Pyroxene does not always preserve its source hydrogen concentration: Clues from some peridotite xenoliths
Introduction
Hydrogen in nominally anhydrous minerals (NAMs) from the Earth’s upper mantle has a fundamental impact on physical and chemical properties of these minerals and geodynamic processes in the upper mantle. For example, hydrogen plays a significant role in controlling the viscosity of peridotites via hydrolytic weakening of olivine (e.g., Mei and Kohlstedt, 2000), and is thus a key factor influencing the long-term stability of cratonic lithosphere (e.g., Li et al., 2008, Peslier et al., 2010). Numerous efforts had therefore been made toward understanding the nature of hydrogen in mantle minerals in the past several decades. On the one hand, experimental petrology under controlled thermodynamic conditions attempted to constrain storage capacity, incorporation mechanism, diffusion coefficient, and partitioning of hydrogen in mantle minerals (e.g., Bai and Kohlstedt, 1992, Ingrin et al., 1995, Kohlstedt et al., 1996, Rauch and Keppler, 2002, Stalder and Skogby, 2002, Aubaud et al., 2004, Hirschmann et al., 2005, Mierdel et al., 2007, Demouchy et al., 2017). On the other hand, investigations on natural peridotites provided us insight into the distribution, actual range, and controlling parameters of hydrogen in the upper mantle (see the comprehensive reviews by Peslier, 2010, Demouchy and Bolfan-Casanova, 2016, Peslier et al., 2017 and references therein).
Despite progresses made so far, there are still many issues to be clarified, among which a key one is whether the measured water concentrations in natural peridotites are representative of the initial values in the upper mantle. Due to fast diffusion of hydrogen in olivine and pyroxene (unless stated otherwise, pyroxene hereinafter refers to orthopyroxene and clinopyroxene) (e.g., Mackwell and Kohlstedt, 1990, Carpenter Woods and Mackwell, 1999, Hercule and Ingrin, 1999, Carpenter Woods et al., 2000), these minerals may lose water on their way to the surface. It has been widely accepted that olivine often loses partially or completely its in-situ water concentration during uplift, as recorded by characteristic hydrogen diffusion profiles (e.g., Demouchy et al., 2006, Peslier and Luhr, 2006, Li et al., 2008, Peslier et al., 2008, Denis et al., 2013) or no detectable OH bands in olivine but a significant amount of hydrogen in coexisting pyroxene (e.g., Xia et al., 2010, Hao et al., 2016, Wang et al., 2016), respectively. In contrast, although the kinetics of hydrogen diffusion in pyroxene is fast enough to allow substantial or complete depletion (e.g., Hercule and Ingrin, 1999, Carpenter Woods and Mackwell, 1999, Carpenter Woods et al., 2000), most previous studies argued that pyroxene preserves the original water concentrations of their mantle source (e.g., Peslier et al., 2002, Yang et al., 2008, Gose et al., 2009, Xia et al., 2010, Yu et al., 2011, Hao et al., 2014, Hao et al., 2016, Warren and Hauri, 2014, Denis et al., 2015, Wang et al., 2016). This argument is based on several lines of reasoning: (1) the water concentrations in pyroxene correlate well with whole-rock and mineral major-element compositions (e.g., Peslier et al., 2002, Hao et al., 2016); (2) the water concentrations in clinopyroxene and orthopyroxene from mantle xenoliths worldwide yield a constant ratio (RCpx/Opx) of ∼1.97–2.40 (e.g., Demouchy and Bolfan-Casanova, 2016); (3) no zonation of water concentrations within pyroxene had been observed (e.g., Xia et al., 2010, Yu et al., 2011, Warren and Hauri, 2014, Wang et al., 2016); and (4) the water concentration in orthopyroxene from abyssal peridotites has the saturation value predicted by experiments carried out at upper-mantle water-saturated conditions (Gose et al., 2009).
Recently, zoned orthopyroxene with high H2O core to low H2O rim has been found in garnet peridotite xenoliths from Sierra Nevada, California, USA (Chin et al., 2016). This zoning has been ascribed to the Al (IV) (Al in the tetrahedral site) contents that show the same pattern and have been demonstrated experimentally (e.g., Rauch and Keppler, 2002, Stalder, 2004, Mierdel et al., 2007) to control the incorporation of hydrogen into pyroxene through the exchange reaction where the Kröger and Vink (1956) notation is applied. Another study has reported significant zonation of hydrogen in orthopyroxene from mantle peridotite xenoliths entrained by the Cenozoic basanite from eastern China (Tian et al., 2017) and claimed that the zoning represents a dehydration profile provoked by the decrease in pressure during the xenolith ascent. It is important to note that correlation with Al contents has not been shown in this study. Denis et al. (2018) reported hydrogen-depleted rims of orthopyroxene from mantle xenoliths from San Carlos, USA, which were attributed to dehydration in response to melt-rock interaction. Additionally, Xu et al. (2019) documented bell-shaped hydrogen concentrations in both olivine and pyroxene and suggested that relatively low temperatures (∼750–900 °C) are crucial to the observed hydrogen diffusion in all these minerals. Although the mechanism remains uncertain, the occurrence of zoned orthopyroxene in natural peridotites shakes the previous common assumption that orthopyroxene preserves the “initial” water concentration prior to exhumation.
With these discrepancies in mind, we carried out an integrated study of mineral and bulk-rock chemistry and fourier transform infrared (FTIR) analyses on mantle minerals within 18 fresh peridotite xenoliths from three localities (Lianshan, Panshishan, and Tashan) in the Nanjing area, eastern China, aiming to shed some new light on the relevant processes and factors that may affect the preservation or loss of hydrogen in these minerals.
Section snippets
Geological background and samples
The North China Craton (NCC) and the South China Block (SCB) represent two major tectonic units in the eastern China. The SCB comprises the Yangtze Craton in the north and the Cathaysia block in the south (Fig. 1a), which were amalgamated ∼880 Ma ago (Li et al., 2009) and separated by the Jiangshan-Shaoxing and the Pingxiang-Yushan fault zone (JS-PYFZ). The Triassic (∼220 Ma) collision between the NCC and the Yangtze Craton (e.g., Li et al., 1993) brought about the Dabie-Sulu orogenic belt,
Sample preparation
For petrographic observation and mineral chemistry analysis, we prepared thin sections by polishing rock slabs to 0.05 μm using colloidal silica. The rock slabs were cut from the cores along a random orientation for most samples, but parallel to the lineation and normal to the foliation (i.e., the XZ section) for samples showing distinct lineation and foliation. For FTIR spectroscopy analysis, doubly polished thick sections (∼130–360 μm; Table 1) were prepared by a final polishing to 1 μm on
Petrology
All the studied samples are spinel-facies peridotites free of hydrous minerals. The mineral modes determined by the point-counting method are broadly consistent with those obtained by the mass-balance method (Table 1). Sample LS3 is an exception, however, and shows large differences in mineral modes determined by the two methods, suggesting that it is modally heterogeneous. Anyhow, the modal compositions of the studied peridotites define them as lherzolite (Table 1; Fig. 3). The predominance of
Partial melting and mantle metasomatism
Partial melting and metasomatism are two fundamental and complementary processes prevailing in the Earth's upper mantle. Evidence in favor of partial melting process in our samples comes from the co-variation plots of modal and chemical parameters (e.g., Al, Mg, and Cr) that are sensitive to melt extraction and relatively immobile after melting (Fig. 6). For instance, Cr#Sp is positively correlated with Mg#Sp (Fig. 6d), while Al2O3 in clinopyroxene is negatively correlated with Mg#Cpx (Fig. 6
Conclusions
This study reported FTIR results and mineral and bulk-rock chemistry of 18 peridotite samples. The following conclusions were reached:
- 1.
The studied peridotites had experienced a low to moderate degree (up to 11%) of partial melting, which is ensued by pervasive metasomatism to various extents.
- 2.
Olivine loses nearly completely its water. The partial preservation of OH bands in some olivine grains is attributed to the inter-site reaction of defects in addition to the more sluggish [Ti]- and
Research data
Research Data associated with this article can be accessed at https://doi.org/10.6084/m9.figshare.9970859.v1.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We thank Luan-Xi Bai, Jin-Lin Li, and Qian Ma for help with sample collection and thin-section preparation. Hua-Ping Ren and Peng-Xiao Li are appreciated for assistance with FTIR measurements and data processing. We are also grateful to Zhao-Chu Hu and Tao Luo for help with LA-ICP-MS analyses, and to Ji-Hao Zhu for help with EMPA analyses. We are indebted to Huai Cheng and Yu Yuan for help with LA-ICP-MS data processing. Constructive and insightful comments by Jollands M. C., an anonymous
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