Experimental Investigations of Chemical Interaction at the Core-Mantle Boundary

地球历史上的一个基本事件是它早期分化为硅酸盐地幔和致密金属地核。地核和地幔之间是否存在显著的化学交换是一个长期存在的问题，对地球化学和地球物理学具有广泛的影响。在地幔底部观察到的复杂地震结构可能与地核边界的化学反应有关，也可能与地核顶部附近的地震结构有关。在海洋岛屿玄武岩中观测到的地球化学特征，主要基于锇同位素，也被解释为反映了来自外核的物质贡献。本研究将通过两种不同的实验研究来解决核心和地幔之间化学交换的重要性，一种旨在解决外核是否是海洋岛屿玄武岩放射性成因锇同位素特征的可信来源，另一种旨在限制核-地幔边界化学交换的动力学。外核是放射性锇源的假设是基于外核的Pt/Os和Re/Os比值因内核的结晶而显著提高。由于Os、Re和Pt之间的固体金属/液体金属分配系数的差异太小，并且随着压力的增加而减小，因此提出的研究将扩展先前对这一概念提出质疑的实验工作。这项新研究将调查这些元素在Fe-FeO和Ni-NiO系统中在高压下的分配，以确定含氧系统是否能够产生适当的分配系数。核幔边界反应的实验研究主要集中在平衡相关系和元素分配上。这些反应的动力学对于评价它们与地球的相关性同样重要。计划开展实验以确定下地幔模拟物质与液态金属之间的反应机理和反应速率，并确定过渡金属在方长石中的扩散速率。提出的研究将进一步测试该小组最近的实验工作结果，该结果表明晶体场效应对部分填充d轨道的过渡金属的扩散速率有很强的影响，详细研究这种效应是该项目的中心目标。

A fundamental event in Earth's history was its early differentiation into a silicate mantle and dense metallic core. Whether there has been significant chemical exchange between the core and mantle subsequently is a long-standing question that has broad implications for geochemistry and geophysics. It is possible that complex seismic structures observed at the base of the mantle, and possibly also near the top of the core, relate to chemical reactions at the core-mantle boundary. Geochemical signatures observed in ocean island basalts, based primarily on osmium isotopes, also have been interpreted to reflect material contributions from the outer core. This study will address the importance of chemical exchange between the core and mantle through two different lines of experimental investigation, one aimed toward resolving whether the outer core is a plausible source of the radiogenic osmium isotope signature in ocean island basalts, and the other at constraining the kinetics of chemical exchange across the core-mantle boundary. The hypothesis that the outer core is a source of radiogenic osmium is based on the idea that Pt/Os and Re/Os ratios in the outer core are raised significantly by crystallization of the inner core. The proposed study will expand on previous experimental work that has cast doubt on this notion, because the differences in solid-metal/liquid-metal partition coefficients among Os, Re and Pt are too small and decrease with pressure. The new study will investigate partitioning of these elements in the Fe-FeO and Ni-NiO systems at high pressures, to determine whether oxygen-bearing systems may be capable of producing the appropriate partition coefficients. Experimental studies of reactions at the core-mantle boundary have focused primarily on equilibrium phase relations and element partitioning. The kinetics of the reactions are equally important for evaluating their relevance to the Earth. It is planned to pursue experiments to determine the mechanisms and rates of reactions between lower-mantle analog materials and liquid metals, and to determine the diffusion rates of transition metals through periclase. The proposed study will further test the results of recent experimental work by this group that indicates that the crystal field effect has a strong influence on the diffusion rates of transition metals with partially filled d orbitals, and it is a central goal of the project to investigate this effect in detail.