The fractionation of Highly Siderophile Elements (HSE) during partial melting, and its implication for the HSE composition of the Primitive Upper Mantle

点击翻译按钮获取中文摘要

The addition of chondritic amounts of Highly Siderophile Elements (Pt, Pd, Rh, Ru, Ir, Os, Re and Au - the HSE) into the Earth's mantle after core/mantle differentiation in what is called the "late veneer", is a widely accepted hypothesis to explain the excess of HSE in the mantle compared to what is expected from metal/silicate-melt equilibrium. Although most mantle peridotites display broadly chondritic HSE patterns, recent studies have shown the existence of peridotites with non-chondritic HSE patterns (e.g. Pd/Ir). An added difficulty is that some authors argue that peridotite suites are extensively refertilized and that variations in their HSE contents may be the result of different degrees of refertilization and sulphide remobilization. The inconsistencies found within the empirical data are aggravated by our poor knowledge of the high-temperature partitioning of the HSE between mantle phases namely their partitioning behaviour between sulphide-melt (here called matte) and silicate melt ( ). Most previous experimental studies give a narrow range of values for ca 103-104, indicating that these elements should not fractionate between each other during partial melting of the mantle, in direct disagreement with empirical observations. These seemingly constant values for have led some authors to cast doubt on the validity of the "late veneer" model and suggest alternate hypotheses, such as core-mantle interaction or the addition of different non-chondritic material to the Earth's mantle. However, from basic thermodynamic considerations, is predicted to be substantially different for the eight HSE, and depends greatly on both oxygen and sulphur fugacity (O2 and S2 respectively). It is the aim of this project to use a novel way of measuring experimentally values of , over a large range of conditions, and to use these values to ascertain the mechanisms for the fractionation of HSE throughout partial melting and refertilization of the mantle and model the HSE composition of the primitive upper mantle (PUM).