Collaborative Research: CSEDI--Origin and Evolution of Highly Siderophile Elements in Earth's Mantle

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The Earth has a metallic core and a separate silicate mantle. They separated from one another very early in Earth's history. This separation is the most fundamental chemical differentiation of the planet. The objective of this project is to learn more about the timing and geochemical details of the separation process(es) because the footprint of the separation appears in the residual abundances in the mantle of elements that should have predominantly joined the core. It is unclear whether the interaction of the core and mantle is ongoing in the sense of a continuing separation, a static steady state, or a reactive regurgitation. It is also the purpose of this project to learn about the state and mechanisms of core-mantle interaction. Much of the project activities are focused on the analysis of the isotopic composition and abundance of siderophile (core joining) element Os in rocks thought to be related to the rise of deep plumes from the core-mantle boundary. If material from the core is incorporated in deep plumes, the isotopic composition of Os may reflect the production of radiogenic Os from radioactive Pt and Re. Pt and Re are enriched in the core compared to the mantle and decay to specific isotopes of Os providing a signature to recognize such interactions. In order to fully evaluate any such signals confirmed, it is necessary to know the geochemical behavior of these siderophile materials across core and mantle systems. Experimental laboratory investigations are directed towards this behavior at high pressure and temperature. Technical problems for laboratory experiments and their analysis, and possibly also for interpreting natural rock chemistry, are presented by the occurrence of noble metal micro-nuggets as the physical host to Os and other Pt-group elements. The investigators have developed a technique using both the isotopic composition and abundance of Os to infer the behavior of Os, stripped of the nugget effect. They propose to extend this technique to Pt, another siderophile element of key importance in understanding the core-mantle system.