Elsevier

Geochimica et Cosmochimica Acta

Volume 292, 1 January 2021, Pages 382-408
Geochimica et Cosmochimica Acta

Pyroxene does not always preserve its source hydrogen concentration: Clues from some peridotite xenoliths

https://doi.org/10.1016/j.gca.2020.10.003Get rights and content

Highlights

  • Widespread metasomatism following partial melting events in the studied peridotites.

  • Pervasive hydrogen diffusional loss in both olivine and pyroxene.

  • Orthopyroxene more likely preserves its in-situ water contents than clinopyroxene.

  • H partitioning between pyroxene of ∼2 doesn't indicate equilibrium partitioning.

Abstract

Water is key to many geodynamical processes in the Earth's upper mantle, yet its preservation in mantle minerals is still debated. To throw some light on this problem, we here carried out an integrated study of whole-rock and mineral chemistry, and hydrogen concentrations in olivine, orthopyroxene, and clinopyroxene within 18 spinel lherzolite samples from three localities (Lianshan, Panshishan, and Tashan) in the Nanjing area, eastern China. Whole-rock and mineral compositions suggest that the studied peridotite samples interacted with melt at different melt/rock ratios following various degrees of partial melting (up to 11%). Fourier transform infrared (FTIR) measurements show that olivine is almost dry (<1 wt ppm H2O) while the cores of orthopyroxene and clinopyroxene contain 14–151 wt ppm H2O and 41–218 wt ppm H2O, respectively. Profile analyses of >70 orthopyroxene grains, which are homogeneous in major-element compositions, covering all the studied samples show hydrogen-depleted rims, indicative of hydrogen diffusional loss. This hydrogen zonation is probably caused by hydrogen chemical diffusion controlled by the mobility of trivalent cations (most likely Al3+) in response to magma degassing or partial melting of peridotite during ascent, or interactions of peridotite with melt, or a combination of these processes. By contrast, no hydrogen zonation is observed in clinopyroxene. Based upon the comparison of chemical compositions (especially Fe and AlIV contents) of clinopyroxene within our samples with those in diffusion experiments, it is inferred that the hydrogen diffusivity in clinopyroxene should be larger than that in orthopyroxene from our samples. This inference points to that clinopyroxene within the studied samples must have experienced diffusional loss of hydrogen as well, suggesting that water concentrations in the lithospheric mantle beneath the study area are probably underestimated. Furthermore, it also implies that orthopyroxene instead of clinopyroxene most likely preserves the in-situ water concentrations at depth, at least at its core. The absence of hydrogen zonation in clinopyroxene can be attributed to its fine-grained nature and fast hydrogen diffusivity. Our FTIR data also show that Lianshan and Tashan samples have water concentration ratio between clinopyroxene and orthopyroxene (RCpx/Opx) of ∼2, similar to mantle xenoliths from eastern China and other localities worldwide, yet Panshishan samples have higher RCpx/Opx values (2.3–5.9). Since hydrogen loss is suggested for both pyroxenes, RCpx/Opx of ∼2 thus cannot be taken as a reliable indicator of preservation of original water concentration of mantle source and equilibrium partitioning of hydrogen between pyroxene, as opposed to previous suggestions.

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 AlSi'+Hi×SiSi×, 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

References (138)

  • Y.-T. Hao et al.

    Partial melting control of water contents in the Cenozoic lithospheric mantle of the Cathaysia block of South China

    Chem. Geol.

    (2014)
  • Y.-T. Hao et al.

    Mantle metasomatism did not modify the initial H2O content in peridotite xenoliths from the Tianchang basalts of eastern China

    Lithos

    (2016)
  • E.H. Hauri et al.

    Partitioning of water during melting of the Earth’s upper mantle at H2O-undersaturated conditions

    Earth Planet. Sci. Lett.

    (2006)
  • E. Hellebrand et al.

    Mantle melting beneath Gakkel Ridge (Arctic Ocean): abyssal peridotite spinel compositions

    Chem. Geol.

    (2002)
  • M.M. Hirschmann et al.

    Storage capacity of H2O in nominally anhydrous minerals in the upper mantle

    Earth Planet. Sci. Lett.

    (2005)
  • M.C. Jollands et al.

    Coupled inter-site reaction and diffusion: Rapid dehydrogenation of silicon vacancies in natural olivine

    Geochim. Cosmochim. Acta

    (2019)
  • V. Le Roux et al.

    The Lherz spinel lherzolite: Refertilized rather than pristine mantle

    Earth Planet. Sci. Lett.

    (2007)
  • P. Li et al.

    Temporal variation of H2O content in the lithospheric mantle beneath the eastern North China Craton: implications for the destruction of cratons

    Gondwana Res.

    (2015)
  • S. Li et al.

    Collision of the North China and Yangtse Blocks and formation of coesite-bearing eclogites: timing and processes

    Chem. Geol.

    (1993)
  • X.H. Li et al.

    Amalgamation between the Yangtze and Cathaysia Blocks in South China: Constraints from SHRIMP U-Pb zircon ages, geochemistry and Nd-Hf isotopes of the Shuangxiwu volcanic rocks

    Precambrian Res.

    (2009)
  • C.Z. Liu et al.

    The Xinchang peridotite xenoliths reveal mantle replacement and accretion in southeastern China

    Lithos

    (2012)
  • Y. Liu et al.

    In situ analysis of major and trace elements of anhydrous minerals by LA-ICP-MS without applying an internal standard

    Chem. Geol.

    (2008)
  • J. Lu et al.

    Petrology and geochemistry of peridotite xenoliths from the Lianshan region: nature and evolution of lithospheric mantle beneath the lower Yangtze block

    Gondwana Res.

    (2013)
  • W.F. McDonough et al.

    The composition of the Earth

    Chem. Geol.

    (1995)
  • O. Müntener et al.

    Refertilization of mantle peridotite in embryonic ocean basins: trace element and Nd isotopic evidence and implications for crust-mantle relationships

    Earth Planet. Sci. Lett.

    (2004)
  • J.A. Padrón-Navarta et al.

    Site-specific hydrogen diffusion rates in forsterite

    Earth Planet. Sci. Lett.

    (2014)
  • L. Patkó et al.

    Extremely low structural hydroxyl contents in upper mantle xenoliths from the Nógrád-Gӧmӧr Volcanic Field (northern Pannonian Basin): geodynamic implications and the role of post-eruptive re-equilibration

    Chem. Geol.

    (2019)
  • A.H. Peslier

    A review of water contents of nominally anhydrous natural minerals in the mantles of Earth, Mars and the Moon

    J. Volc. Geotherm. Res.

    (2010)
  • A.H. Peslier et al.

    Hydrogen loss from olivines in mantle xenoliths from Simcoe (USA) and Mexico: Mafic alkalic magma ascent rates and water budget of the sub-continental lithosphere

    Earth Planet. Sci. Lett.

    (2006)
  • A.H. Peslier et al.

    Low water contents in pyroxenes from spinel-peridotites of the oxidized, sub-arc mantle wedge

    Earth Planet. Sci. Lett.

    (2002)
  • A.H. Peslier et al.

    Fast kimberlite ascent rates estimated from hydrogen diffusion profiles in xenolithic mantle olivines from southern Africa

    Geochim. Cosmochim. Acta

    (2008)
  • A.H. Peslier et al.

    Water disequilibrium in olivines from Hawaiian peridotites: recent metasomatism, H diffusion and magma ascent rates

    Geochim. Cosmochim. Acta

    (2015)
  • G.B. Piccardo et al.

    Melt/peridotite interaction in the Southern Lanzo peridotite: Field, textural and geochemical evidence

    Lithos

    (2007)
  • L. Reisberg et al.

    Re-Os and S systematics of spinel peridotite xenoliths from east central China: evidence for contrasting effects of melt percolation

    Earth Planet Sci. Lett.

    (2005)
  • L.A. Schaffer et al.

    Effects of melting, subduction-related metasomatism, and sub-solidus equilibration on the distribution of water contents in the mantle beneath the Rio Grande Rift

    Geochim. Cosmochim. Acta

    (2019)
  • L. Ackerman et al.

    Geochemical and petrological constraints on mantle composition of the Ohře (Eger) rift, Bohemian Massif: peridotite xenoliths from the České Středohoří Volcanic complex and northern Bohemia

    Int. J. Earth Sci.

    (2014)
  • J. Adam et al.

    Crystal/melt partitioning of water and other volatiles during the near-solidus melting of mantle peridotite: comparisons with non-volatile incompatible elements and implications for the generation of intraplate magmatism

    Am. Mineral.

    (2016)
  • C. Aubaud et al.

    Hydrogen partition coefficients between nominally anhydrous minerals and basaltic melts

    Geophys. Res. Lett.

    (2004)
  • Q. Bai et al.

    Substantial hydrogen solubility in olivine and implications for water storage in the mantle

    Nature

    (1992)
  • L. Beccaluva et al.

    Multistage evolution of the European lithospheric mantle: new evidence from Sardinian peridotite xenoliths

    Contrib. Mineral. Petrol.

    (2001)
  • M. Bedini et al.

    Evolution of LILE-enriched small melt fractions in the lithospheric mantle: a case study from the East Africa Rift

    Earth Planet. Sci. Lett.

    (1997)
  • D.R. Bell et al.

    Quantitative analysis of trace OH in garnet and pyroxenes

    Am. Mineral.

    (1995)
  • A.J. Berry et al.

    Fingerprinting the water site in mantle olivine

    Geology

    (2005)
  • Bodinier J. L. and Godard M. (2004) Orogenic, ophiolitic, and abyssal peridotites. In Treatise on Geochemistry 2 (eds....
  • J.L. Bodinier et al.

    Mechanisms of mantle metasomatism: Geochemical evidence from the Lherz orogenic peridotite

    J. Petrol.

    (1990)
  • C. Bonadiman et al.

    Water contents of pyroxenes in intraplate lithospheric mantle

    Eur. J. Mineral.

    (2009)
  • G.P. Brey et al.

    Geothermobarometry in four phase spinel lherzolites II: a practical assessment of existing thermobarometers

    J. Petrol.

    (1990)
  • G.D. Bromiley et al.

    Hydrogen solubility and speciation in natural, gem-quality chromian diopside

    Am. Mineral.

    (2004)
  • Carpenter Woods S. J. and Mackwell S. J. (1999) Hydrogen diffusion in enstatite. Bayerisches Forschungsinstitut für...
  • S. Carpenter Woods et al.

    Hydrogen in diopside: diffusion profiles

    Am. Mineral.

    (2000)
  • Cited by (7)

    • H diffusion in orthopyroxene and the retention of mantle water signatures

      2021, Geochimica et Cosmochimica Acta
      Citation Excerpt :

      Measurements of natural mantle orthopyroxene have frequently suggested that it is a relatively reliable (compared to olivine) host of water, based on the absence of H diffusion profiles in samples which displayed clear evidence of H-loss in coexisting olivine (Gose et al., 2009; Peslier, 2010; Xia et al., 2010; Warren and Hauri, 2014; Demouchy and Bolfan-Casanova, 2016; Xia et al., 2019). This suggestion has been challenged in more recent studies of natural mantle orthopyroxene (Tian et al., 2017; Denis et al., 2018; Tollan and Hermann, 2019; Xu et al., 2019; Wang et al., 2021) which have found strong evidence for rapid diffusion of H, broadly supporting inferences from experiments (Stalder and Skogby, 2003; Stalder and Behrens, 2006). The specific conditions that govern the reliability of orthopyroxene as a recorder of mantle H remain unclear, thus warrant further research.

    View all citing articles on Scopus
    View full text