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 in minerals from peridotite xenoliths from Shangzhi, NE China: products of recent melt/fluid-peridotite interaction
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DOI：
10.1007/s11631-018-0276-5
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发表时间：
2018-05
期刊：
影响因子：
1.6
通讯作者：
Sheng-Hua Zhou;Songyue Yu;Ting Zhou;Jiangbo Lan;Jian Kang;Liemeng Chen;Jun-Hao Hu
中科院分区：
文献类型：
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作者：
Sheng-Hua Zhou;Songyue Yu;Ting Zhou;Jiangbo Lan;Jian Kang;Liemeng Chen;Jun-Hao Hu
锂元素和同位素不平衡在大陆和海洋地幔捕虏体中经常观察到，但其起源仍然存在争议。本文对中国东北上治地区新生代玄武岩中夹带的变交代橄榄岩捕虏体进行了元素和Li同位素联合研究，为这一问题提供了新的思路。上枝橄榄岩橄榄石中Li浓度（0.3 ~ 2.7 ppm）和δ7Li（多为2‰~ 6‰）与正常地幔值相近，与熔融萃取指标（如模态橄榄石和全岩MgO）呈大致负相关。这些特征与不同程度的部分熔化相一致。而上志捕虏体斜辉石中Li富集显著（0.9 ~ 6.1 ppm）， δ7Li（- 13.8‰~ 7.7‰）相对于正常地幔值偏轻。这种特征可以解释为在很短的时间内（几分钟到几个小时）从硅酸盐熔体或富锂流体中发生的锂扩散。此外，在一些散装样品中保存的轻锂同位素组成也表明这些渗透熔体/流体没有足够的时间与散装橄榄岩进行同位素平衡。因此，我们强调，上芝地幔捕虏体中的锂同位素分馏主要与硅酸盐熔体或富锂流体的Li扩散有关，这种扩散发生在它们被夹带进入宿主岩浆之前或同时。
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.