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的金属核心。核幔之间是否发生了重大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具有广泛的意义。在地幔底部观察到的复杂地震结构可能与核幔边界的化学反应有关，也可能在地核顶部附近观察到。在洋岛玄武岩中观察到的地球化学特征也被解释为反映了来自外核的物质贡献，这些特征主要基于Os同位素。这项研究将通过两条不同的实验研究路线来讨论核和地幔之间化学交换的重要性，一条是为了确定外核是否是洋岛玄武岩中放射性成因Os同位素特征的可信来源，另一条是限制跨核-地幔边界的化学交换动力学。外核是放射源的假设是基于外核中的Pt/Os和Re/Os比值因内核的结晶而显着提高的观点。这项拟议的研究将在之前对这一概念提出质疑的实验工作的基础上进行扩展，因为Os、Re和Pt之间的固体-金属/液态-金属分配系数的差异太小，并且随着压力的增加而减小。这项新的研究将调查这些元素在高压下在Fe-FeO和Ni-NiO系统中的分配，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估它们与地球的相关性同样重要。计划进行实验，以确定下地幔模拟物质与液态金属之间的反应机制和速度，并确定过渡金属通过方镁石的扩散速度。这项拟议的研究将进一步检验该小组最近的实验结果，该实验结果表明，晶场效应对部分填充d轨道的过渡金属的扩散速度有很强的影响，详细研究这种影响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.