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

Abstract

Metasomatism induced by melts/fluids is ubiquitous in the lithospheric mantle and can potentially modify the initial water content of the mantle. However, the preservation of correlations between H2O content and partial melting indices (e.g., Yb content in clinopyroxene, Cr/(Cr+Al) in spinel) and the lack of correlations between H2O content and metasomatic indices (e.g., La/Yb in clinopyroxene) in peridotite xenoliths from several localities suggest that variations in the initial H2O content were controlled by partial melting processes rather than by subsequent metasomatic event(s) (Hao et al., 2014; Denis et al., 2015). However, the applied partition coefficients of H2O between peridotite and melt (Dperidotite/melt=0.1–0.3) in the partial melting models cast doubts on the reasonability of such explanations. Whether metasomatism always modifies the initial H2O content of the lithospheric mantle remained a topic of debate. In this paper, we measure major and trace element concentrations and H2O contents of minerals in the peridotite xenoliths hosted by the Tianchang Cenozoic basalts in eastern China by using electron microprobe, laser-ablation ICP-MS and Fourier transform infrared spectroscopy, respectively. The H2O contents (weight in ppm) of clinopyroxene, orthopyroxene and olivine are 70–280ppm, 35–140ppm and below detection limit (<2ppm H2O), respectively. Although water diffusion during xenolith ascent cannot be excluded for olivine, pyroxenes largely retain the initial H2O content of the mantle source, as supported by (1) the correlation between H2O content and major element content of pyroxene, and (2) the equilibrium H2O partitioning between clinopyroxene and orthopyroxene. The calculated whole-rock H2O contents range from 14 to 93ppm (average 52±25ppm) assuming 0.1 for the H2O partition coefficient between olivine and clinopyroxene. Although no hydrous minerals are found, the enrichment in light rare earth elements and large ion lithophile elements of clinopyroxene indicates cryptic mantle metasomatism. However, variations between the H2O contents of the whole rocks and the metasomatic index (La/Yb ratio in clinopyroxene) are not correlated, suggesting that mantle metasomatism did not modify the initial H2O contents after the melting event. Instead, the H2O content correlates with the melting indices demonstrating that partial melting is the primary factor controlling the variations in the initial H2O contents. Notably, variations in whole rock H2O contents of the Tianchang peridotites can be modeled as a simple melting process of a MORB mantle source using the reported partition coefficients of H2O (Dperidotite/melt=0.005–0.03), providing a robust example suggesting that metasomatism does not always change the initial H2O content in the lithospheric mantle.