliths and samples (Day et al., 2015). The ‘age corrected 3He/4He ratios’ (R*/Ra) show minor changes with the measured ones assuming ≤1% addition for all samples. However, at 10% addition, the calculated 4He* exceed measured He concentrations in many transitional chromitite samples and has significant effects on the

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measured 3He/4He for most samples but except for massive chromitites and nodular chromitites in Kangjinla (Table 2).

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We discount release of radiogenic 40Ar from matrix, because 40Ar is much less mobile than He and seems unlikely to diffuse from matrix to inclusion by the vacuum crushing method

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(Tolstikhin et al., 2010).

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5.3. Source of noble gases in the two different groups of samples

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5.3.1. Slab-derived He, Ne and Ar dominate Group I samples

5.3.1.1 Helium

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The 3He/4He ratios of 0.81 to 2.36 Ra for Group I samples (Fig. 6a) are higher than radiogenic

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He production values (~0.05 Ra, Andrews, 1985), suggesting the presence of mantle volatile in the micro-fluid inclusions. There is no apparent correlation between 4He concentrations and the measured 3He/4He ratios (Fig. 6a), which suggests that in situ produced 4He either from matrix or

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fluid inclusion itself is not a significant component in these samples. Given the absence of atmospheric He contribution to the all chromitite samples, the observed He isotope compositions

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can be regarded as a simple binary mixing (Fig. 6b) between mantle-derived and radiogenic He. Such a mixing process recorded by the chromitites when they crystallized can be achieved by dehydration or melting of the subducted slab in a SSZ setting, which was also indicated by the major element data. The radiogenic He is more likely derived from subducted oceanic crust and/or sediments. Increasingly radiogenic helium is readily accumulated in old oceanic crust and sediments as a function of their age (Staudacher and Allègre, 1988). The analyses of 11 Ma gabbros from drilling cores revealed rather radiogenic He with 3He/4He ratios down to 2.2 Ra, and these gabbros will generate very radiogenic 3He/4He ratios (<1 Ra) on a 100 Ma time scale (Moreira et al., 2003).

ACCEPTED MANUSCRIPT Hence, a rather amount of radiogenic 4He accumulated by ingrowth can be expected in the composition of slab-derived fluids. The mantle-derived He possibly comes from the subducted slab itself (as it was once generated in the mid-oceanic ridge, MORB-type He and radiogenic He can be synchronously present in oceanic crust, Moreira et al., 2003) and ore-hosting peridotite rocks (the Luobusa

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harzburgites) when parental magmas for the chromitites pass through and react with them. The latter has potential to retain MORB-type He but with extreme low helium concentrations due to

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feeding MORB or melt extraction events (Zhou et al., 1996), because helium behaves incompatibly and strongly partition into basaltic melts during partial melting, leading to its severe

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depletion in the residual mantle rocks without fractionating 3He/4He ratios (e.g. Gonnermann and Mukhopadhyay, 2009; Graham, 2002).

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Assuming the subducted slab has a radiogenic He of 0.05 Ra and the mantle-derived component has a MORB-type He of 8 Ra, then approximately 70%-90% He in Group I samples

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are crustal-derived. Although the mantle-derived component may have more than one origin, it was well mixed with the crustal-derived component before trapping by the mafic magma from

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which the chromitites crystallized. Most modern arc-related volcanism samples have MORB-type

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helium isotope compositions with a mean global 3He/4He ratio around 5 Ra, which suggests negligible influence from subducted slab (Hilton et al., 2002). Hence, the chromitites were probably formed in a forearc setting where radiogenic helium will be efficiently extracted from the

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oceanic crust and sediments (Hopp and Ionov, 2011).

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5.3.1.2 Ne and Ar

In comparison with helium, neon and argon have higher concentrations in air and they are more soluble in seawater (Ozima and Podosek, 2002), so it is expected that they are more concentrated in seawater and susceptible to capture by the supracrustal rocks via water-rock interaction (Dai et al., 2016). During the Ne and Ar uptaking on the surface and subsequent devolatilization during crustal subduction, there seems little or no significant fractionation on isotopic ratios but on elemental ratios, as demonstrated by studies of noble gas isotopes in deep-sea sediments, oceanic crust (Matsuda and Nagao, 1986; Moreira et al., 2003; Staudacher and Allègre, 1988) and mantle xenoliths (Hopp and Ionov, 2011). Recycling of atmospheric or

ACCEPTED MANUSCRIPT seawater-like heavy noble gases by subduction has been demonstrated in the study of convecting mantle (Holland and Ballentine, 2006), xenoliths from subcontinental mantle (Broadley et al., 2016; Yamamoto et al., 2004), mantle wedge peridotites (Hopp and Ionov, 2011; Matsumoto et al., 2001; Sumino et al., 2010), forearc serpentinites (Kendrick et al., 2013; Kendrick et al., 2011), ultrahigh-pressure terrane (Baldwin and Das, 2015), continental basalts (Xu et al., 2014) and

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orogenic mafic intrusives (Dai et al., 2016). Therefore, subduction is an efficient mechanism to carry the supracrustal noble gas components into the mantle.

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Despite their overwhelming air-like isotopic composition, Ne and Ar of Group I samples are not likely due to laboratory air contamination (as discussed above), instead they could be recycled

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in origin. Atmospheric neon isotope ratios and slightly higher 40Ar/36Ar ratios (up to 340) than air (40Ar/36Ar = 295.5) support the predominance of air-like or seawater-derived noble gas in the

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micro-fluid inclusions. Assuming a binary mixing between atmospheric argon (40Ar/36Ar = 295.5) and MORB-type mantle argon (40Ar/36Ar = 10000 ~ 40000, Burnard et al., 1997), the measured Ar/36Ar corresponds to >99% atmospheric argon. The atmospheric

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40

36

Ar has a broadly positive

correlation with the mantle-derived 3He for nodular samples (Fig. 7b), which suggests a source

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mixing process through subduction but not due to shallow air contamination (Broadley et al., 2016;

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Matsumoto et al., 2001). The correlation is not so strong for massive and transitional samples, which might be a consequence of limitation of available data and the possibility of some unknown fractionation process between 3He and 36Ar for these samples. In contrast to the linear correlation 40

Ar/36Ar found in the Horoman ultramafic complex (Matsumoto et al.,

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between 3He/36Ar and

2001) and Baker Rock xenoliths from the Western Antarctic Rift System (Broadley et al., 2016),

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there is no fixed linear correlation for Group I samples in this study due to the predominance of atmospheric Ar isotopic ratios (Fig. 7a). The wide distribution range of and argon. In the plot of

20

Ne/36Ar also indicates a subduction-related origin of neon

20

Ne/36Ar versus

38

Ar/36Ar (Fig. 8), the

38

Ar/36Ar values of samples

whose 20Ne/36Ar below air are of atmospheric 38Ar/36Ar within error, suggesting no argon isotope fractionation, and the corresponding

20

Ne/36Ar may be a mixture between air and sedimentary

components (Baldwin and Das, 2015; Matsuda and Nagao, 1986) or serpentinites (Jackson et al., 2015; Kendrick et al., 2013; Kendrick et al., 2011). However, the 38Ar/36Ar values of samples with 20

Ne/36Ar higher than air are not well within error of atmospheric

38

Ar/36Ar ratios. Given the

ACCEPTED MANUSCRIPT systematic differences of 38Ar/36Ar between first and repeat experiments for samples L1, L2, L3 and L5 (Table 2), we suggest that this could be caused by instrumental effects, but not true isotope fractionation. The high 20Ne/36Ar values (higher than air) in these samples cannot be explained by addition of mantle-derived

20

Ne/36Ar because the atmospheric neon and argon isotope

compositions indicate little contribution from mantle. Instead, the enrichment of 20Ne relative to 36

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Ar could be a consequence of complicated elemental fractionation in subduction zone perhaps

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5.3.2. Secondary noble gases preserved in Group II samples

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involving melt degassing at early stages of melt extraction (Hopp and Ionov, 2011).

Group II samples yield much more radiogenic helium with 3He/4He ratios of 0.03 to 0.3 Ra,

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and the 3He/4He seem to decrease with the increase of 4He concentrations (Fig. 6a). All Group II samples have higher 4He/40Ar for a given 3He/40Ar on the Group I samples correlation (Fig. 6b).

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This could be due to mixing of another fluid component with high He/Ar, although the correlation is poor. They also show slightly more radiogenic argon, than Group I samples (Fig. 7a). As it is suggested above that the addition of 4He* and

40

Ar from matrix can be ruled out, whereas the

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addition of radiogenic 4He and 40Ar could be from other processes. Given the intrusive carbonate

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veins and trails of hydrated minerals (no fresh olivine was found so far) in Group II samples, it is suggested that the noble gas compositions of these samples may have been modified, at least on specimen scale, by hydrothermal activity which probably occurred during or after emplacement of

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the peridotites and chromitites. Coupled with intrusion of CO2-rich fluids, radiogenic 4He and 40Ar

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were also added into the rocks but without any change on their non-radiogenic isotopes. We also found remarkable high CO2 and possible H2O contents during purification of these samples. It should be noted that the samples with lower 3He/4He also have lower 40Ar/36Ar (Fig. 6a and Fig. 7a), indicating the addition of radiogenic 4He and 40Ar were not proportionally related. This may be caused by introduction of different types of fluids in terms of their radiogenic components. However, more data from Group II samples are needed to validate the deduction. The Luobusa listwanites indicate little major element mobility in the ultramafic protoliths during hydrothermal alteration, except for the addition of CO2 and H2O (Robinson et al., 2005). Nevertheless, the composition of chromite grains can be modified during post-magmatic alteration,

ACCEPTED MANUSCRIPT with significantly higher Cr# and FeO (Zhang et al., 2015; Zhou et al., 2014). Group II samples in this study, however, do not show apparent modification in their major element compositions (Table 1; Fig. 5a-d), except for the volatile compositions. Possible reasons can be: (1) The hydrothermal fluids are not in large scale when intruding the chromitites; (2) Relatively low temperature (e.g., ~300℃, Robinson et al., 2005) of the fluids and insufficient interaction time with the chromitites failed to initiate the mobility of major elements; (3) The fluids contain little

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cation species like Fe2+/Fe3+ and Al3+ that can be exchanged with the chromitites. Overall, the

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composition characteristics of Group II samples indicate that volatile species (including noble gases) in the rocks are more vulnerable to the modifications posed by post-magmatic alteration

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than major elements, as expected.

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5.4. Noble gas data and current models for origin of the Luobusa podiform chromitites

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The noble gas data coupled with major element data in this study suggest that the Luobusa podiform chromitites were formed at subduction zone by melt-rock interaction (Arai and Yurimoto, 1994; Zhou et al., 1994; Zhou et al., 1996) and some of the massive chromitites have experienced

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hydrothermal metamorphism during or after the emplacement stage. The recycled noble gas

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signature in primary samples (Group I samples) are consistent with the existence of crustally-dervied minerals (Robinson et al., 2015; Yamamoto et al., 2013) and the observation of large fractionation in Li isotopes (Su et al., 2016), and it highlights the importance of subducted

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fluid and volatiles in the chromitite genesis. Several recent models were proposed, focusing on the explanation of rare but important UHP

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minerals in the Luobusa chromitites, represented by the ‘deep crystallization model’ (Xiong et al., 2015; Xu et al., 2015; Yang et al., 2015), the ‘deep recycling model’ (Arai, 2013; Griffin et al., 2016; McGowan et al., 2015) and the ‘late metamorphism model’ (Huang et al., 2014). Despite the distinct explanations in terms of UHP phenomenon among these models, a formation or modification stage at subduction zone is needed to explain the major and trace element characters of the chromitites. Here we make a brief discussion based on noble gas signatures and/or the possible constraints on these models. In the ‘deep crystallization model’ (Xiong et al., 2015; Xu et al., 2015; Yang et al., 2015), the

ACCEPTED MANUSCRIPT authors suggest that chromite grains or small-scale chromitites first crystallized and encapsulated the UHP minerals like diamonds at the top of transition zone mantle, and were later modified by SSZ fluids and melts at subduction zone. In this case, it is not clear that the noble gases preserved in chromitites record signatures either from transition zone mantle or subduction zone, because noble gas characteristics in transition zone mantle have not been documented. Despite the

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possibility of recycling atmospheric noble gases to great mantle depth (>200 km) transported by serpentinites in cold slab (Kendrick et al., 2011), we argue that the observed noble gases probably

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come from the later subduction-zone stage in this model. However, future noble gas study of the ophiolitic diamonds (Yang et al., 2014) may shed light on resolving this problem.

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Similar situation is encountered in the ‘deep recycling model’ (Griffin et al., 2016; McGowan et al., 2015). We are also not clear whether the primary noble gas compositions can be persevered

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in the subsequent UHP metamorphism at transition zone mantle (~410-660 km) following the shallow formation of the chromitites at subduction zone, due to the insufficient knowledge of

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noble gas behavior in transition zone mantle. Given the extreme high pressure (>15 Gpa) and temperature (>1300℃) at transition zone mantle and long stagnant time (>150 m.y.) of the

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chromitites and hosting peridotite massifs in this model, noble gases, as volatiles and not like

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other solid elements which were apparently unaffected, may easily get equilibration with the surrounding deep mantle materials. If this speculation is correct, then the UHP-chromitite samples may provide invaluable information of noble gases from transition zone mantle.

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In the ‘late metamorphism model’ (Huang et al., 2014), it is suggested that the disseminated and massive chromitites are secondary in origin due to high-pressure or UHP metamorphism, and

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both the chromitites and hosting peridotites experienced serpentinization after the formation of magmatic nodular chromitites but before the UHP metamorphism event. However, our noble gas data do not support such a hypothesis, as the noble gas signatures of Group I samples have no systematic differences among the chromitites of different structure types, and the olivines keep consistent with their chromites in terms of noble gas compositions. Otherwise, if this model is correct, some processes poorly understood so far are required to maintain the consistency in noble gas compositions in these samples during serpentinization and subsequent high-pressure metamorphism. Nevertheless, note that the transitional chromitite samples used in this study are not the typical disseminated chromitites described by Huang et al. (2014), future noble gas studies

ACCEPTED MANUSCRIPT of disseminated chromitites and more massive chromitite samples should be considered to give more constraints.

6. Conclusions Noble gas isotope compositions released from fluid inclusions by crushing chromite and olivine separates from the Luobusa and Kangjinla chromitites of Tibet provide geochemical

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constraints on the origin of ophiolitic chromitites. The noble gas isotope signatures preserved in carbonate-free Group I samples show a mixed composition between a mantle component and a

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crustal component with the atmospheric noble gas signature. In contrast, carbonate-bearing Group 40

Ar/36Ar, probably

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II samples host much more radiogenic 3He/4He and moderately radiogenic

recording the hydrothermal metamorphism events during or after the emplacement stage of the

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chromitites and hosting peridotites. A comparison of these two group samples indicates that the primary noble gas signatures reflecting the characteristics of ore-forming fluids can be preserved in

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fresh chromite and olivine grains.

The principal conclusion of this study is the extensive effect of subduction-related fluids and

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volatiles on the formation of Luobusa chromitites. The most favorable tectonic setting is subduction

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zone (probably forearc) where favorable conditions are present for the formation of the chromitites (Matveev and Ballhaus, 2002). The noble gas analysis of chromitites is a useful tool to trace the origin of fluids/melts responsible for the formation of chromitites. Future systematic noble gas

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studies of the mantle peridotites and UHP minerals like diamonds may provide better constraints on the UHP metamorphism history. Moreover, we note that the study of ophiolitic chromitites,

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commonly occur in ancient subduction zones throughout the world, may be extremely useful targets to investigate the recycling of supracrustal components including noble gases into the mantle.

Acknowledgments The authors thank Prof. Jingsui Yang for providing Kangjinla samples, Prof. Meifu Zhou, Rendeng Shi, Benxun Su, Chuanzhou Liu and William Griffin for their constructive suggestions.

We thank Dr. Qian Mao and Yuguang Ma for assistance with EPMA analyses. The authors gratefully acknowledge comments by anonymous reviewers, which have significantly improved the manuscript. This work is supported by funds from the National Natural Science Foundation of

ACCEPTED MANUSCRIPT China (41425031), the Ministry of Science and Technology of People’s Republic of China (2016YFC0600105) and the Chinese Academy of Sciences (XDB18030505).

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Figure captions

Fig. 1. Sketch map of the Luobusa ophiolite, Tibet (adopted from Xu et al., 2015). (a) Location of the study area within Yarlung-Zangbo Suture and Tibetan tectonic division; (b) geological

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units of the Luobusa ophiolite and its surrounding rocks.

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Fig. 2. Photographs of hand specimen with different textures in Luobusa and Kangjinla chromitites

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and photomicrographs of representative sample. Panels (a) to (g) Nodular chromitite ores, names marked with L means samples from Luobusa and 07y means samples from Kangjinla;

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(h) to (l) transitional chromitite ores, with smaller and denser spheric textures; (m) and (n) selected massive chromitite ores, with intrusive carbonates; (o) photomicrograph of L1 thin

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section showing porphyroclastic textures, orthopyroxene is surrounded by small olivine grains and euhedral-subhedral chromite; (p) euhedral chromite dispersed in olivine matrix

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has olivine inclusions. Chr-chromite, Ol-olivine, Opx-orthopyroxene.

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Fig. 3. Backscattered electron (BSE) images of chromitite samples. (a) Olivine inclusion in the chromite with serpentine cover (sample L1); (b) disperse clinopyroxene occurs along olivine

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and chromite rim or as inclusions in olivine (sample L2); (c) carbonate veins and hydrated mineral in massive chromitite L6; (d) hydrated minerals with spherical or negative crystal have

a

linear

distribution

in

massive

chromitite

L7.

Chr-chromite,

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shapes

Cpx-clinopyroxene, Ol-olivine, Serp-serpentine.

Fig. 4. Photomicrographs of olivine under plane-polarized light. (a) Nodular chromitites (L1). Round, tiny (generally <1 or 2 μm) and scattered primary micro-inclusions are pervasive in olivine. The brown or black ones are likely solid inclusions whereas the bright ones (pointed by yellow arrows) could be fluid inclusions. They are too small to determine their constitutions. The orientated needle-shape inclusions are likely to be clinopyroxene (Huang et al., 2014; Yamamoto et al., 2009). (b) Transitional chromitites (L5). Euhedral

ACCEPTED MANUSCRIPT chromite grains are common in olivine. Note the inflected trail of inclusions which could be secondary origin, derived from trapped fluids during the healing of a crack (Miura et al., 2011).

Fig. 5. Major element compositions of chromite and olivine. (a) Cr# versus Mg# of

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spinel/chromite in the harzburgites, dunites, and chromitites from Luobusa and Kangjinla. (b) FeO versus MgO of chromite. (c) Cr# versus TiO2 of spinel/chromite in the three

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lithologies (fields after Pagé and Barnes, 2009). (d) Al2O3 versus TiO2 of chromite from chromitites (fields after Kamenetsky et al., 2001). (e) NiO versus Fo of olivine. (f) FeO

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versus MgO of olivine. MORB, mid-oceanic ridge basalt; OIB, ocean island basalt; LIP, large igneous province basalt; ARC, arc-related volcanic rock; BON, boninite; Chr*,

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euhedral chromite inclusions in olivine; Ol*, small olivine inclusions in chromite. The grey triangles, asterisk and circles represent data of spinel/chromite or olivine in the

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Luobusa/Kangjinla harzburgites, dunites and chromitites in literature, respectively (Huang et al., 2014; Xiao et al., 2016; Xiong et al., 2015; Xu et al., 2011; Zhou et al., 1996; Zhou

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et al., 2014).

Fig. 6. (a) Helium concentration versus 3He/4He of chomite and olivine separates of different structure-type podiform chromitites from the Luobusa and Kangjinla district. Fields of

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MORB (8±1 Ra) and SCLM (6.1±0.9 Ra) are from Day et al. (2015). Data of carbonate-free Group I samples show relatively uniform and high proportion of mantle He.

4

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However, carbonate-bearing Group II samples have more radiogenic helium component. (b) He/40Ar versus 3He/40Ar. Group I samples lie on a binary mixing trajectory given by the

line (least squares fit to the data): 4He/40Ar = (3.67±0.22)×105 * 3He/40Ar + (0.03±0.006) (r2 = 0.90). Group II samples have higher 4He/40Ar for a given 3He/40Ar on the Group I samples correlation.

Fig. 7. (a) 40Ar/36Ar versus 3He/36Ar for all samples. Mixing line between air and MORB is shown (3He/36Ar =0.7 at

40

Ar/36Ar = 40000, Burnard et al., 1997; Moreira et al., 1998). (b) 3He

and 36Ar of nodular chromitites, showing a broadly positive correlation between these two

ACCEPTED MANUSCRIPT isotopes.

Fig. 8. 20Ne/36Ar versus 38Ar/36Ar of the chromitite samples. Red stars represent air and seawater

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values based on Ozima and Podosek (2002) and Kendrick et al. (2013), respectively.

ACCEPTED MANUSCRIPT

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CE

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D

MA

NU

SC

RI

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Figure 1

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AC

CE

PT E

D

MA

NU

SC

RI

PT

Figure 2

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CE

PT E

D

MA

NU

SC

RI

PT

Figure 3

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

NU

SC

RI

PT

Figure 4

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CE

PT E

D

MA

NU

SC

RI

PT

Figure 5

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

NU

SC

RI

PT

Figure 6

ACCEPTED MANUSCRIPT

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CE

PT E

D

MA

NU

SC

RI

PT

Figure 7

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CE

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MA

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SC

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Figure 8

ACCEPTED MANUSCRIPT Table 1 Major-element composition of chromite and associated minerals from the Luobusa and Kangjinla podiform chromitites K Samp

Min

Na

Z Ca

Si

M

M

Cr2

Al2

Fe

Ti

Ni

2 le

eral

2O

O

O2

gO

nO

O3

O3

O

O2

O

Nodular chromitite 14.

0.

59.

10.

15.

0.

0.

99.

67

79

5

01

0

2

10

13

44

15

0.0

0.

0.0

0.0

12.

0.

58.

7

01

0

0

35

17

86

0.0

0.

0.0

41.

52.

0.

0.0

3

01

3

83

90

09

0.0

0.

0.0

42.

54.

0.

4

01

2

16

11

0.0

0.

0.0

0.0

14.

1

01

1

0.0

0.

0.0

0.0

0

01

0

0.0

0.

0

10

6

.8

.7

9.9

RI

05

18.

0.

0.

0.

99.

60

79

3

08

19

07

09

8

.2

.9

0.0

5.3

0.

0.

0.

10

94

1

0

6

01

48

02

0.8

.6

0.1

0.0

3.4

0.

0.

0.

10

96

05

5

0

7

00

60

01

0.6

.5

0.

59.

10.

14.

0.

0.

0.

99.

68

78

11

27

72

63

19

10

02

4

.8

.8

13.

0.

58.

10.

16.

0.

0.

0.

99.

63

78

0

06

13

56

60

79

18

07

07

5

.2

.8

0.0

42.

53.

0.

0.0

0.0

4.2

0.

0.

0.

10

95

00

1

15

62

05

8

0

8

01

56

00

0.8

.7

0.2

0.

24.

54.

18.

0.

0.5

0.5

0.8

0.

0.

0.

10

97

1

00

71

89

10

03

5

2

9

06

06

03

0.1

.3

0.0

0.

0.0

0.0

15.

0.

59.

10.

13.

0.

0.

0.

99.

74

78

4

01

1

3

68

09

17

64

42

19

14

06

5

.5

.9

0.0

0.

0.0

42.

54.

0.

0.0

0.0

3.4

0.

0.

0.

10

96

1

00

7

16

12

05

2

0

7

01

58

01

0.5

.5

0.0

0.

0.0

0.0

14.

0.

59.

10.

14.

0.

0.

0.

99.

70

79

1

00

3

9

68

14

41

23

83

19

11

04

8

.1

.6

MA

D 35

CE

PT E

0

SC

18

AC

Ol

0.

35

Chr

*

#

0.0

Ol*

Chr

g#

0.0

Ol

L2

tal

0.

NU

*

Cr

0.0 Chr Chr

M

O

PT

O

L1

To n

Cpx

L4

Chr

Ol 07y-4 Chr 45

0.0

0.

0.0

42.

53.

0.

0.1

0.0

4.2

0.

0.

0.

10

95

0

01

3

07

77

07

1

0

3

01

52

02

0.8

.8

0.0

0.

0.0

41.

54.

0.

0.2

0.0

3.0

0.

0.

0.

10

96

0

00

2

82

05

04

5

0

7

00

69

03

0.0

.9

0.1

0.

25.

55.

18.

0.

0.1

0.3

0.8

0.

0.

0.

10

97

4

00

25

20

30

02

9

3

7

03

05

03

0.4

.4

0.0

0.

0.0

0.0

15.

0.

59.

10.

13.

1

01

3

3

39

11

18

81

50

PT

ACCEPTED MANUSCRIPT

0.0

0.

0.0

0.0

13.

0.

58.

10.

16.

0.

1

01

2

5

47

13

47

23

65

0.0

0.

0.0

41.

53.

0.

0.1

0.0

1

00

3

91

80

06

1

0.0

0.

0.0

42.

54.

0.

0

00

0

08

20

05

0.0

0.

0.0

0.0

15.

2

01

1

1

44

0.0

0.

0.0

42.

2

01

2

01

0.0

0.

0.0

1

00

0.0

Ol

Ol*

Cpx 07y-4

0.

0.

0.

99.

73

78

23

13

05

5

.2

.6

0.

0.

99.

65

79

22

09

04

4

.0

.3

3.6

0.

0.

0.

10

96

1

1

00

59

02

0.2

.4

0.3

0.0

2.7

0.

0.

0.

10

97

3

1

3

01

72

00

0.1

.3

0.

59.

10.

13.

0.

0.

0.

99.

73

78

09

50

87

43

21

13

04

8

.2

.6

0.

0.0

0.0

3.0

0.

0.

0.

10

97

34

04

2

0

0

01

71

02

0.2

.0

0.0

15.

0.

59.

10.

13.

0.

0.

0.

99.

74

78

1

2

53

09

41

75

08

21

15

07

3

.0

.8

0.

0.0

41.

54.

0.

0.1

0.0

2.8

0.

0.

0.

99.

97

1

01

2

88

04

05

3

0

8

01

65

02

7

.1

0.0

0.

0.0

0.0

15.

0.

57.

12.

13.

0.

0.

0.

99.

72

75

1

00

0

0

23

11

94

38

61

20

12

05

7

.0

.8

0.0

0.

0.0

42.

54.

0.

0.0

0.0

3.7

0.

0.

0.

10

96

0

01

2

34

25

05

3

0

5

00

62

01

1.1

.3

0.0

0.

0.0

0.0

15.

0.

59.

11.

13.

0.

0.

0.

10

73

Chr *

07y-4

MA

Ol*

07y-4 60

AC

Ol

CE

Chr

54.

PT E

Ol

D

Chr 59

NU

Ol

SC

48

RI

Chr

Nodular-massive chromitite L3

Chr

Ol L5

Chr

78

ACCEPTED MANUSCRIPT 1

01

1

2

70

10

76

09

53

22

13

06

0.6

.7

.3

0.0

0.

0.0

42.

54.

0.

0.0

0.0

3.4

0.

0.

0.

10

96

0

00

2

41

69

05

2

0

8

01

63

02

1.3

.6

0.0

0.

0.0

0.0

15.

0.

57.

12.

13.

0.

0.

0.

99.

73

75

1

00

1

1

67

09

74

65

21

19

12

05

8

.7

.4

0.0

0.

0.0

41.

53.

0.

0.0

0.0

3.8

0.

0.

0.

10

96

0

00

6

87

80

06

1

0

8

0.0

0.

0.0

0.0

15.

0.

59.

10.

13.

1

01

1

1

71

10

47

63

0.0

0.

0.0

42.

54.

0.

0.0

1

02

2

36

83

04

2

0.0

0.

0.0

0.0

15.

0.

59.

0

00

1

2

18

09

0.0

0.

0.0

0.0

14.

0.

1

01

1

0

91

0.0

0.

0.0

42.

54.

0

00

1

03

0.0

0.

0.0

41.

0

01

2

Ol

L8

Chr

0.3

64

20

12

05

0.0

.3

0.0

2.9

0.

0.

0.

10

97

0

4

00

65

00

0.9

.1

10.

13.

0.

0.

0.

99.

72

78

45

86

61

20

13

06

6

.2

.6

59.

10.

14.

0.

0.

0.

99.

71

78

10

07

80

26

21

12

08

6

.2

.6

0.

0.0

0.0

3.1

0.

0.

0.

10

96

05

1

0

1

01

74

02

0.2

.9

54.

0.

0.1

0.0

2.6

0.

0.

0.

10

97

90

51

03

7

0

3

01

86

02

0.2

.4

44

07y-4

Chr *

Ol*

L7

Chr

64

01

.1

0.

0.

0.

10

74

79 .0

0.0

0.

0.0

0.0

15.

0.

60.

10.

12.

0.

0.

0.

99.

74

78

1

01

1

0

62

10

08

87

79

19

17

05

9

.1

.8

0.0

0.

0.0

0.0

15.

0.

57.

12.

13.

0.

0.

0.

99.

73

75

1

01

0

0

67

08

51

79

50

16

15

04

9

.6

.1

0.0

0.

0.0

0.0

16.

0.

58.

13.

12.

0.

0.

0.

10

74

75

0

00

1

1

05

20

75

12

75

19

15

00

1.2

.4

.0

0.0

0.

0.0

0.2

16.

0.

61.

10.

12.

0.

0.

0.

10

77

79

4

01

2

2

68

20

07

30

18

21

20

00

1.1

.5

.9

0.0

0.

0.0

0.1

16.

0.

60.

10.

12.

0.

0.

0.

10

76

79

AC

L6

CE

Massive chromitite

17

PT E

Ol

D

54

MA

Chr

NU

Ol

SC

Chr

01

RI

07y-4

PT

Ol

Chr

LBS1 Chr 3-34 LBS1 Chr 3-39 LBS1

Chr

ACCEPTED MANUSCRIPT 3-58 LBS1

0

00

8

8

50

19

61

80

09

21

18

00

0.8

.9

.0

0.0

0.

0.0

0.0

16.

0.

60.

11.

12.

0.

0.

0.

10

77

78

1

00

0

1

53

20

80

00

00

23

20

00

1.0

.2

.8

0.0

0.

0.0

0.0

16.

0.

60.

10.

12.

0.

0.

0.

10

78

79

0

00

0

3

73

21

80

63

10

22

20

00

0.9

.1

.3

Chr 3-59 LBS1 Chr 3-60

PT

FeO is total iron, Mg# = 100Mg/(Mg+Fe2+), Fe2+ is calculated based on microprobe analyses assuming stoichiometry, Cr# = 100Cr/(Cr+Al).

RI

Nodular-massive chromitites mean the transitional type between nodular and massive chromitites. Chr* = euhedral chromite grains dispersed in the matrix or included in olivine, Ol* = small olivine

SC

inclusion in chromite.

Data of massive chromitites (except for L6 and L7) are from Xiao et al. (unpublished).

AC

CE

PT E

D

MA

NU

Mineral abbreviations: Chr=chromite, Ol=olivine, Cpx=clinopyroxene.

ACCEPTED MANUSCRIPT Table 2 He, Ne and Ar concentration and isotopic data of chromite and olivine separates from the Luobusa and Kangjinla chromitites by vacuum crushing method

H

g

R

E

e

20

E

21

Sa

36

N

A

H

e in

ht

/

rr

R

o

mpl er e al

(g

ac

r

(× 1

Ne

rr

Ne

/22

o

/22

Ne

r

Ne

0-

)

9

)

0

.

9

9.7

.

1

9

8

2

3

hr 6

0

0

/36

o

/36

o

N

e

Ar

r

Ar

2. 3

9

)

AC

.

0.0

3

0

30

30

2

8

9

2

13

33

7

1

5

6

*/

*/

*/

e

R

R

R

*d

a

a

a

(×

(0

1 09

)

3.

%

%

)

)

.

0.

6

17

0

2

9

2.

2.

7.

8

0

6

0

3

5

7

8

4

9

0.

.

0.

6

19

0

4

9

2.

2.

5.

6

0

2

0

3

5

2

9

2

6

0

2.

0 1

0

9.8

.

0.0

7

2

28

7

7

1.

0 %

0.

17

30

04

6

10

6

2

0.

.

2.

6

18

0

9

9

2.

2.

3.

8

0

0

0

0

1

4

8

6

6

1.

1.

7.

0 4

C

.1 )

2

9 L2

(1 (1

5

1 9

24

92

0

. 9

H

0

9.8

0 0.

R

6

2

8

0-

R

0

1

1

8

30

1

R

0

.

8

0.0

r

(×

4

0

CE

1.

Ol

rr

40

2

hr

Ar

NU 2.

D

5

0

E r

PT E

3. C

Ra

38

rr

MA

0

C

10

E

Ar

)

chromitite

L1

(×

40

e/2

-11

Nodular

L1

4

PT

ei

20

RI

4

SC

M

w

0

9

9.7

0

0.0

15

61

33

2

0.

0

0.

6

ACCEPTED MANUSCRIPT hr

4

8

.

0

5

.

9

0

0

3

9

0

29

0

5

4

17

.

1

9

8

9

7

9

0

0

0

1

5

1

0.

6

0 6 0

0

R

hr

1.

1 .

5

8

.

0.0

3

0

29

1 0

8

0. 9.8

3

39

32

7

8

28

5

19

0

3

9

1.

1.

4.

5

0

4

0

8

9

9

4

5

1

9

1.

1.

2.

0

6

7

2

6

0

0

0 5

5

.

PT

C

RI

2. L2

0

2

2 .

0

6

.

0.0

2

2

29

8 1

0

9.9

6

14

3 5

0

0 1.

2.

0 1

.

L4

6

1

hr

3 1

3

9.8

8

3

CE

2. R

hr

2. 1

AC 1

6

0.0

2

29

0 7

5

0 0 3 0

97

32

2

4

8

6

14

0.

.

0.

6

19

0

5

9

2.

2.

4.

0

0

4

0

1

3

4

9

0

3

9 0

0 9.7

.

0.0

3

0

29

4

0

0

18

4

1

.

3

25

6 4.

5

0

C

1 .

29

6

3

L4

.

PT E

C

20

MA

Ol

0

0.

NU

1.

D

1.

0

SC

0

64

33

2

2

22

5

2 6

0.

.

0.

6

19

0

2

9

2.

2.

4.

5

0

3

0

1

2

0

1

1

8

9

1.

1.

1.

0

3

4

5

9

1

9

6 2 0

0 07y

1.

1.

C -44

0

3

0. 9.8

.

0.0

7

0

30

5 0

5

1

5 .

hr 5

0

9

30

3

9

72

8 7

77

.

6 1.

2

18

0 9

9

0

5

0 1

ACCEPTED MANUSCRIPT 0 0 0.

1.

n 6

. Ol

5

3

.

n.d

n.

n.

32

.

d

.

d.

d.

2

1 1

9

n.d

3

4

0.

.

1.

6

18

0

8

9

1.

1.

1.

7

0

2

0

3

3

5

3

5

0

2 0

. 4 0 5

. 7

8

6

0

28

5 0

5

0.0

5

0.

3

8

n.d

.

n.d

n.

n.

31

.

d

.

d.

d.

5

6

8

2 8

0 1.

. 0

1

hr 9

0 2

6

0

C -45

0 8

.

9.

6

18

0

5

9

1.

1.

2.

8

5

0

7

7

0

5

7

0

0 1 0

0.

.

2.

6

18

0

9

9

0.

0.

1.

8

0

7

0

8

9

0

8

0

9

9

1.

1.

1.

0

1

1

2

6

7

4

9

1.

1.

1.

0

2

2

2

1

2

9

1.

1.

1.

2

2

3

4

5

1

4 0

0

9.8

3

5

0

2 0.

.

0.0

10

96

32

0

29

7

2

1

.

6 1.

6

18

0 3

9

0

5

0 1 0

1

0

1 0.

.

0

9.7

.

0.0

10

99

31

0

7

9

0

29

8

5

9

6

0

2

hr 9R

4

1.

AC

1.

0.

0

CE

6

07y

1

1.

C -45

.

PT E

07y

6

n

0 9

8

5

. 4

18

7

0 Ol

32

50

9

0.

11

SC

0 hr

.

NU

-44

9.8

RI

1.

MA

1. C

D

07y

0

PT

0

.

6 8.

2

18

0 7

1

8

0

5

0 1

0 07y

1. C

-46

0

0

2

2

4

0

1

.

8.

0. .

hr 0

1

1. 3

9.8

.

0.0

16

31

80 0

6

2

0

29

8 94

6

18

8

9 0

6

6 6

0

5

0 0

0

ACCEPTED MANUSCRIPT 4 0 0 07y

1. 2 7

1 0.

.

0

9.8

.

0.0

0

6

9

0

29

6

0

2

hr 0R

0

1.

C -46

1

14

32

50

3

73

.

6 0.

2

18

0

9

1.

1.

1.

0

2

2

3

8

9

7

7

8

9

0

5

5

PT

1 0

0 5 . Ol

4

5

.

n.d

n.

n.

32

.

d

.

d.

d.

1

8 1

9

0. n.d

9

2

6 2

.

0 1.

C 9

9.7

.

1

1

2

1

3

hr 8

2

3

0

R

hr

1. 1

6 7

30

0

0

5

1.

1.

1.

9

0

4

0

5

6

8

9

1

0

2

0.

.

0.

6

18

0

7

9

1.

1.

5

0

4

0

3

9

8

8

0

.

0.0

3

1

28

18

33

1

1

5

5

0

AC Ol

9

2

9.9

1

0.

.

0.

6

19

0

0

9

1.

8.

7

0

4

0

2

0

0

0

0 2 0

0 0.

29

8

1

6

1

0

.

4

0

4

CE

3. C

18

12

0

L3

0.0

D

L3

.

6

0

PT E

1.

0

MA

ive chromitite

3.

18

NU

Nodular-mass

.

RI

1.

SC

0.

n

0

1. .

6

10.

.

0.0

2

2

00

2

29

7

12

29

55

9

5

6

5 1

0.

.

2.

6

18

0

0

9

1.

1.

8

0

0

0

7

9

7

7

2.

2.

6 4

L5

C

1.

2.

0

1

9.8

0

0.0

30

48

29

5

0.

0

1.

6

4.

ACCEPTED MANUSCRIPT hr

7

1

.

4

7

8

1

8

3

.

28

6

5

18

.

4

9

1

2

0

4

0

6

0

9

9

8

0.

6

1

4

3

0 2 0

0

R

hr

5 1

2.

0. .

8

9.7

.

0.0

0

8

9

0

30

2

64

33

4

3

14

5

6

0

1

9

2.

2.

0.

6

0

2

0

2

4

3

5

2

3

2

2 .

7

3

0.0

2

2

25

0 1

2

.

3

6

3 4

3

0 L8

9

1. .

1

9.7

1

2

5

8

hr 6

0

3

0

1. L8

C

R

hr

4

0.0

0

31

0.

.

4.

6

19

0

7

9

1.

1.

2.

0

0

3

0

3

3

0

3

8

1

29

6

9

4 0

28

32

5

3

4

5

0.

.

0.

6

19

0

0

9

1.

4.

5

0

3

0

9

3

4

1

2 0

0

1.

.

1

9.7

.

0.0

2

2

3

1

26

8

AC

1

29

0

9

CE

1

.

PT E

3. C

31

MA

Ol

9.8

NU

1.

D

0.

0

SC

0

1

19

3 7

.

PT

C

RI

2. L5

0

26

31

5

7

4

5

0.

6

0

0

9

1.

4.

0

3

0

9

3

7

8

19

6

3

. 0.

4 2 0

0 1. Ol

0 8

0

1. .

8

9.8

.

0.0

1

4

6

2

29

9

95

29

0

8

9

0 9

6

0.

.

4.

6

1

18

0

1

9

1.

2.

0.

9

0

0

0

9

0

6

2

7

4

1 3

ACCEPTED MANUSCRIPT 0 07y

1. 0 6

0

.

2

9.7

.

0.0

0

5

6

0

29

6

0

2

hr 4

1

1.

C -44

0

32

30

93

2

38

6

0.

.

6.

6

18

0

0

9

1.

1.

1.

9

0

3

0

2

2

3

3

4

0

3 6

1 0 0.0

0

4

7

0

28

6

0

1. 1 1

.

6

9.9

.

0.0

0

4

3

0

28

0 7

9

4R

0 9

1

9.7

1

8

1.

AC

Ol

0 2

0

4

9

1.

1.

1.

3

0

2

2

3

7

8

5

RI

19

29

0

8

6

.

0.0

0

30

0.

.

4.

6

19

0

3

9

1.

1.

1

0

8

0

0

1

2

3

0

37

30

7

0

22

2

0.

.

4.

6

18

0

4

9

0.

0.

6.

9

0

6

0

9

9

1

2

9

4

1

0 9.8

.

0.0

7

0

30

9 0

1

0

1

1

.

2

1

0

0

1.

6

7

CE

8

8

PT E

. 9

5.

0

0.

hr

32

20

1

C -45

6

0

0 0.

4

.

0.

0

9

07y

60

6

1.

hr 4

30

7

C -45

35 32

0 07y

PT

.

SC

5

9.7

2

hr 4R

1

NU

1

.

1.

C -44

0

MA

1.

1

D

07y

0

17

29

38

8

11

2

9 7

0.

.

1.

6

18

0

5

9

1.

1.

1.

8

0

4

0

2

2

8

1

5

5

9 1

Massive chromitite C

2.

0.

0

3

9.8

0

0.0

L6

92

40

33 hr

4

2

.

0

1

.

30

0.

0

2.

6

0.

0.

0.

18

.

3

9

2

2

3

7 5

8

ACCEPTED MANUSCRIPT 2

7

0

7

1

2

9

2

0

3

0

7

8

5

0 3 0

0 C

R

hr

0.

1 .

2

2

.

0.0

0

0

30

7 0

8

9.8

1

11

42

63

0

15

6

0.

.

0.

6

19

0

1

9

0.

0.

0.

0

2

2

3

1

2

4

PT

2. L6

0

8

1

2

5

0

0

RI

1 0

2.

b

0.0

34

78

0

1

0

0

30

5

6

1

0 2

.

4

1

1

8

0

hr

0 1 0 2.

C hr

1

.

0

2

AC

L7 R#3

C

1

8

0

hr

n.d .

3

9

0

hr -34

0.

0.

0.

4

0

1

0

1

1

1

0

0

0

0.

0.

0.

1

1

1

0

0

0

0.

0.

0.

1

1

3

2

3

2

0.

0.

0.

1

1

1

0

0

4

0.

0.

0.

3

3

3

0

0

2

n

n 6

n.

.

n.

.

n. 9

d.

d

d.

d

d. 0

.

.

n

n

.

6 n.d

n.

n.

n.

.

n.

.

n.

d

9 .

d.

d.

d.

d

d.

d

d.

.

.

.

n

n

n

2 .

6 n.d

.

n.d

n.

n.

n.

.

n.

.

n.

5 0

9 .

d

.

d.

d.

d.

d

d.

d

d.

8

0 .

1

.

.

0

0 0.

. 5

9

1

0.

C S13

d.

8

0

0

0 1.

d.

n.

n

1 LB

.

n.

0

0

0.

1

d

n.d

.

1

2.

.

19

1

0

8

n.d

6

1

. 1

R#2

0.

2

CE

L7

n

2.

6

NU

0 0.

.

34

5

3

0.

SC

.

0

C R#1

9.9

MA

L7

7

1

hr 9

.

D

0

0

0.

C L7

2

PT E

1.

0

3

9.6

.

0.0

19

38

71 0

5

4

1

30

5 12

19

6

0

4

6

1

9

2

0

0 4

1

9. .

0

ACCEPTED MANUSCRIPT 3 0 LB

0 1.

S13

C

-34

hr

6 5

1

0

1

0.

0. .

3

9.7

.

0.0

0

0

5

1

31

1

0

2

19

38

40

0

67

6 0.

6

19

0

9

0.

0.

0.

0

2

2

2

7

7

9

3

7

R

.

5

0

2

0

PT

4 0

.

n.d

n.

n.

34

0

5

.

d

.

d.

d.

4

0

5

.

2

3 .

0 LB

1.

0.

C S13

8 0

.

7

n.d

.

0

1

.

d

8

hr -58

n

4

LB

1.

0.

C 5

8

hr 1

C S13

6

n.

31

.

d

.

d.

d.

2

n

-60

.

6

0

0

0

0

3

3

3

.

3.

6

0

6

9

0.

0.

9.

0

9

0

8

9

8

5

3

2

0.

.

9.

6

19

0

0

9

0.

0.

1.

1

0

4

0

8

8

1

1

3

4

9

0.

0.

1.

0

9

9

1

1

3

7

0 1 . n.d

.

n.d

n.

n.

31

.

d

.

d.

d.

2

0. 2

6 1.

0 19

3 7

1 0

.

8 4

a

R: Repeat crushing experiment.

b

#1: Step-crushing number.

c

R/Ra: Ratio of 3He/4He in sample (R) over 3He/4He of air (1.4×10-6).

d 4

2

19

2

1

1

0

4

0 0

0.

0

3

9

0.

0 n.

0

hr

0.

2

2

0

0.

9

4

n.d

.

AC 1.

6

0.

0

.

6

LB

d.

4

0 3

d.

n.d

CE

-59

.

31

2

.

S13

n.

0

n

PT E

0

.

n.

6

19

D

8

n.d

2.

0.

RI

9

n.d

0

hr -39

8

SC

6

.

0.

C S13

n

NU

0.

4

MA

LB

0

He* and R*/Ra: Radiogenic ingrowth estimate is calculated using equation (1) with assumed U,

ACCEPTED MANUSCRIPT Th contents and formation age to derive an 'age corrected 3He/4He' (R*). R*/Ra is shown at 0.1%, 1%, and 10% addition to illustrate the effect of efficient (10%) and inefficient (1% or less) addition of ingrown 4He to intrinsic 3He/4He in the crystals (after Day et al., 2015). Unit of concentrations is cm3 STP g-1. Errors of concentrations are generally within 10%.

AC

CE

PT E

D

MA

NU

SC

RI

PT

Chr=chromite, Ol=olivine, n.d.=not determined.