全球不同构造环境蛇纹岩全岩和矿物微量元素组成及非传统稳定同位素(Fe-Zn-Cu同位素)特征  

Mineral and whole-rock trace elements and non-traditional stable isotopes(Fe-Zn-Cu isotopes)of serpentinites from different tectonic settings

作　　者：杨凯 戴紧根[1] 沈洁 张文仓 赵玲玲 YANG Kai;DAI Jingen;SHEN Jie;ZHANG Wencang;ZHAO Lingling(School of Earth Science and Resources,China University of Geosciences(Beijing),Beijing100083,China)

机构地区：[1]中国地质大学(北京)地球科学与资源学院,北京100083

出　　处：《地质学报》2022年第12期4149-4166,共18页Acta Geologica Sinica

基　　金：国家自然科学基金委创新群体项目(编号42121002);111引智基地项目(编号B18048)联合资助的成果。

摘　　要：蛇纹岩对地球深部和浅部的元素循环以及氧化还原状态调节具有非常重要的作用。蛇纹岩中的流体活动性元素(fluid-mobile element,FME)是揭示地幔岩石水化、脱水以及元素循环的关键。本文系统收集和分析了前人报道的不同构造环境的蛇纹岩矿物化学、全岩微量元素和非传统稳定同位素(Fe、Zn、Cu)的组成特征,试图从多个角度总结蛇纹岩脱水过程的元素迁移规律及流体性质。蛇纹岩主要矿物蛇纹石微量元素含量具有以下主要特征:①不同变质程度的蛇纹岩中的蛇纹石既包含轻稀土元素(light rare earth element,LREE)富集,又包含LREE亏损的特征;②纤蛇纹石的REE和微量元素分布在利蛇纹石和叶蛇纹石的范围内,利蛇纹石重稀土元素(heavy rare earth element,HREE)整体上略高于叶蛇纹石且更加富集FME;③通过中度不相容元素与REE含量相结合,能够较好地区分橄榄石和辉石蛇纹石化所形成的蛇纹石,即辉石形成的蛇纹石富集相容元素(如Sc、Zn、Cr、Y和Ti等)并具有较高的HREE,而橄榄石形成的蛇纹石则表现为平坦且整体较低的REE分布型式。在蛇纹岩全岩微量元素和稀土元素(rare earth element,REE)含量方面,不同构造环境的蛇纹岩具有较大范围的重叠,但也有一定的差异:①慢速扩张的印度洋中脊蛇纹岩REE和微量元素含量要整体高于快速扩张的大西洋中脊和太平洋中脊的蛇纹岩;②马里亚纳蛇纹岩泥相比于蛇纹岩和蛇纹石化纯橄岩具有更高的REE和微量元素,而蛇纹石化纯橄岩相比于蛇纹岩则具有相对低的REE及流体不活动性元素含量。因此,利用微量元素的含量在区分不同环境的蛇纹岩方面存在一定的困难。但是,碱金属元素与U元素含量及其相应的比值,则可以较明显区分出大洋蛇纹岩和弧前蛇纹岩。目前已发表的蛇纹岩Fe、Zn、Cu同位素数据表明:①蛇绿岩中的蛇纹岩Fe和Zn同位素的分馏与�Serpentinite is essential in chemical exchanges between Earth's interior and surface,and plays a fundamental role in cycling of redox sensitive elements.Fluid-mobile element(FME)of serpentinite is critical for revealing the processes of mantle rock hydration,dehydration,and element recycling.Here we compile mineral and whole-rock trace elements and non-traditional stable isotopic compositions(Fe,Zn,Cu)of serpentinites from various tectonic settings.Trace elements of serpentine indicate that the occurrence of both LREE-enriched and LREE-depleted patterns in serpentines experienced by different degrees of serpentinization.The contents of REE and trace element of chrysotile are compared with those of lizardite and antigorite,while the contents of HREE and FME of lizardite are slightly higher than those of antigorite.The moderately incompatible elements together with REE could be useful to discriminate serpentines formed after olivine and pyroxene.Serpentine originated from pyroxene is enriched in compatible elements such as Sc,Zn,Cr,Y and Ti,while serpentine generated from olivine displays nearly flat REE patterns.The whole-rock trace element and REE patterns are overlapped for serpentinites formed among different settings,but there are also some differences.The REE and trace elements of the serpentinite from the slow-spreading Indian Mid-Ocean Ridge are higher than those from the Atlantic Mid-Ocean ridge and the Pacific Mid-Ocean ridge.Compared with serpentinite and serpentinized dunite,the Mariana serpentinite mud has higher REE and trace elements,while the serpentinized dunite has lower REE and fluid-immobile element.It is difficult to distinguish serpentinite in different environments by using absolute element concentrations.However,the alkali-U element ratios can distinguish mid-oceanic serpentinite from forearc serpentinite.We summarize the data of Fe,Zn and Cu isotopes of serpentinite.The fractionation of Fe and Zn isotopes of ophiolitic serpentinite is closely related to serpentinite devolatilization.Theδ56

关 键 词：蛇纹岩 元素循环 微量元素 非传统稳定同位素 

分 类 号：P597[天文地球—地球化学] P588.12[天文地球—地质学]