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

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.