Quantifying the chemistry of sulfide in the core and its influence on the composition of the silicate Earth

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The chemical composition of the Earth and rocky planets of the inner solar system is traditionally thought to be the same as primitive "chondritic" meteorites, amongst the earliest material formed in our solar system. Recent work, however, shows that the Earth does not have a chondrite-like composition, but instead appear to have lost a substantial part (some 10%) of its mass early in solar system history. The material that has been lost is highly enriched in so-called "incompatible" elements (that is, those elements with a large size (ionic radius) that preferentially go into silicate melts) including uranium, thorium and potassium, which together are responsible for generating much of Earth's internal heat through radioactive decay.The isotope signature observed in the modern silicate Earth indicates that this material must have been lost from Earth in the first 100 million years of its history. During this time metal separated from silicate in the growing Earth, forming a metallic core at the centre of the planet, leaving a silicate mantle above and crust at the surface. Some have argued that the loss of "incompatible" element rich material this was due to the removal of Earth's earliest crust through collisions with other growing planets, but this leaves the Earth without its full complement of heat-producing elements. Others have argued that these elements might be stored in a hidden reservoir deep in the silicate mantle, but so far no chemical or thermal trace of this reservoir has been observed. Moreover, models suggest that a high concentration of heat-producing elements at the base of the mantle may prohibit a functioning geodynamo (generation of the Earths magnetic field in the liquid outer core). Each of these hypotheses has very different implications for the chemical, dynamic and thermal evolution of Earth, but each poses problems that are difficult to circumvent. Seismic data from earthquakes and experimental work indicates that the Earths metallic core is principally composed of Fe-Ni metal, but also includes "lighter" elements, chief amongst which is sulphide. Experimental and isotope data suggest that sulphur was added late to the core either as a S-rich metal or as sulfide. Normally the "incompatible" are not expected to be incorporated into Fe-Ni metal, however, remarkably our own preliminary experimental data indicate that they are enriched in sulfide. At the same time new stable isotope data are also consistent with the incorporation of "incompatible" into sulfide and subsequent migration to the core. That sulfide in the core provides an incompatible element enriched reservoir capable of balancing the composition of the silicate Earth, offers an elegant solution to the non-chondritic Earth. At once reconciling the problem of planetary depletion of the heat-producing elements and providing a heat-source for the geodynamo.The overall aims of this project are (1) to quantify the role of sulfide during Earth's growth and core formation through high-pressure experiments that simulate the conditions of core formation. (2) Assess the influence of sulfide in the core on the composition of Earth's silicate mantle, and (3) the potential influence of continent formation and recycling using neodymium stable isotopes.