The hydrous properties of subcontinental lithospheric mantle: Constraints from water content and hydrogen isotope composition of phenocrysts from Cenozoic continental basalt in North China

Accepted Manuscript The hydrous properties of subcontinental lithospheric mantle: constraints from water content and hydrogen isotope composition of phenocrysts from Cenozoic continental basalt in North China Zheng Xu, Yong-Fei Zheng, Zi-Fu Zhao, Bing Gong PII: DOI: Reference:

S0016-7037(13)00721-7 http://dx.doi.org/10.1016/j.gca.2013.12.025 GCA 8603

To appear in:

Geochimica et Cosmochimica Acta

Please cite this article as: Xu, Z., Zheng, Y-F., Zhao, Z-F., Gong, B., The hydrous properties of subcontinental lithospheric mantle: constraints from water content and hydrogen isotope composition of phenocrysts from Cenozoic continental basalt in North China, Geochimica et Cosmochimica Acta (2013), doi: http://dx.doi.org/10.1016/j.gca. 2013.12.025

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The hydrous properties of subcontinental lithospheric mantle: constraints from water content and hydrogen isotope composition of phenocrysts from Cenozoic continental basalt in North China

Zheng Xu*, Yong-Fei Zheng, Zi-Fu Zhao, Bing Gong CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China

*

Corresponding author. E-mail: [email protected]

Abstract To investigate the hydrous properties of subcontinental lithospheric mantle, phenocrysts from Cenozoic continental basalt in North China were analyzed by FTIR for structural hydroxyl content and by TC/EA-MS for total water content and hydrogen isotope composition. The concentration of structural hydroxyl, while not detectable in olivine, ranges from 62 to 913 ppm in clinopyroxene phenocrysts. Both olivine and clinopyroxene phenocrysts exhibit high total water contents, indicating the presence of molecular water as nanoscale inclusions. The majority of clinopyroxene phenocrysts contain less total water than olivine phenocrysts, and all the clinopyroxene phenocrysts exhibit higher δD values than olivine phenocrysts, suggesting that the molecular water in olivine is rather D-poor. The phenocryst minerals exhibit δD values of -145 to -94‰ that are negatively correlated with their total water contents. The initial basaltic melt is inferred to contain 0.6 wt% total water, which increases to 3.5-6.7 wt% due to fractional crystallization, rendering the melt water-saturated in the late stage of its evolution. Also during this process, structural hydroxyl in the melt is partly converted to molecular water, leading to the depletion of D in the phenocrysts and the systematic variation of structural hydroxyl content with the Mg number of clinopyroxene. The mantle source of the basaltic melt is inferred to have a structural hydroxyl content of 290 ppm, which is slightly higher than that of MORB-type mantle but similar to that of OIB-type mantle. Thus, we suggest that the subcontinental lithospheric mantle of North China is rather hydrous and D-depleted, probably due to the entrainment of recycling subducted crustal material.

Keywords: Structural hydroxyl; molecular water; hydrogen isotopes; mineral phenocrysts; continental basalt; slab-mantle interaction

1. Introduction Water plays a crucial role in subduction-zone magmatism (Grove et al., 2012; and references therein). Addition of water to the mantle wedge not only brings fluid-mobile incompatible trace elements into the mantle wedge, but also facilitates melting by reducing mantle solidus (Hirose and Kawamoto, 1995; Hirth and Kohlstedt, 1996). The distribution of water in the mantle is highly heterogeneous, varying from 180-540 ppm in subarc mantle (Hirschmann, 2006) to only tens of ppm in subcontinent lithospheric mantle (e.g., Grant et al., 2007; Li et al., 2008; Bonadiman et al., 2009; Peslier, 2010; Xia et al., 2010). In the lack of direct samples of the mantle, water content of different types of mantle sources can also be inferred from their derivatives, the basalts produced from partial melting. Studies on oceanic arc basalts (Benjamin et al., 2007; Wade et al., 2008; Kelley et al., 2010; Parman et al., 2011) have revealed that the subarc mantle has been considerably hydrated by subducted crustal material (hydrothermally altered basalts and sediments), but little is known for continental basalts (Xia et al., 2013a). Nominally anhydrous minerals (NAMs) such as clinopyroxene (Cpx) and olivine (Ol) often occur as phenocrysts in oceanic and continental basalts. They are major constituent minerals in the mantle as well as the crust. Mafic to felsic NAMs can incorporate significant amounts of water, not only in the form of structural hydroxyl (OH) in crystal lattices (Bell and Rossman, 1992; Hirschmann et al., 2005; Peslier, 2010) but also in the form of molecular water (H2O) in crystal defects (e.g., Su et al., 2002; Wu et al., 2004; Meng et al., 2009; Kawamoto et al., 2013). Mantle-derived NAMs may contain structural hydroxyls of 100 to 1000 ppm when normalized to H2O (Hirschmann et al., 2005; Peslier, 2010; Xia et al., 2013a). The partition coefficient of water between Cpx and basaltic melt is generally >0.01 (Aubaud et al., 2004; Hauri et al., 2006), implying that >100-1000 ppm structural water in Cpx is in equilibrium with a basaltic melt containing 1-10 wt% H2O. In silicate melts, both structural hydroxyl and molecular water are present as dissolved species and can be readily interconverted (e.g., Zhang and Stolper, 1991; Zhang and Behrens, 2000; Yamashita et al., 2008). To investigate the water budget of basalt and the property of its mantle source, it is indispensable to quantify the concentration of total water (structural hydroxyl + molecular water) and water speciation in both NAMs and melts. Hydrogen isotope analysis is a useful means for tracing the evolution of subarc mantle, particularly in recognizing the contribution from subducted oceanic crust. While stable isotope fractionation is small at mantle melting (Shaw et al., 2008; Bindeman et al., 2012), it is significant during the dehydration of crustal rocks as they are subducted to mantle depths. The released water is enriched in deuterium relative to the dehydrated rocks because of negative hydrogen isotope fractionations between minerals and water at thermodynamic equilibrium (e.g., Vennemann and O’Neil, 1996; Chacko et al., 1999; Xu and Zheng, 1999). As a consequence, the mantle source is

usually enriched in deuterium with the alteration by aqueous fluid derived from the dehydration of subducting oceanic crust, but is depleted in deuterium with the metasomatism by hydrous melt derived from partial melting of the crustal rocks. Such a contrast in the hydrogen isotope composition of mantle sources may be inherited by their derivatives, oceanic and continental basalts (Poreda, 1985; Hochstaedter et al., 1990; Hauri, 2002; Kingsley et al., 2002; Shaw et al., 2008, 2012). While there have been many studies on the water content and hydrogen isotope composition of NAMs from xenolith peridotites enclosed by continental basalts, much less attention has been paid to phenocryst NAMs in continental basalts themselves. Phenocrysts in continental basalts are generally crystallized from the melts during their ascent from mantle sources to crustal levels prior to eruption. Hardly altered by meteoric-hydrothermal fluid on the surface nor contaminated by crustal material, they provide valuable information about the geochemical composition of basaltic melts and their mantle sources. In this contribution, we report structural and total water contents as well as hydrogen isotope composition of clinopyroxene and olivine phenocrysts in Cenozoic continental basalts from North China. In combination with previous geochemical studies of major-trace elements and stable-radiogenic isotopes, our results provide insights into the hydrous properties of subcontinental lithospheric mantle (SCLM) and the slab-mantle interaction in oceanic subduction factory.

2. Geological setting and samples Cenozoic volcanic rocks are widespread along the coastal provinces and adjacent offshore shelf extending over 4000 km from north to south along the eastern edge of China continent. The predominant lithology of these volcanic rocks is alkali basalt, with minor tholeiite (Zhou and Armstrong, 1982; Zou et al., 2000). These continental basalts erupted throughout the Cenozoic, with the majority occurring in Neogene to Pleistocene (Chi, 1988). They are primarily distributed along regional faults such as the Tan-Lu fault (Fig. 1). Mantle and crustal xenoliths are common in these Cenozoic basalts, with occasional occurrence of megacrysts.

The geochemistry of Cenozoic continental basalts has been studied extensively (e.g., Xu et al., 2005, 2012; Tang et al., 2006; Liu et al., 2008a; Chen et al., 2009; Zhang et al., 2009; Zeng et al., 2010, 2011; Wang et al., 2011). Most of them exhibit not only variable degrees of depletion in radiogenic isotopes relative to bulk silicate earth, but also OIB-like trace element distribution patterns on the primitive mantle-normalized spidergram. Therefore, these basalts are not likely to be derived from partial melting of the depleted MORB mantle (i.e., the normal asthenospheric mantle). Crustal contamination during magma ascent through the continental crust is ruled out based on the

decoupling between major-trace elements and radiogenic isotopes. Therefore, the basalts must be derived from some special mantle sources that are enriched in melt-mobile incompatible trace elements, such as LILE and LREE. This kind of mantle sources may again have been developed from the metasomatism by crustally derived melts (e.g., Zhang et al., 2009; Wang et al., 2011; Xu et al., 2012). Thus, understanding the origin of crustal melts and the nature of mantle sources will shed light on the geodynamic mechanism for the destruction of the North China Craton (Gao et al., 2004; Menzies et al., 2007; Zheng and Wu, 2009; Zhu et al., 2012). Twenty-one basalt samples were collected from the southeastern part of the North China Craton, with thirteen from the Changle-Linqu area in western Shandong province and the rest from the Hefei basin in Anhui province (Fig. 1). These basalts are generally porphyritic, containing olivine, clinopyroxene and plagioclase phenocrysts. Olivine phenocrysts, subhedral to anhedral, are chemically different from those in the peridotite xenoliths enclosed by Cenozoic basalts from the same areas, especially in terms of MgO and CaO contents. Discrete clinopyroxene phenocrysts are subhedral and slightly smaller than olivine. Plagioclase phenocrysts also occur in a few samples (e.g., 07CL10 and 07CL12). They are tabular, with small olivine and clinopyroxene grains filling the space between them, and exhibit polysynthetic twinning. The matrix looks glassy, suggesting rapid solidification of basaltic melt, but observations under an optical microscopy reveal that it is actually composed of assemblage of olivine, clinopyroxene and plagioclase microcrystals.

3. Analytical methods 3.1 Mineral major and trace elements The major element compositions of clinopyroxene phenocrysts were measured by EPMA-760 electron microprobe (EMP) at CAS Key Laboratory of Crust-Mantle Materials and Environments in University of Science and Technology of China, Hefei. Acceleration voltage and probe diameter were 15 kV and 1 μm, respectively. Peak and background counting times for each element were 10 s and 5 s, respectively. Analytical errors for major elements are better than ±2.5%. Trace element analysis was conducted by LA-ICP-MS at State Key Laboratory of Geological Processes and Mineral Resources in China University of Geosciences, Wuhan. Instrument setup and data reduction are similar to those described by Liu et al. (2008b). Laser sampling was performed using GeoLas 2005 with a 44μm spot size. An Agilent 7700x ICP-MS was used to measure ion-signal intensities. For each analysis, a background acquisition of approximately 20-30s (gas blank) was followed by 50s data acquisition on the sample. Element contents were calibrated against multiple-reference materials (BCR-2G, BIR-1G, BHVO-2G and NIST610) exempt of applying an internal standardization (Liu et al., 2008b). Off-line selection and integration of background and sample signals, and time-drift correction and quantitative calibration were

performed by ICPMSDataCal 8.4 (Liu et al., 2008b, 2010). Analytical precisions (1σ) for most elements are better than ±5%.

3.2 Structural hydroxyl contents The concentration of water in the form of structural hydroxyl (OH) was measured on double-polished thin sections of phenocrysts by Nicolet 5700 Fourier Transform Infrared Spectrometry (FTIR) at the CAS Key Laboratory of Crust-Mantle Materials and Environments in University of Science and Technology of China, Hefei. The measurement utilized a KBr beamsplitter, a liquid N2 cooled MCT-A detector, and an unpolarized infrared beam is ~15 µm in diameter. Detailed analytical procedures are described in Xia et al. (2010, 2013a). For each analysis, 512 scans at a resolution of 4 cm-1 were accumulated on a clean spot free of fluid or mineral inclusion; a background spectrum was collected after 3-4 sample analyses. Linear absorbances of OH bands (peak height) relative to a tangential baseline (Fig. 3) are always lower than 0.22, fulfilling the criterion for Beer-Lambert law based unpolarized analysis (Kovács et al., 2008). Hydroxyl concentration was hence quantified by c = A/(I×t), where c, A, I and t denote the OH content (in ppm H2O), the integrated absorbance (in cm-1) of OH band, the integral absorption coefficient (in ppm-1 cm-2) and the sample thickness (in cm), respectively. Note that the integrated area between 3000-3800 cm-1 and the tangential baseline needs to be multiplied by a factor of 3 to obtain the values of A (Kovács et al., 2008; Sambridge et al., 2008). The chosen values of I were 5.32 ppm-1 cm-2 for olivine (Bell et al., 2003) and 7.09 ppm-1 cm-2 for clinopyroxene (Bell et al., 1995). According to Xia et al. (2013a), the analytical uncertainty of our unpolarized FTIR analyses is less than ±27%.

3.3 Total water contents and hydrogen isotope ratios Total water contents and hydrogen isotope ratios were measured on a MAT-253 mass spectrometer using a thermal combustion/elemental analyzer – mass spectrometer (TC/EA-MS) online technique at the CAS Key Laboratory of Crust-Mantle Materials and Environments in University of Science and Technology of China, Hefei. Detailed analytical procedures are available in Gong et al. (2007a, b). About 4-10 mg olivine and clinopyroxene grains of 40-60 µm size were coated by silver foil and preheated at 90°C for 12 h in low vacuum to remove water absorbed on mineral surfaces. Then the sample was placed in a ceramic tube filled with coarse glassy carbon, inserted into a furnace and heated under high vacuum at 1450°C. The water released from the sample reacted with carbon to produce H2 and CO, which were carried by a continuous flow of He and separated by gas chromatography. Finally, H2 was directed to the mass spectrometer for measuring H contents and hydrogen isotope ratios. Benzoic acid (C7H6O2) with 5 wt% H and

NBS-30 biotite (δD = -65.7‰) and IAEA-CH-7 polyethylene (δD = -100.3‰) were used as the standards. Garnet 04BXL02, with a total water content of 522 ± 11 ppm and a δD value of -93 ± 2‰ (Gong et al., 2007b), was also used as a laboratory standard. Analytical errors for δD values and water contents are generally better than ±1‰ and ±10% (1 σ), respectively.

4. Results 4.1. Chemical composition of mineral phenocrysts The major element compositions of Cpx phenocrysts in the alkali basalts from Changle-Linqu are presented in Tables 1 and 2. Most of them are diopside (En = 34-41, Fs = 11-16, Wo = 47-52), with SiO2 contents of 44 to 50 wt% and Mg# values of 66 to 83. They have high contents of TiO2 (1.17-4.45 wt%) and moderate Na2O (0.27-0.62 wt%). The major element composition of Ol phenocrysts was already reported in Wang et al. (2011) and Xu et al. (2012), showing a large variation of Mg# values from 55 to 88. This suggests that many of them were crystallized from evolved basaltic melts rather than from primitive melts. The Cpx phenocrysts exhibit spoon-shaped REE patterns with enrichment of MREE relative to both LREE and HREE (Fig. 2a). On the other hand, the Cpx phenocrysts show depletion of many melt-mobile incompatible trace elements relative to REE on the spidergram (Fig. 2b), with significant negative anomalies of Ba, K and Pb but no negative anomalies of Nb and Ta.

4.2. Structural hydroxyl content of phenocrysts Water incorporated into the crystal structure of olivine accounts for absorption peaks at 3572 cm-1 and 3525 cm-1 in FTIR spectra (Berry et al., 2005; Demouchy et al., 2006; Peslier and Luhr, 2006; Grant et al., 2007; Li et al., 2008). These peaks are absent for the Ol phenocrysts from Cenozoic continental basalts, indicating that the structural hydroxyl is below detection limit (<1 ppm). Among the known OH absorption bands between 3000 and 3800 cm-1 in Cpx, those at 3610-3630 cm-1 are detected for the Cpx phenocrysts from the continental basalts (Fig. 3a-c), whereas those at 3530-3540 cm-1 are ambiguous. The obtained structural hydroxyl contents range from 62 to 913 ppm (Table 1). Some analyses on Cpx yield absorption bands at 3640-3700 cm-1 (Fig. 3d), which are probably due to the presence of Amphibole (Amp) inclusions in Cpx; these Cpx phenocrysts are not used for calculating structural hydroxyl contents. < Figure 3>

4.3. Total water contents and δD values of phenocrysts The total water contents of phenocryst minerals, as determined by TC/EA-MS, are 1224 to 2808 ppm for olivines and 1215 to 1854 ppm for clinopyroxene (Table 4). When coexisting Ol and Cpx phenocrysts have been analyzed, the Ol phenocrysts show higher total water contents than Cpx, with the differences ranging from 53 to 1112 ppm. There is a positive correlation between total water content in Ol and that in Cpx, with a different slope for alkali basalts and trachybasalts (Fig. 4a).

Olivine phenocrysts have δD values of -145 to -104‰ and Cpx phenocrysts have δD values of -108 to -94‰ (Table 4). When Cpx and Ol phenocrysts coexist, Cpx show higher δD values than olivine, with the differences ranging from ~5 to ~11‰ (Fig. 4b). The majority of δD values fall in the range of -120 to -100‰, much lower than the common δD values of -80 to -60‰ for the normal mantle.

5. The hydrous properties of subcontinental lithospheric mantle The spoon-shaped REE patterns are evident for the Cpx phenocrysts in the continental basalts (Fig. 2a). This suggests that their mantle sources have already experienced two sequential processes: (1) metasomatism by a crustal component lead to enrichment of LREE and MREE relative to HREE; (2) early melt extraction before the continental basaltic magmatism resulted in depletion of LREE relative to MREE and the negative anomalies of Ba, K and Pb (Fig. 2b). In contrast, radiogenic isotope compositions should not change during partial melting and fractional crystallization. In this regard, the mantle sources of Cenozoic continental basalts in eastern China are substantially of asthenospheric origin in terms of low (87Sr/86Sr)i ratios and positive εNd(t) values (e.g., Zhou and Armstrong, 1982; Zou et al., 2000). Thus, the mantle sources were originally transformed from normal MORB-type mantle (Zhang et al., 2009; Zheng and Wu, 2009). Based on the nature of the crustal component in the mantle sources, the metasomatic agent was suggested to be the hydrous melt derived from partial melting of subducting oceanic crust (Zhang et al., 2009; Wang et al., 2011; Xu et al., 2012). If this interpretation is correct, metasomatism via melt-peridotite reaction is expected to occur at the slab-mantle interface in oceanic subduction channel, a shear zone between subducting oceanic slab and the overlying mantle wedge (Zheng, 2012). The new mantle being produced will have a fertile lithology (pyroxene-rich), and be enriched in melt-mobile incompatible trace elements (including water) and depleted in radiogenic isotopes. Therefore, the validity of the above geodynamic model can be tested by the hydrous properties of mantle sources for the continental basalts.

5.1 Structural hydroxyl in basaltic melt In this study, FTIR analysis does not detect structural hydroxyl in Ol phenocrysts, regardless of their Mg# values. There is also no FTIR peaks associated with the molecular water and hydrous mineral in the Ol phenocrysts, suggesting the absence of either fluid or hydrous mineral inclusions at micron scale in the scanned area. Structural hydroxyl was also not detected in Ol from xenolith peridotites enclosed by Cenozoic continental basalts from North China Craton (Xia et al., 2010, 2013b). However, significant amounts of structural hydroxyl have been reported in Ol from mantle-derived samples elsewhere across the world. These include 0.5-7.2 ppm in phenocrysts in mid-ocean ridge basalts from the North Atlantic Volcanic Province (Jamtveit et al., 2001), 0.5-2.1 ppm in phenocrysts from island arc basalts from Cyprus and Kamchatka (Matveev et al., 2005), 0-40 ppm in olivines from peridotite xenoliths in the South China Block (Yu et al., 2011), and 0-54 ppm in olivines from peridotite xenoliths from other localities (Demouchy et al., 2006; Peslier and Luhr, 2006; Grant et al., 2007; Li et al., 2008; Bonadiman et al., 2009). The absence of detectable OH in phenocryst and xenolith olivines is usually ascribed to water loss during basalt eruption. For the extremely low content of structural hydroxyl in the xenolith olivines from the North China Craton, however, Xia et al. (2010) ascribed it to water loss from the mantle source due to heating by the convective asthenospheric mantle. In contrast to olivine, the Cpx phenocrysts analyzed in this study contain structural hydroxyl of 62 to 913 ppm (Table 1). These values are similar to the 500±250 ppm for Cpx in MORB-type mantle but lower than the >1000 ppm for Cpx in OIB-type mantle (Hirth and Kohlstedt, 1996; Dixon et al., 2004; Peslier et al., 2010). Furthermore, they are lower than the 336 to 957 ppm for Cpx in xenolith peridotites from craton mantle (Bell and Rossman, 1992; Grant et al., 2007; Li et al., 2008), but higher than the 5 to 550 ppm for Cpx from xenolith peridotites in cratonic mantle (Peslier et al., 2002; Peslier and Luhr, 2006; Demouchy et al., 2006; Grant et al., 2007; Li et al., 2008; Bonadiman et al., 2009; Peslier et al., 2012). The structural hydroxyl contents of Cpx phenocrysts from our continental basalts do not demonstrate a simple correlation with Mg# values (Fig. 5). Nevertheless, the data can be categorized into several groups. Clinopyroxenes that have Mg# values between 75 and 78 show an increase of structural hydroxyl content from 66 ppm at Mg# of 78 to 797-823 ppm at Mg# around 75. This is attributed to fractional crystallization because progressive crystallization of phenocryst NAMs renders the residual melt, and the Cpx phenocrysts crystallized from it, progressively more enriched in water. Clinopyroxenes with Mg# <75 and structural hydroxyl >600 ppm do not show a significant variation in structural hydroxyl content, suggesting water saturation in the melt. The data

with Mg# <75 and structural hydroxyl <600 ppm can be ascribed firstly to magma degassing (array B-D in Fig. 5) and then to the effect of fractional crystallization (array D-E in Fig. 5).

Using the initial Cpx structural hydroxyl content of 66 ppm, the highest Mg# for Cpx and a water partition coefficient of 0.017 between Cpx and melt (DCpx/melt) according to O’Leary et al. (2010), we estimate that the initial basaltic melt may contain 0.5 wt% water. By contrast, the water-saturated melt is inferred to contain 3.5-6.7 wt%, based on the highest Cpx structural hydroxyl contents of 760-913 ppm and the Mg# dependent DCpx/melt values of 0.014-0.023. Experimental studies have found that the solubility of structural hydroxyl in olivine is much lower than that in clinopyroxene, with typical DCpx/Ol values ranging from 9-22 (Aubaud et al., 2004; Hirschmann et al., 2005; Grant et al., 2007; Tenner et al., 2009; Kovács et al., 2012). Assuming that structural hydroxyl contents of 66 to 129 ppm in the high Mg# Cpx represent the primary value, Ol in equilibrium with Cpx would contain ~3 ppm to ~14 ppm structural hydroxyl, much higher than the <1 ppm concentration in coexisting Ol phenocrysts as determined by FTIR. This discrepancy suggests disequilibrium partitioning of water between coexisting Cpx and Ol phenocrysts. The extremely low content of structural hydroxyl in the Ol phenocrysts could be explained by magma degassing prior to eruption to the surface (e.g., Demouchy et al., 2006; Peslier and Luhr, 2006; Peslier et al., 2008). Experimental studies showed that hydrogen diffusion in olivine is faster than in clinopyroxene at high temperatures (Hercule and Ingrin, 1999; Ingrin and Skogby, 2000; Woods et al., 2000; Demouchy and Mackwell, 2003, 2006), which could result in preferential water loss from Ol phenocrysts relative to Cpx phenocrysts. However, the melt is much more susceptible to losing water than the phenocryst NAMs during magma ascent. The extremely low contents of structural hydroxyl in the Ol phenocrysts are more likely due to crystallization from water-poor basaltic melts. Therefore, the coexisting Cpx and Ol phenocrysts are probably not in thermodynamic equilibrium in terms of structural hydroxyl contents; otherwise, water partition coefficient would have to be lower than ~0.0001 between Ol and melt but higher than ~100 between Cpx and Ol. Hence, we argue that earlier melt extractions may have dehydrated the mantle source, leading to the production of water-poor primary basalts. On the other hand, the variable structural hydroxyl contents of 66 to 913 ppm for the Cpx phenocrysts suggest that most of them were crystallized relatively later from basaltic melts with variable water contents.

5.2 Total water in basaltic melt The TC/EA-MS analyses yield much more total water than the structural hydroxyl in the Ol and Cpx phenocrysts (Table 4). Because fluid and hydrous mineral inclusions were neither observed under optical microscope nor detected in FTIR analyses of Ol phenocrysts, this difference suggests

that there is significant amount of molecular water in nano-scale fluid inclusions in the Ol phenocrysts. On the other hand, Amp inclusions were detected in some Cpx phenocrysts by FTIR analyses (Fig. 3b). Thus it appears that Amp crystallized earlier than Cpx from the melt. The Amp-bearing Cpx phenocrysts exhibit larger variations not only in Mg# values but also in CaO and Al2O3 contents than those of the Amp-absent Cpx phenocrysts (Tables 1 and 2; Fig. 6). This implies that the Amp-bearing Cpx phenocrysts crystallized throughout the evolution process during the ascent of basaltic melts. These melts would be locally oversaturated with magmatic water prior to the crystallization of Amp-bearing Cpx phenocrysts. When Amp inclusions are present, they may contribute to the total water in Cpx phenocrysts. Even for those Cpx phenocrysts that do not contain Amp inclusions, an extrinsic contribution (i.e., molecular water from nanoscale fluid inclusions) cannot be precluded.

Molecular water has higher diffusivity than structural hydroxyl in silicate melts (Zhang and Behrens, 1991; Behrens and Nowak, 1997; Zhang, 1999; Zhang and Behrens, 2000; Okumura and Nakashima, 2006; Ni et al., 2009) and minerals (Wang et al., 1996; Gong et al., 2007b; Chen et al., 2007, 2011). Furthermore, structural hydroxyl and molecular water in silicate melts may be interconverted with changing P-T conditions and total water content (Zhang, 1999; Mysen and Fogel, 2010; Ni et al., 2013). In general, the proportion of structural hydroxyl increases with pressure (Hui et al., 2008). As magma rises to the surface, hydroxyl will be partly transformed to molecular water, which contributes to promoting water saturation of the melt.

5.3 The hydrous property of SCLM sources Assuming a water content of 0.5 wt% for the initial melt, a water content of 290 ppm can be inferred for the mantle sources of our continental basalts based on the following equation: Csource = Cmelt×[D + F×(1 − D)], where F = 0.05 and Dmantle/melt = 0.010 (Dixon et al., 1988). In view of pyroxene-rich mantle sources as suggested by Fe/Mn and Zn/Fe ratios for these basalts as well as by the compositions of their olivine phenocrysts (Xu et al., 2012), the mantle sources even have the potential to contain more water because DCpx/melt is much higher than DOl/melt (Aubaud et al., 2004, 2008; Grant et al., 2007). The inferred water content of the mantle sources is reasonable in recognition of that it is slightly higher than the ~200 ppm as the upper limit of MORB-type mantle (Dixon et al., 1988, 2002; Michael, 1988; Jambon, 1994; Danyushevshy et al., 2000; Saal et al., 2002; Simons et al., 2002) and close to the lower limit of 300-1000 ppm for OIB-type mantle (Dixon et al., 1997, 2002; Jamtveit et al., 2001; Hauri, 2002; Nichols et al., 2002; Seaman et al., 2004; Tenner et al., 2012). The trend in water content between the three types of mantle sources is

also consistent with their fertile property in lithochemistry (for example, high CaO, Al2O3 and low MgO). In order to constrain the water content of SCLM in the North China Craton, a number of studies have analyzed the structural hydroxyl contents of NAMs from xenolith peridotites enclosed by Cenozoic continental basalts. Firstly, Aubaud et al. (2007) reported low average water contents of 91 ppm and 147 ppm for xenolith Cpx and orthopyroxene (Opx) from Damaping and Hannuoba, respectively. Similar results were also obtained for lherzolite xenoliths enclosed by Cenozoic continental basalts from Hannuoba, Beishan, Fanshi, Nushan, Fangshan, Penglai, Qixia and Changle (Yang et al., 2008; Xia et al., 2010, 2013b), with Cpx water contents of 27 to 255 ppm. These water contents are systematically lower than those of 62 to 913 ppm for Cpx phenocrysts from the continental basalts measured in the present study (Table 1). Nevertheless, the measured Opx water contents and estimated whole-rock water contents of lherzolite xenoliths from these areas are similar to those of refractory harzburgite xenoliths from Hebi in the North China Craton. Thus, Xia et al. (2010, 2013b) suggested that these lherzolite and harzburgite xenoliths are relics of the old SCLM in the North China Craton after thinning rather than the newly accreted SCLM from the normal asthenospheric mantle. A similar conclusion was also reached by studies of trace elements and Re-Os isotopes in these mantle xenoliths (Gao et al., 2002; Xu and Bodinier, 2004). Hao et al. (2012) studied the structural hydroxyl contents of Cpx in xenolith lherzolites from Beiyan and Changle. Primary water contents of 450-557 ppm were found for Cpx, which fall in the range of 500±250 ppm for Cpx from the MORB-type mantle. These xenoliths were suggested to represent young mantle accreted from the normal asthenosphere after the thinning of SCLM in the North China Craton. Trace element, Sr-Nd and Re-Os isotope studies also indicate that these xenoliths were derived from the isotopically depleted MORB-type mantle sources that were affected by crustal metasomatism (Xiao et al., 2010; Xiao and Zhang, 2011). In summary, the low water contents of xenolith peridotites suggest water-poor mantle sources for the Cenozoic continental basalts in the North China Craton. This can be ascribed to the effect of melt extraction on the hydrous properties of SCLM sources. This is because the melt extraction is a very effective mechanism to deplete the mantle sources in water because water is much more compatible in melt than in crystalline minerals. In this regard, the very low contents of structural hydroxyl in phenocryst and xenolith olivines from the North China Craton can be ascribed to their crystallization from water-poor mantle sources. Nevertheless, the Mesozoic SCLM would be originally hydrous due to the crustal metasomatism by hydrous melt derived from partial melting of the subducting paleo-Pacific oceanic crust (Zhang et al., 2009; Zheng and Wu, 2009). Whole-rock element and isotope studies have demonstrated that the mantle sources of these Cenozoic continental basalts were generated through reaction of the juvenile SCLM-wedge peridotite with

felsic melts derived from the subducting oceanic crust (Wang et al., 2011; Xu et al., 2012). In this regard, the water of mantle sources for these basalts is also binary in origin.

6. Hydrogen isotope fractionation during subduction-zone magmatism 6.1 Hydrogen isotopes in basaltic melt The phenocryst NAMs have δD values of -145 to -94‰ (Table 4), which are lower than those of -72±10‰ for fresh MORB (Shilobreeva et al., 2011). Meteoric-hydrothermal alteration during or after the eruption of basaltic magmas could be a potential mechanism to cause the D depletion. High-latitude meteoric water is normally characterized by low δD values of < -100‰ (Hoefs, 2009). However, the Cenozoic continental basalts used in this study were collected from the southeastern part of the North China Craton at middle latitudes of 32-37°N, where the meteoric water exhibits mantle-like δD values of -80 to -60‰ (Zhang et al., 1995). Thus, the low δD values < -100‰ for the phenocryst NAMs cannot be ascribed to meteoric-hydrothermal alteration. The phenocryst NAMs from the continental basalts have δ18O values of 4.1 to 6.6‰ (Wang et al., 2011; Xu et al., 2012). As illustrated in Fig 7a, there is no correlation between total water content and δ18O value for these phenocryst NAMs. If the water contents were promoted by surface water-rock interaction at low temperatures during or after the eruption, there should be a significant amount of high δ18O hydrous mineral inclusions, which are not observed. Thus, once phenocryst minerals have crystallized from basaltic melts, they are unlikely to lose water by diffusion or gain water by hydrothermal alteration. Therefore, we suggest that the high water contents of phenocryst NAMs are primary in origin.

The low δD values of -145 to -94‰ for the phenocryst NAMs fall within the δD range of minerals from xenolith peridotites enclosed by Cenozoic continental basalts in the North China Craton (Zhang et al., 2007) as well as Loihi and Koolau melt inclusions in phenocryst NAMs from Hawiian OIB (Hauri et al., 2002). As illustrated in Fig 7b, there is generally a negatively correlated trend between total water contents and δD values for olivines, but no systematic δD variation between the different basalt compositions. The negative trend can be explained by water species transformation during fractional crystallization, by which molecular water depleted in D becomes increasingly concentrated in evolving basaltic melts. This mechanism is different from magma degassing that results in simultaneous decreases of water contents and δD values in melt (e.g., Kyser and O'Neil, 1984; Taylor and Sheppard, 1986; Hauri et al., 2002; Kingsley et al., 2002). Because molecular water is depleted in D relative to structural hydroxyl (Chen et al., 2007; Gong et al., 2007b), decreasing molecular water in melt may result in an increase of melt δD values but a decrease of water content in the melt of the late stage of fractional crystallization. This accounts for

the negative correlation between total water contents and δD values for the NAMs. For coexisting Cpx and Ol phenocrysts, the majority of Cpx exhibits lower total water contents than Ol (Fig. 4a), but Cpx always has higher δD values than Ol (Fig. 4b). This is consistent with the predominance of D-poor molecular water in the Ol phenocrysts. As such, the highest δD values of -108 to -94‰ may provide a proxy for the initial δD values of basaltic melt, and these low δD values reflect the incorporation of a crustal component.

6.2 Hydrogen isotope fractionation in oceanic subduction factory Many studies have shown that the origin of oceanic basalts with δD values distinct from fresh MORB is related to recycling of the subducted oceanic crust (e.g., Demény et al., 2004, 2008; Shaw et al., 2008, 2012; Bindeman et al., 2012). The subduction of oceanic crust plays an essential role in the heterogeneous hydrogen isotope composition in the mantle. This involves several steps of hydrogen isotope exchange in oceanic subduction-zone factory: (1) between mineral and seawater during alteration of the fresh MORB; (2) between mineral and fluid during metamorphic dehydration of the subducting oceanic crust; (3) between mineral and melt during partial melting of the dehydrated crustal rocks; (4) between mineral and melt during crustal metasomatism of the mantle wedge, and (5) between melt and mineral during fractional crystallization of the basaltic melt. Although the magnitude of hydrogen isotope fractionation varies between these different processes, supracrustal hydrogen isotope signatures are certainly transferred to mantle sources via the melt-peridotite reaction at the slab-mantle interface in subduction zones. Shilobreeva et al. (2011) reported δD values of -64 to -25‰ for altered oceanic basalts from ODP/IODP Hole 1256D. Most altered oceanic basalts from East Pacific Rise show δD values higher than -57‰, higher than those of fresh MORB. Most authigenic marine sediments and seafloor serpentinites also exhibit high δD values of -30±25‰ (France-Lanord and Sheppard, 1992; Lécuyer and O’Neil, 1994). During water-rock interaction, some hydrous minerals such as smectite, caledonite and Fe-oxyhydroxides commonly form at <150°C, whereas other hydrous minerals such as chlorite, amphibole, epidote, prehnite and zeolite form at >250°C (Shilobreeva et al., 2011). All hydrous minerals are characterized by negative hydrogen isotope fractionation factors relative to seawater in the temperature range of seawater-hydrothermal alteration, typically smaller than -50‰ (Graham et al., 1980, 1984, 1987; Capuano, 1992; Chacko et al., 2001; Hyeong and Capuano, 2004). Assuming a δD value of 0‰ for seawater, hydrous minerals originating from alteration would have δD values higher than -50‰, which are higher than those of fresh MORB. This suggests the presence of a D-depleted component in the mantle source of normal MORB. When the oceanic crust is subducted to a depth of >50 km, it releases significant amounts of water due to the breakdown of hydrous minerals (e.g., Schmidt and Poli, 1998; Rüpke et al., 2002).

Experimental studies have determined that mineral-water hydrogen isotope fractionation factors are a function of temperature (Suzuoki and Epstein, 1976; Graham et al., 1980, 1984; Gilg and Sheppard, 1996; Vennemann and O’Neil, 1996; Xu and Zheng, 1999; Horita et al., 2002; Saccocia et al., 2009). For most hydrous minerals except epidote (Chacko et al., 1999), the fractionation factors increase with increasing temperature. For instance, at a pressure of 50 MPa, ΔDserpentine-water (the fractionation factor between serpentine and water) is -32‰, -29‰ and -20‰ at 250°C, 350°C and 450°C, respectively (Saccocia et al., 2009). However, altered oceanic basalts have δD values of -64 to -25‰ (Shilobreeva et al., 2011), implying that such magnitudes of hydrogen isotope fractionation cannot be ascribed to seawater-hydrothermal alteration alone. On the other hand, altered oceanic basalts undergo metamorphic dehydration at subarc depths and partial melting at greater depths. Such processes can cause significant hydrogen isotope fractionations, not only between released fluid and dehydrated mineral, but also between derived melt and restitic mineral. Because the metamorphic dehydration at subarc depths is generally caused by the breakdown of hydrous minerals, this process produces D-rich aqueous fluids because of negative ΔDmineral-water values. This fractionation has been confirmed by hydrogen isotope analyses of island arc basalts (IAB) and backarc basin basalts (BABB) derived from partial melting of fluid-altered mantle wedges, which yield δD values similar to, or higher than, those for altered oceanic basalts (Poreda, 1985; Hochstaedter et al., 1990; Shaw et al., 2008). As a consequence, the dehydrated minerals are relatively depleted in D, and partial melting of which gives rise to hydrous melts depleted in D because of the negative ΔDmineral-water values between minerals and water dissolved in the hydrous melts. Taken together, the effects of alteration, dehydration and melting can quantitatively account for the negative δD values of -80 to -60‰ for common MORB (Hoefs, 2009). In addition, pressure is one critical variable that affects hydrogen isotope fractionation factors because vibrational frequencies of water change significantly with pressure (Driesner, 1997). Both theoretical calculations (Driesner, 1997) and isotope exchange experiments (Horita et al., 2002) indicate that pressure enhances hydrogen isotope fractionation. In other words, for others being equal, water in equilibrium with a mineral has a lower δD value at a higher pressure. Dehydration and melting of subducting oceanic crust beneath the craton mantle wedge may continue at much higher pressures (>2.0 GPa) than the mineral-water exchange experiments (Ringwood, 1990; Rüpke et al., 2002; Zheng, 2012). Therefore, a more pronounced pressure effect can be expected. At subarc mantle depths, the subducting slab may release the supercritical fluid (SF) that is completely miscible between aqueous fluid and hydrous melt (Hermann et al., 2006; Mibe et al., 2011; Zheng et al., 2011; Kawamoto et al., 2012). In this case, much larger hydrogen isotope fractionations may arise from the pressure effect on the SF. Since equilibrium ΔDmineral-water values

for hydrous minerals such as serpentine, epidote and amphibole at T > 400°C are smaller than -30‰ (Suzuoki and Epstein, 1976; Chacko et al., 1999; Saccocia et al., 2009), the pressure effect on ΔDmineral-SF values may be as large as -40 to -20‰. Therefore, ΔDmineral-SF values at depths of >100 km would be as large as -80 to -60‰. Only such magnitudes of hydrogen isotope fractionation can account for the δD values of -108 to -94‰ of the phenocryst minerals from the continental basalts (Table 4). Melt inclusions in phenocryst minerals of OIB from Koolau volcano and glasses from Manus basin have low δD values of < -100‰, which were interpreted to be inherited from the partial melt of subducting dehydrated oceanic crust (Hauri, 2002; Shaw et al., 2012). The low δD values for the continental basalts in this study are also likely to be inherited from the subducting dehydrated oceanic crust. A similar conclusion was also reached by the studies of trace elements and oxygen isotopes in these continental basalts (Wang et al., 2011; Xu et al., 2012). The depletion of radiogenic Sr-Nd-Hf isotopes and the low δ18O values for these basalts suggest that their mantle sources could be the young SCLM, generated by reaction of the MORB-like mantle wedge with a hydrous melt derived from partial melting of the dehydrated paleo-Pacific oceanic crust. In view of rapid hydrogen diffusion in minerals under upper mantle conditions (Zhao and Zheng, 2007), the low δD mantle sources could be isotopically exchanged with the asthenospheric mantle to acquire normal mantle δD values. In this regard, the preservation of low δD values suggests that the metasomatic

mantle

domains

would

have

been

stored

in

the

SCLM

above

the

lithosphere-asthenosphere boundary prior to partial melting for the continental basaltic magmatism. Assuming a δD value of -30‰ for the altered oceanic crust, a ΔDmineral-water of -50‰ and 80% loss of water (Kogiso et al., 1997) during the dehydration and melting of subducting oceanic crust, the calculated δD value for the dehydrated oceanic crust is as low as -150‰. Incorporation of this low δD component into the mantle wedge would result in sufficiently low δD mantle sources and basaltic melts. This mechanism is also consistent with the low δD values for common OIB, because their mantle sources contain a similar dehydrated subducted oceanic crust (Hofmann, 1997). Nevertheless, the very low δD crustal components in mantle sources cannot survive the mantle convection because of the fast rate of hydrogen diffusion under the asthenospheric conditions (Zhao and Zheng, 2007).

7. Conclusions We have identified disequilibrium partitioning of structural hydroxyl between phenocryst minerals from the Cenozoic continental basalts in North China. While water cannot be detected in the crystal structure of olivine phenocrysts, clinopyroxene phenocrysts contain variable amounts of structural hydroxyl from 62 to 913 ppm. The relationships between the structural hydroxyl contents

and Mg numbers of clinopyroxene phenocrysts are interpreted to reflect the combined effects of fractional crystallization and water species transformation during the ascent of basaltic melts. The basaltic melts are estimated to have an initial water content of 0.5 wt% that significantly increases to 3.5-6.7 wt%, such that the melts become water-saturated in the late stage of fractional crystallization. The structural hydroxyl content of mantle sources is estimated to be about 290 ppm, which is higher than that of the normal MORB-type mantle but close to that of OIB-type mantle. Both olivine and clinopyroxene phenocrysts exhibit variably high contents of total water, indicating the presence of molecular water in nano-scale fluid inclusions in the phenocryst NAMs. Water species would transform from structural hydroxyl to molecular water in basaltic melts during fractional crystallization, resulting in the local saturation of total water in the basaltic melts prior to eruption, as recorded by the occurrence of amphibole inclusions in clinopyroxene phenocrysts. The Cpx phenocrysts are consistently enriched in D relative to coexisting Ol phenocrysts. The initial δD values of -108 to -94‰ are estimated for these phenocryst NAMs considering the hydrogen isotope fractionation due to water species transformation during fractional crystallization. Because the subducting oceanic crust beneath the craton mantle wedge dehydrates at much higher pressures than those for the mineral-water hydrogen isotope exchange experiments, the pressure effect on hydrogen isotope fractionation during devolatization of the subducting oceanic crust may be much more significant. This can produce low δD hydrous melt/supercritical fluid, which metasomatizes the normal δD mantle wedge to generate the low δD mantle sources for the continental basalts. Therefore, the slab-mantle interaction in oceanic subduction channel is a key process to transfer crustal water and hydrogen isotope signatures to the mantle sources of intraplate basalts.

Acknowledgments This study was supported by funds from the Natural Science Foundation of China (41303005, 91014007 and 41221062). Thanks are due to J. Tang and Q.-L. Yang for their assistance with the field sampling, to R.-X. Chen for their assistance with TC/EA-MS analyses, to J. Liu, P. Li and H. Chen for their assistance with FTIR analyses and to M. Feng and J.-L. Xu for their assistance with EMP analyses. We are grateful to A.R.L. Nichols, Y. Tatsumi and one anonymous reviewer for their comments that greatly helped improvement of the presentation. We thank Dr. Huaiwei Ni for his help with improvement of the English presentation.

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Figure captions Figure 1. Sketch map of geology in North China showing the distribution of Cenozoic continental basalts, and locations of the samples used in this study.

Figure 2. Spidergrams of trace element distribution for Cpx phenocrysts in Cenozoic continental basalts from the North China Craton. (a) Chondrite-normalized REE patterns (chondrite REE composition after McDonough and Sun, 1995). (b) Primitive mantle-normalized trace element patterns (primitive mantle composition after McDonough and Sun, 1995).

Figure 3. Typical FTIR spectra for Cpx phenocrysts in Cenozoic continental basalts from the North China Craton. (a-c) Structural hydroxyl, and (d) molecular water and mineral inclusions. All spectra are normalized to 1 mm thickness.

Figure 4. Relationships between total water contents and hydrogen isotope compositions of olivine and clinopyroxene phenocrysts from Cenozoic continental basalts in the North China Craton.

Figure 5. The relationship between structural hydroxyl contents and Mg# values for Cpx phenocrysts in Cenozoic continental basalts from the North China Craton. Dash-dot arrays A-B-C and D-E denote the effect of fractional crystallization, and dashed array B-D denotes the effect of magma degassing.

Figure 6. The relationships between (a) CaO and (b) Al2O3 contents and Mg# values for Amp-present and Amp-absent Cpx phenocrysts in Cenozoic continental basalts from the North China Craton. Red triangles denote the Cpx phenocrysts with Amp peaks in FTIR spectra, and black diamonds denote the Cpx phenocrysts in which Amp peaks are absent in FTIR spectra.

Figure 7. The relationships between (a) hydrogen, and (b) oxygen isotope compositions and total water contents of phenocryst minerals in Cenozoic continental basalts from the North China Craton. Oxygen isotope data are from Wang et al. (2011) and Xu et al. (2012).

Fig. 1

Fig. 2

Fig. 3

Fig. 4 33

Fig 5

34

Fig. 6 35

Fig. 7 36

Table 1 Major element composition and structural water content of amphibole-absent clinopyroxene phenocrysts in Cenozoic basalts from the North China Craton Spot

SiO2

TiO2

Al2O3

FeO

MnO

MgO

CaO

Na2O

K2 O

Cr2O3

NiO

sum

Mg#

En

Fs

Wo

H2 O (ppm)

DCpx/melt HO

07CL05B-6-2

43.89

4.26

9.10

7.15

0.08

11.02

23.17

0.47

0.01

0.11

0.06

99.32

73

35

13

52

348

0.056

07CL05B-6-4

44.72

3.99

8.25

7.00

0.08

11.41

23.24

0.46

0.00

0.11

0.00

99.27

74

36

12

52

376

0.046

07CL10B-1-4

50.11

2.07

3.04

8.92

0.13

13.00

22.37

0.32

0.00

0.00

0.01

99.96

72

38

15

47

913

0.014

07CL10B-1-6

47.99

3.03

4.51

9.43

0.19

11.16

22.15

0.43

0.00

0.00

0.00

98.88

68

34

16

49

760

0.018

07CL10B-2-4

48.39

2.25

5.50

7.05

0.08

12.67

22.35

0.36

0.00

0.49

0.00

99.14

76

39

12

49

797

0.020

07CL10B-3-2

46.01

3.62

6.46

8.55

0.12

11.63

22.17

0.43

0.00

0.07

0.05

99.12

71

36

15

49

487

0.033

07CL10B-3-5

48.52

2.55

5.53

7.29

0.13

12.74

22.57

0.27

0.01

0.22

0.03

99.84

75

38

12

49

848

0.020

07CL12B-4-4

47.74

2.92

5.47

7.68

0.09

12.38

22.88

0.40

0.00

0.01

0.00

99.56

74

37

13

50

823

0.023

07CL12B-7-4

48.26

2.58

4.77

7.13

0.11

13.05

22.38

0.43

0.01

0.05

0.02

98.78

76

39

12

48

420

0.020

07CL12B-10-1

49.81

2.67

4.08

7.89

0.14

10.72

19.83

0.55

0.01

4.68

0.05

100.40

71

36

15

48

62

0.014

07CL12B-12-3

48.57

1.93

5.13

6.25

0.09

13.38

21.90

0.31

0.00

0.95

0.08

98.60

79

41

11

48

421

0.019

07CL12B-12-4

49.31

2.12

4.81

6.78

0.12

13.31

22.07

0.31

0.00

0.14

0.09

99.06

78

40

12

48

717

0.016

07CL13B-2-2

47.57

3.33

4.86

8.21

0.14

12.26

22.49

0.47

0.00

0.01

0.06

99.40

72

37

14

49

96

0.023

07CL13B-2-3

49.47

2.06

4.78

6.63

0.06

13.47

22.54

0.30

0.00

0.46

0.03

99.81

78

40

11

48

129

0.017

07CL13B-2-6

47.50

3.01

5.45

8.08

0.14

12.33

22.51

0.52

0.00

0.03

0.05

99.61

73

37

14

49

178

0.025

07CL13B-7-1

47.95

3.03

5.77

7.38

0.12

12.78

21.90

0.38

0.00

0.13

0.00

99.43

76

39

13

48

74

0.023

07CL13B-7-2

49.75

2.29

2.91

8.65

0.12

12.74

21.75

0.47

0.00

0.04

0.00

98.72

72

38

15

47

63

0.013

07CL13B-8-1

48.26

3.04

5.32

8.15

0.14

12.22

21.89

0.42

0.00

0.04

0.06

99.54

73

37

14

48

305

0.021

07CL13B-9-1

48.87

1.90

4.75

6.51

0.10

13.30

22.14

0.39

0.00

0.78

0.00

98.73

78

40

11

48

66

0.018

a

DCpx/melt was calculated according to O’Leary et al. (2010). HO 2

37

2

Table 2 Major element composition of amphibole-present clinopyroxene phenocrysts in Cenozoic basalts from the North China Craton spot

SiO2

TiO2

Al2O3

FeO

MnO

MgO

CaO

Na2O

K2 O

Cr2O3

NiO

sum

Mg#

En

Fs

Wo

07CL01B-4-1

47.48

2.89

4.08

10.76

0.12

12.12

21.54

0.62

0.00

0.10

0.04

99.75

66

36

18

46

07CL10B-6-6

49.40

1.86

3.98

7.08

0.16

13.44

22.35

0.33

0.01

0.28

0.02

98.90

77

40

12

48

07CL10B-7-1

49.96

1.91

3.03

8.30

0.11

13.41

22.29

0.37

0.00

0.00

0.05

99.42

74

39

14

47

07CL10B-7-2

46.53

3.21

5.43

9.20

0.14

11.31

22.27

0.51

0.00

0.00

0.00

98.60

68

35

16

49

07CL12B-1-3

49.56

2.16

4.83

6.63

0.13

13.64

22.70

0.39

0.00

0.38

0.05

100.45

78

40

11

48

07CL12B-1-4

51.31

1.74

2.98

7.01

0.16

13.85

22.55

0.39

0.01

0.05

0.02

100.08

78

41

12

48

07CL12B-3-1

48.94

2.37

4.40

7.34

0.10

13.30

22.48

0.42

0.00

0.07

0.04

99.46

76

40

12

48

07CL12B-3-2

47.09

3.50

5.47

7.86

0.11

12.25

22.44

0.35

0.01

0.07

0.02

99.17

73

37

13

49

07CL12B-3-6

49.56

2.12

4.68

6.73

0.12

13.60

22.88

0.33

0.01

0.24

0.00

100.27

78

40

11

49

07CL12B-4-1

48.71

2.65

5.15

7.11

0.07

13.27

22.68

0.36

0.01

0.08

0.00

100.08

77

40

12

49

07CL12B-4-2

51.84

1.17

2.93

5.39

0.09

15.04

22.68

0.34

0.00

0.55

0.01

100.04

83

44

9

47

07CL12B-4-5

51.31

1.18

3.02

5.60

0.10

14.87

22.79

0.33

0.00

0.58

0.03

99.79

82

43

9

48

07CL12B-5-1

48.29

2.39

5.40

6.92

0.10

12.96

22.71

0.35

0.00

0.11

0.02

99.25

77

39

12

49

07CL12B-6-2

46.98

3.64

5.42

8.19

0.10

12.28

22.10

0.51

0.00

0.05

0.00

99.27

72

37

14

48

07CL12B-7-1

48.82

2.65

4.49

7.64

0.11

12.96

22.42

0.45

0.00

0.05

0.00

99.58

75

39

13

48

07CL12B-7-3

48.53

2.66

4.82

7.58

0.13

13.00

22.59

0.48

0.00

0.07

0.00

99.86

75

39

13

48

07CL12B-8-3

51.39

1.54

2.89

6.57

0.09

14.40

22.47

0.35

0.00

0.04

0.05

99.79

79

42

11

47

07CL12B-8-5

48.32

2.42

4.81

7.23

0.11

13.19

22.40

0.35

0.00

0.05

0.04

98.91

76

39

12

48

07CL13B-1-2

48.12

2.42

4.95

7.16

0.12

13.17

22.59

0.38

0.00

0.16

0.05

99.12

76

39

12

48

07CL13B-1-5

48.48

2.36

4.96

6.84

0.14

13.47

22.51

0.37

0.00

0.23

0.04

99.39

78

40

11

48

07CL13B-1-6

50.51

1.49

2.63

6.45

0.10

14.34

22.25

0.35

0.00

0.17

0.00

98.28

80

42

11

47

07CL13B-2-1

48.16

2.37

5.99

6.38

0.08

12.94

22.58

0.38

0.00

0.70

0.03

99.61

78

39

11

49

07CL13B-2-4

45.62

4.45

6.31

8.55

0.07

11.37

22.53

0.60

0.00

0.03

0.00

99.53

70

35

15

50

07CL13B-2-5

48.71

2.32

4.73

6.88

0.05

13.37

22.64

0.33

0.00

0.15

0.01

99.19

77

40

12

49

07CL13B-3-3

48.02

2.46

4.97

6.94

0.07

13.44

22.57

0.34

0.01

0.21

0.00

99.03

77

40

12

48

38

Table 3 Trace element composition of amphibole-absent clinopyroxene phenocrysts in Cenozoic basalts from the North China Craton 07CL05B -6-2

07CL05B -6-4

Cs (ppm) Rb Ba Th U Nb Ta K La Ce Pr Pb Nd Sr Zr Hf Sm Eu Ti Gd Tb Dy Ho Y Er Yb Lu Ba/Nb

Spot

07CL12B -7-4

0.15 0.23 0.04 4.02 0.83 36.65 13.86 45.87 7.91 0.11 38.63 171.75 328.31 10.71 10.81 3.63 25502 11.63 1.43 7.68 1.55 34.86 3.55 2.24 0.30

0.10 0.06 0.31 0.03 3.31 1.01 8.53 12.66 39.98 6.77 0.09 37.08 155.49 337.11 12.52 9.97 3.20 26733 9.08 1.46 7.52 1.32 33.60 3.16 2.27 0.31

07CL12B -4-4 0.03 0.02 0.002 0.02 0.003 0.35 0.07 5.79 3.38 12.52 2.24 0.04 13.91 74.21 74.37 3.02 5.23 1.55 12007 4.74 0.75 4.12 0.65 17.64 1.52 1.05 0.15

0.04 0.05 0.01 0.55 0.12 8.52 3.60 13.15 2.51 0.04 13.88 67.95 83.35 4.07 4.76 1.45 13361 5.29 0.68 4.33 0.73 17.93 1.66 1.14 0.15

07CL12B12-3 0.02 0.07 0.02 0.05 0.003 0.40 0.06 6.32 3.81 13.31 2.45 0.02 13.90 73.86 71.86 3.36 4.91 1.48 12171 4.70 0.80 3.97 0.65 17.06 1.61 1.24 0.18

0.036

0.019

0.006

0.080

0.055

07CL12B12-4

0.04 0.01 0.46 0.07 6.15 4.02 15.01 2.72 0.05 15.36 74.42 76.58 3.77 5.41 1.72 13404 5.98 0.78 4.73 0.75 19.91 1.87 1.21 0.17

07CL13B -2-2

07CL13B -2-3

0.09

0.04 0.04 0.03 0.01 0.28 0.04 4.67 2.72 9.55 1.98 0.03 10.63 67.00 45.39 2.23 3.57 1.03 8989 3.96 0.55 3.26 0.56 13.15 1.42 1.12 0.15

0.08 0.04 0.01 0.49 0.10 7.07 3.84 13.98 2.50 0.05 13.47 73.39 77.49 3.86 4.24 1.60 12840 4.64 0.84 3.92 0.78 17.98 1.91 1.42 0.20

0.138

0.161

0.04 0.003 0.48 0.12 4.25 3.68 13.48 2.29 0.03 13.36 66.77 88.85 4.20 4.45 1.57 14789 5.17 0.82 4.32 0.77 18.89 2.01 1.31 0.19

39

07CL13B -2-6

07CL13B -7-1 0.004

07CL13B -8-1

0.10 0.04 0.01 0.57 0.21 6.85 4.64 16.69 3.07 0.08 17.84 71.59 105.96 4.69 5.42 1.67 16907 6.86 0.90 5.01 0.81 21.69 2.31 1.34 0.22

07CL13B -7-2 0.01 0.04 0.05 0.01 0.003 0.16 0.03 4.62 2.35 8.86 1.52 0.03 9.93 61.53 39.48 1.85 2.41 0.87 7929 3.46 0.46 2.96 0.51 11.89 1.34 0.80 0.10

07CL13B -9-1 0.04

0.05 0.03 0.09 0.004 0.69 0.14 9.21 5.73 21.68 3.90 0.09 21.95 81.16 124.40 5.35 7.58 1.87 17360 7.22 1.14 6.00 1.02 25.42 2.71 2.02 0.24

0.03 0.08 0.01 0.71 0.10 9.58 3.26 12.79 2.35 0.05 13.64 72.37 75.47 3.57 4.46 1.26 12039 5.07 0.74 4.01 0.75 16.71 1.74 1.24 0.14

0.183

0.329

0.037

0.038

Table 4 Total water content and hydrogen isotope composition of phenocryst minerals in Cenozoic basalts from the North China Craton Sample

δD (‰)

Total water (ppm) Olivine

Clinopyroxene

Olivine

Clinopyroxene

Changle-Linqu alkali basalt 07CL01

1620

-106

07CL02

1753

-107

07CL06

1761

-114

07CL07

1251

-104

07CL10

2808

-119

07CL11

1674

-124

07CL12

2484

1665

-114

-110

07CL13

1611

1215

-111

-102

Changle-Linqu basanite 07CL03

1475

-110

07CL04

1693

-108

07CL05

2136

-110

07CL08

2700

-145

07CL09

1602

-123

Hefei trachybasalt 05HF02

2966

05HF04

1854

-113

1647

-108 -98

05HF08

1382

1562

-104

-94

05HF09

1706

1647

-113

-102

05HF10

1224

-117

05HF15

1323

-115

05HF17

1751

-113

05HF18

1728

-124

Hefei basanite

×

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