地幔橄榄岩中橄榄石的指示意义

Indicative Significance of Olivine in Mantle Peridotites

橄榄石是地幔橄榄岩和辉石岩的主要组成矿物,但也经常以斑晶和捕虏晶的形式出现在玄武质岩石中。结合近年来在地幔橄榄岩的主要元素(如Mg和Fe)组成特征以及Li、Mg和Fe稳定同位素地球化学方面的研究成果,重点对橄榄石的地球化学特征与华北克拉通岩石圈地幔演化过程之间的联系进行了讨论,旨在加深对华北克拉通岩石圈地幔演化过程的理解。现有研究表明:地幔橄榄岩中橄榄石的矿物学特征、元素和同位素地球化学组成能够很好地指示岩石圈地幔的特征及其演化过程,因而具有重要的意义。对于克拉通地区的地幔橄榄岩来说,橄榄石的Mg#通常可以指示岩石圈地幔的属性,古老、难熔的地幔橄榄岩中的橄榄石一般具有较高的Mg#(>92),而新生的岩石圈地幔橄榄岩中的橄榄石则具有较低的Mg#(<91)。因此,地幔橄榄岩中橄榄石的Mg#在一定程度上具有年龄意义。橄榄岩中橄榄石的Li、Mg和Fe同位素组成也可以明确指示岩石圈地幔的属性及其所经历的演化过程,正常地幔的δ7Li、δ26Mg和δ57Fe组成相对均一,如果上述同位素组成偏离正常地幔值,则说明岩石圈地幔经历了熔体/流体的交代作用。华北克拉通地区地幔橄榄岩捕虏体中橄榄石的Li、Mg和Fe同位素组成研究表明:该区的岩石圈地幔经历了多个阶段、不同来源的熔体/流体的改造过程。

Since normal mantle has relatively homogeneous δ7Li,δ26Mg and δ57Fe values,the anomalies of these isotopic ratios indicate that the lithospheric mantle must have experienced metasomatism of melts/fluids.The Li,Mg and Fe isotopic characteristics of olivines in peridotite xenoliths from the North China Craton imply that the lithospheric mantle beneath the craton has experienced multistage modification by melts/fluids derived from different sources.Olivine is major mineral in mantle peridotites and pyroxenites.Moreover,olivines usually occur in basaltic rocks as phenocrysts and xenocrysts.Based on the summary of the latest developments of the signatures of major-element（such as Mg and Fe） compositions and Li-Mg-Fe isotopic geochemistry of olivines in the mantle peridotites from the North China Craton,the relationship between olivine geochemistry and the evolution of lithospheric mantle beneath the craton is discussed in detailed.The aim is to deepen the understanding of the evolution history of the lithospheric mantle beneath the North China Craton.The current investigations have shown that the characteristics of mineralogy,elemental and isotopic geochemistry of olivines in mantle-derived peridotites can well indicate the signature of lithospheric mantle and its evolution history,displaying important significance.Olivine Mg# in mantle peridotites can usually reflect the property of lithospheric mantle beneath cratonic areas.The olivines in peridotites from ancient and refractory lithospheric mantle are usually high in Mg#（92）,while those from newly-accreted lithospheric mantle are relatively low in Mg#（91）.As a result,olivine Mg# of mantle peridotites has certain age significance.The Li,Mg and Fe isotopic compositions of olivines in mantle peridotites can efficiently imply the signature of lithospheric mantle and its evolutionary processes.