亲铁元素异常——后增薄层模型

Anomalies of Siderophile Elements-Late-Veneer Model

地幔中铂族元素丰度是基于地球形成理论所得计算值的两三百倍,无论是地幔中还是球粒陨石中,铂族元素之间的比值是相近的;对于这一现象有很多解释,其中被普遍接受的是:地核形成后(核幔分异完成后),大约占现在地球质量0.5%～1%的球粒陨石物质加入地幔,即可造成现在地幔中的铂族元素在丰度与比值方面的特征,这就是后增薄层模型(Late veneermodel)。但是,这一模型受到很多地幔样品铂族元素具非球粒陨石比值和高压下Co、Ni亲铁性测定结果的挑战。最近的地幔橄榄岩各相铂族元素精确测定、高压下Pt、Pd金属相与硅酸盐分配系数的测定,以及Re Os同位素的研究都支持后增薄层模型。

The observed abundances of platinumgroup elements(highly siderophile elements, PGEs) in the mantle are twothree orders of magnitude more than those calculated, furthermore, the relative abundances of PGEs in the mantle were roughly equal to those measured for chondritic meteorites. There are several explanations about this, among those the Lateveneer model is widely accepted. This model suggests that 0.5%-1% of primitive chondritic material arrived at the Earth after core formation was completed. The lateveneer model has been challenged, as measurements of mantle peridotites with nonchondrite PGE ratios, and the results of high pressure experiments of Co and Ni, that are difficult to reconcile with a primitive extraterrestrial source. However, recently new PGEs measurement of different phases of mantle peridotites, results for the partitioning of platinum and palladium between metal and silicate magmas at high pressures and temperatures, and the isotopic advances of ReOs, all support the lateveneer model.