山东蓬莱新生代岩石圈地幔的异常锂同位素组成：橄榄岩包体的离子探针分析

应用离子探针技术对山东蓬莱新生代玄武岩中橄榄岩包体的主要组成矿物进行了原位测定，结果显示了强烈偏离正 常地幔矿物组成和平衡的Li 含量和同位素比值。橄榄石、斜方辉石和单斜辉石的Li 含量和δ7Li 分别为（2.0~10.4）×10-6， （2.8~23.8）×10-6 和（3.3~59.7）×10-6；-28.6‰ ~+0.5‰，-12.7‰ ~+12.8‰和-41.5‰ ~+22.6‰。这种强烈的非平衡体系反 映了最近强烈和复杂的地幔交代作用。辉石的高Li（~ 60×10-6）和δ7Li（~ +20‰）反映了最近富Li地幔熔体/ 流体的特征， 而橄榄石的中等Li 富集（<5×10-6）和强烈的δ7Li 亏损（~ -20‰）则可能记录了更早一期低δ7Li 的地幔交代。同位素剖面 的扩散反演以及Li 强烈富集与LREE 相对亏损之间的解耦显示了两期地幔交代作用的短暂性和近期性。因此，我们认为华 北东部克拉通上地幔中存在与再循环有关的锂同位素异常组分，它们的形成应该与中生代以来太平洋板块向东亚大陆下的 俯冲有关. 这些组分能够在地幔中较长的保存直至最近被热扰动所释放，并被橄榄岩包体所记录。

Anomalous Lithium Isotopic Compositions of the Cenozoic Lithospheric Mantle Beneath Penglai,Shandong Province:The ion Probe Analyses of Peridotite Xenoliths

Li contents and isotopic compositions of minerals in the peridotite xenoliths hosted by Cenozoic basalts from Penglai,Shandong Province have been measured using ion probe.The results show that the Li abundance ranges from 2.0 to 10.4×10-6,2.8 to 23.8×10-6 and 3.3 to 59.7×10-6 in olivine(ol),orthopyroxene(opx) and clinopyroxene(cpx),respectively,and the δ 7 Li values varies from-28.6 ‰ to +0.5 ‰,-12.7 ‰ to +12.8‰ and-41.5 ‰ to +22.6 ‰ in ol,opx and cpx,respectively.Both the Li contents and isotopic ratios deviate strongly from the normal mantle range and the equilibrated trend.These extreme disequilibria imply that the strong Li enrichments were induced by strong mantle metasomatic events.The high Li and δ 7 Li enrichments of pyroxene reflected the nature of the recent mantle metasomatism that fed them,whereas the moderate Li enrichment and strong δ 7 Li depletion in the olivines may record a relatively ancient low-δ 7 Li signature.The reproduction of the measured profiles and the decoupling of Li enrichment from the LREE-depleted pattern in cpx suggest that both enrichment processes were relatively recent and transient.Accordingly,we conclude that the"pristine"recycled components with anomalous Li composition existed in the upper mantle beneath the eastern North China Craton(NCC).The fractionation of Li isotopic compositions in the recycled components was linked to the subduction of the Pacific plate beneath East Asia since the Mesozoic,and the recycled components were reactivated recently to feed these anomalous Li enrichments as recorded by the Penglai peridotite xenoliths.