岩浆过程中铁同位素的地球化学行为  

Fe Isotope Geochemistry of Magmatic System

作　　者：朱祥坤[1] 孙剑[1] 王跃[1] 

机构地区：[1]中国地质科学院地质研究所国土资源部同位素地质重点实验室,北京100037

出　　处：《地球科学与环境学报》2016年第1期1-10,共10页Journal of Earth Sciences and Environment

基　　金：国家重点基础研究发展计划("九七三"计划)项目(2012CB416806);国家自然科学基金项目(41403009;41203006;41273025)

摘　　要：铁同位素在岩浆过程中可以发生明显的分馏。相对于地幔橄榄岩(δ56 Fe平均值约为0‰),玄武岩略微富集铁的重同位素(δ56 Fe平均值约为0.1‰)。相对于中基性岩浆岩,酸性岩浆岩更加富集铁的重同位素。岩浆过程中的铁同位素分馏主要受铁的价态或氧逸度所控制。通常岩浆岩中的三价铁矿物(如磁铁矿等)相对于二价铁矿物(如橄榄石、辉石等)富集铁的重同位素。地幔橄榄岩部分熔融过程中,由于Fe3+比Fe2+更加不相容,熔体比固体相更加富集Fe3+,因而部分熔融产物(通常为玄武岩)相对富集铁的重同位素。在结晶分异过程及低氧逸度条件下,橄榄石、辉石等含Fe2+的矿物结晶导致熔体更加富集Fe3+,熔体铁同位素组成变重;在高氧逸度条件下,磁铁矿等含Fe3+的矿物结晶促使熔体更加富集Fe2+,熔体铁同位素组成变轻。在流体出溶过程中,出溶的流体相对富集铁的轻同位素,残余的岩浆铁同位素组成变重。现有研究表明,铁同位素在示踪岩浆作用方面具有很大潜力。Fe isotopes fractionate during magmatic processes.Comparing to mantle xenoliths（withδ56 Fe average value of ca.0‰）,basalts incorporate heavier Fe isotopes（withδ56 Fe average value of 0.1‰）.Relative to basic and intermediate magmatite,acidic magmatites show heavy Fe isotope enrichments.The Fe isotope fractionation during magmatic processes is controlled mainly by the oxidation states.Normally,Fe3＋-bearing minerals（e.g.,magmatite）incorporate heavy Fe isotopes than Fe2＋-bearing minerals（e.g.,olivine,pyroxene）.During partial melting process,Fe3＋is enriched in the melt preferentially,leading to heavy Fe isotopes enriched into the melt.As a result,the partial melting product（normally basalt）incorporates heavy Fe isotopes relate to the residue.During magmatic differentiation,the crystallization of Fe2＋-bearing minerals such as olivine and pyroxene in low oxygen fugacity condition leads to enrichment of Fe3＋thus heavy Fe isotopes in the residual melt;the crystallization of Fe3＋-bearing minerals such as magnetite in high oxygen fugacity results in enrichment of Fe2＋thus light Fe isotopes in residual melt.During fluid exsolution,the exsolved fluid preferentially incorporates light Fe isotopes.It shows that Fe isotope is a powerful tracer for studying magmatic processes.

关 键 词：地球化学 铁同位素 岩浆过程 部分熔融 结晶分异 流体出溶 氧逸度 火成岩 

分 类 号：P597[天文地球—地球化学]