大陆岩石圈地幔定年

大陆岩石圈地幔是伴随地壳熔体抽取而形成的低密度地幔残留,是联系软流圈与地壳的重要纽带。虽然大陆岩石圈地幔一直是固体地球科学研究的重要内容,但对其形成时代的准确厘定则是当前研究的难点。传统方法是采用岩石圈地幔中橄榄岩主要元素的贫瘠程度来对岩石圈进行定年,即古老的岩石圈地幔Al2O3和CaO含量低,而MgO含量高(同时导致橄榄石高Fo值),而年轻岩石圈地幔的特征正好相反。显然,这种间接的方法不能给出确切的年龄信息。尽管Sr-Nd和锆石U-Pb等同位素方法也被用来进行地幔橄榄岩定年,但岩石圈地幔通常具有的较高温度使上述同位素体系不能保持封闭,因而给出的年龄大多与岩石圈地幔的形成时代无关。近几年发展起来的Re-Os同位素技术是目前进行岩石圈地幔定年的最理想工具,但也存在一系列需要研究的问题。文中对这一方法的基本原理、发展现状和存在的问题进行了全面的介绍。同时,根据目前获得的Os同位素资料,对中国东部岩石圈地幔的时代进行了讨论,并简要论述所获得的资料对岩石圈减薄研究的启示。

Dating the subcontinental lithospheric mantle

As the low-density residue after crustal melt extraction, the subcontinental lithospheric mantle (SCLM) serves a key connection between the asthenosphere and crust. However, the determination of the formation age of the SCLM is very difficult, compared to other geological entities in the solid earth sciences. It is traditionally proposed that the ancient SCLM is low in Al2O3 and CaO, and high in MgO (resulting in high Fo value for the olivine), but the juvenile SCLM shows the opposite features. Therefore, this indirect method cannot provide a reliable time for the SCLM formation. Sr-Nd and zircon U-Pb isotopic methods have been also applied in an attempt to date the SCLM, but the high temperature of the SCLM make these isotopic systems open during mantle evolution and the ages obtained are sometimes unrelated to the real SCLM ages. However, it has recently been shown that the Re-Os isotopic technique can be considered as the most reliable method to date the SCLM, although some problems still exist and need to be resolved. In this paper, a comprehensive overview of the Re-Os isotopic system, including its principles, advantages and disadvantages, are given. At the same time, we also discuss the SCLM age beneath eastern China, based on the obtained Os isotopic data, and their implications to the lithospheric thinning in the area.