Lithium elemental and isotopic disequilibrium in minerals from peridotite xenoliths from Shangzhi, NE China： products of recent melt/fluid-peridotite interaction  

作　　者：Shenghua Zhou Songyue Yu Ting Zhou Jiangbo Lan Jian Kang Liemeng Chen Junhao Hu 

机构地区：[1]State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences,Guiyang 550081, PR China [2]University of Chinese Academy of Sciences, Beijing 100049,PR China

出　　处：《Acta Geochimica》2018年第5期769-789,共21页地球化学学报（英文）

基　　金：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)

摘　　要：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 δ7 Li (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 δ7 Li (-13.8‰ to7.7‰) 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.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 δ^7Li （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 δ^7Li （- 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 composi- tions 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.

关 键 词：Mantle peridotite Li isotope Mantle metasomatism Northeastern China 

分 类 号：P588.125[天文地球—岩石学] P597[天文地球—地质学]