特提斯洋对流上地幔Os同位素组成估算的新方法及初步运用

A new estimation method for Os isotopic composition of convecting upper mantle and its application to Tethyan oceanic upper mantle

对流上地幔Os同位素组成的准确估算是运用Re-Os同位素体系探讨地幔演化的基础。前人研究主要是以地幔橄榄岩为研究对象,由于地幔橄榄岩Os同位素存在明显的不均一性,因而直接影响估算值的准确性。对流上地幔中包含的不同亏损程度的难熔组分在部分熔融过程中难以熔融,对形成的熔体相的Os同位素组成贡献很少或者没有。因此,与对流上地幔具有相同的Os同位素组成初始值的早期分离结晶岩石(如堆晶岩),结合堆晶岩中锆石的准确定年,可以用来估算对流上地幔Os同位素组成。本文根据这一方法测试了那曲地区弧后盆地堆晶岩的Os同位素组成和锆石U-Pb年龄,推测那曲地区新特提斯洋对流上地幔Os同位素组成为碳质球粒陨石型的。根据这一模型,对比了罗布莎和东巧铬铁矿岩、含矿围岩以及不含矿围岩的Os同位素特征,揭示出矿石及围岩均具有古老大陆岩石圈地幔信息,而不含矿围岩(泽当岩体)的Os同位素组成为碳质球粒陨石型的,无古老大陆岩石圈地幔信息。

It is essential to obtain precise Os isotopic composition of convecting upper mantle before the application of Re-Os isotope system. However, published estimates for convecting upper mantle, especially calculation from mantle peridotites, are hindered by ubiquitous mantle heterogeneity. Given the ancient refractory domains contribute little to MORB formation,early- crystallizing dunite which have the same initial 187Os/188Os ratios with mantle source, can be used to estimate the Os isotopic composition of convecting upper mantle based on zircon U-Pb ages. A carbonaceous chondritic mantle source for the Nagqu ophiolite are indicated by Cr-spinel Os isotopes and zircon U-Pb ages from cumulate dunite and gabbro in the same cumulate section. According to this model, ancient mantle events are recorded in chromitites and host rocks in Luobusa and Dongqiao, which may related to ancient subcontinental lithospheric mantle. In contrast, the chromitite-free peridotites （Zedang lherzolite ） show carbonaceous chondritic Os isotopic composition without ancient mantle events information.