大洋地幔橄榄岩-铬铁矿中的金刚石和深地幔再循环  

Diamond in oceanic peridotites-chromitites and recycled in deep mantle

作　　者：杨经绥 YANG Jingsui(School of Earth Sciences and Engineering,Nanjing University,Nanjing 210023,Jiangsu,China;Center for Advanced Research on Mantle(CARMA),Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources,Beijing 100037,China)

机构地区：[1]南京大学地球科学与工程学院,江苏南京210023 [2]自然资源部深地动力学重点实验室地幔研究中心,北京100037

出　　处：《地质力学学报》2020年第5期731-741,共11页Journal of Geomechanics

基　　金：国家自然基金项目(41720104009)。

摘　　要：全球多地蛇绿岩型地幔橄榄岩和铬铁矿中发现微粒金刚石,并在中国西藏南部和俄罗斯乌拉尔北部的蛇绿岩铬铁矿中发现原位产出的金刚石,认为是地球上金刚石的一种新的产出类型,不同于金伯利岩型金刚石和超高压变质型金刚石。它们与呈斯石英假象的柯石英、高压相的铬铁矿和青松矿等高压矿物以及碳硅石和单质矿物等强还原矿物伴生,指示蛇绿岩中的这些矿物组合形成于深度150~300 km或者更深的地幔。金刚石具有很轻的C同位素组成(δ13C-18‰^-28‰),并出现多种含Mn矿物和壳源成分包裹体。研究认为它们曾是早期深俯冲的地壳物质,达到>300 km深部地幔或地幔过渡带后,经历了熔融并产生新的流体,后者在上升过程中结晶成新的超高压、强还原矿物组合,通过地幔对流或地幔柱作用被带回到浅部地幔,由此建立了一个俯冲物质深地幔再循环的新模式。蛇绿岩型地幔橄榄岩和铬铁矿中发现金刚石等深部矿物,质疑了蛇绿岩铬铁矿形成于浅部地幔的已有认识,引发了一系列新的科学问题,提出了新的研究方向。Microdiamonds have been recovered from mantle rocks and associated podiform chromitites in many ophiolites across the world and,in particular,in-situ diamonds were found in ophiolitic chromitites in Southern Tibet and Northern Ural.Microdiamonds in ophiolites present a new type occurrence of diamonds on Earth,different from those diamonds occurring in kimberlites and ultrahigh pressure metamorphic belts.The discoveries of pressuresensitive minerals such as coesites with stishovite pseudomorph,high-pressure facies chromitites and Qingsongites(BN)indicate that ophiolitic chromitite may form at depths of>150~300 km or even deeper in the mantle.The very light C isotope composition(δ13 C-18‰to-28‰)of these ophiolitic diamonds,Mn-bearing mineral inclusions observed in these diamonds and coesite occurring in chromite all indicate the recycling of ancient continental or oceanic materials into the deep mantle(>300 km)or down to the mantle transition zone via subduction.These new observations and data strongly suggest that microdiamonds and their host podiform chromitite may have formed near the transition zone in the deep mantle,and that they were then transported upward into shallow mantle depths by convection processes.Thus,a new model has been proposed for deep subduction and recycling of oceanic crust in deep mantle.The discovery of diamonds and other UHP minerals from peridotites and chromitities in ophiolites doubts the current shallow genesis of ophiolitic chromitites and raises a serious of new scientific questions which leads to a new research direction.

关 键 词：金刚石 铬铁矿 蛇绿岩 超高压矿物 强还原矿物 深地幔 深俯冲作用 

分 类 号：P588.3[天文地球—岩石学] P542.5[天文地球—地质学]