铼－锇同位素体系在基性—超基性岩中应用现状的综述

综述了该同位素体系晚侏罗世以来，尤其是现代地幔岩石的研究成表明，地幔在晚侏罗世以来在１８７Ｏｓ／１８６Ｏｓ比值方面显示出非均一性，其比值从０．９０～１．２６，但此类岩石的１８７Ｏｓ／１８６Ｏｓ比值分别与各个不同研究者所确定的地幔演化线相一致；来自较古老正常地幔或贫化地幔的岩石，在锇同位素方面体现为１８７Ｏｓ／１８６Ｏｓ初始比值接近或低于Ｒｅ－Ｏｓ同位素体系地幔演化线值。在此类岩石中，Ｒｅ－Ｏｓ同位素体系与Ｓｍ－Ｎｄ同位素体系之间存在着三种可能的关系：①正相关关系；②负相关关系；③无相关关系。据认为，第一种关系是由羽状地幔端元和富集亲石元素而且贫铼之次大陆型岩石圈地幔端元混合而成；第二种关系则是由羽状地幔端元与地壳物质混合而成；第三种关系则是锇同位素成分相同但钕同位素成分明显不同的两端元物质混合而成。在第三方面，有古老俯冲大洋壳物质参与之基性－超基性岩石具有高于地幔演化线的初始１８７Ｏｓ／１８６Ｏｓ比值。具有此种锇同位素成分特征的岩石包括大洋岛弧玄武岩（ＯＩＢ）、洋中脊玄武岩（ＭＯＲＢ）以及榴辉岩。①

APPLICATION OF THE Re-Os SYSTEM TO BASIC AND ULTRABASIC ROCKS

The application of the Re-Os system to basic and ultrabasic rocks has attained the following important results. Firstly,it is indicated by this system that the modern mantle has been inhomogeneous in187Os/186Os ratios since the late Jurassic.Secondly,ancient rocks derived from the normal mantle or depleted mantle had initial187Os/186Os ratios close to or lower than that of the mantle evolution. These rocks may have positive or negative or no correlations between the Re-Os and Sin-Nd systematics.The positive correlation was caused by the mixing of an end member from the plume mantle and the other end member from the subcontinental lithospheric mantle,whereas the negative correlation may result from the mixing of an end member from the plume mantle with the other end member from the crustal materials. The rocks without any correlation between the Re-Os and Sm-Nd systems may be formed by the mixing of two end members with the same or similar187Os/186Os ratios but quite different143Nd/144Nd ratios.Thirdly,the basic and ultrabasic rocks which had been involved with ancient subducted oceanic crusts have higher initial187Os/186Os ratios than that of the mantle evolution.