Lithium elemental and isotopic disequilibrium in minerals from peridotite xenoliths from Shangzhi, NE China: products of recent melt/fluid-peridotite interaction
Lithium elemental and isotopic disequilibrium has frequently been observed in the continental and oceanic mantle xenoliths, but its origin remains controversial. Here, we present a combined elemental and Li isotopic study on variably metasomatised peridotite xenoliths entrained in the Cenozoic basalts from Shangzhi in Northeast (NE) China that provides insight into this issue. Li concentration (0.3–2.7 ppm) and δ7Li (mostly 2‰–6‰) in olivine from the Shangzhi peridotites are similar to the normal mantle values and show roughly negative correlations with the indices of melt extraction (such as modal olivine and whole rock MgO). These features are consistent with variable degrees of partial melting. In contrast, clinopyroxene from the Shangzhi xenoliths shows significant Li enrichment (0.9–6.1 ppm) and anomalously light δ7Li (− 13.8‰ to 7.7‰) relative to normal mantle values. Such features can be explained by Li diffusion from silicate melts or Li-rich fluids occurring over a very short time (several minutes to several hours). Moreover, the light Li isotopic compositions preserved in some bulk samples also indicate that these percolated melts/fluids have not had enough time to isotopically equilibrate with the bulk peridotite. We thus emphasize that Li isotopic fractionation in the Shangzhi mantle xenoliths is mainly related to Li diffusion from silicate melts or Li-rich fluids that took place shortly before or coincident with their entrainment into the host magmas.
KeywordsMantle peridotite Li isotope Mantle metasomatism Northeastern China
We thank Prof. Yi-Lin Xiao and Hai-Yang Liu from the University of Science and Technology of China and Ting Zhou from the Institute of Geochemistry, Chinese Academy of Sciences for their assistance in analysis of Li isotopes. This study was funded by the strategic priority research program (B) of the Chinese Academy of Sciences (XDB18000000), NSFC (41573009; 41373042, 41203031). Open research fund of the State Key Laboratory of Ore Deposit Geochemistry of China (SKLODG Grant # 201204). We thank three anonymous reviewers for their constructive reviews which greatly improved the quality of this manuscript.
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