Si and Fe Isotope Geochemistry at High Pressure and Temperature

Carnegie Institution Of Washington, Washington DC

Investigators

Abstract

Intellectual Merit. One of the main goals in earth science research is to determine the chemical composition of the Earth and its primary reservoirs. Isotopic and chemical mass balance calculations have been used extensively to better constrain fundamental issues such as the structure of mantle convection and determination of the light element contents in the core. Much work has focused on chemical partitioning experiments over a large range of pressures and temperatures. Although it has been assumed that pressure was not an important variable for isotopic fractionation among coexisting phases, new calculations of vibrational frequencies suggest that this assumption may not be true, and high P conditions may significantly influence stable isotopic variations within planetary bodies. It is well known that pressure can produce dramatic changes in material properties and mineral equilibria, but its effect on isotopic fractionation has not been well characterized. With the advent of new high-precision isotope ratio measurement techniques and development of high-pressure apparatus, there is great potential for interdisciplinary research in this field. It is proposed to investigate the effect of pressure on isotope fractionation, and specifically under conditions of core formation, as a function of mineralogy throughout the earth's mantle. Stable isotope geochemistry and experimental petrology/mineral physics methods will be applied to determine fractionation mechanisms. Isotopically spiked experiments will be conducted at high P-T conditions and run products analyzed to determine isotopic fractionation between coexisting phases. The results will be applied to understand the composition of the Earth's core and its interaction with the overlying mantle. Specific research questions include: [1] how does pressure affect isotopic fractionation of the non-traditional stable isotopes (Si, Fe)? [2] do fractionation factors change throughout the Earth's mantle as a function of pressure and hence mineralogy? [3] do large-scale processes such as core formation leave a stable isotopic signature that can be measured today? Broad Impacts. This project is defining a new field in the geosciences, necessitating the need for training in both mineral physics/experimental petrology and isotope geochemistry. A new female Staff member is named as a PI on this proposal and is responsible for much of the work therein. Results from the proposed study will be disseminated primarily through scientific journals and by participation at scientific conferences from both fields.