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.
The goal of this proposal was to determine the Si and Fe isotopic fractionation at high pressure and temperature. We quickly determined the Si isotopic fractionation between metal and silicate at high temperature and 1 and 7 GPa. Higher pressure experiments have been tried several times but have not resulted in equilibrium fractionation factors yet. The smaller sample size was a bit of an experimental barrier (due to non equilibrium conditions) but we have now turned to conducting the experiments in Japan with colleagues who have a large volume multi- anvil press. The Fe isotope fractionation has been found at high temperature using four different starting compositions, in all cases a fractionation was found. This has implications for Earth and Mars differentiation processes. In total more than 100 experiments were conducted, probed, and analyzed for isotopic composition. We have learned a tremendous amount about how to conduct these experiments so as to come to isotopic equilibrium in the most efficient and effective way. We have also found that sulfur has a large effect on the iron isotope fractionation. This has implications for determining the amount of sulfur in a planetary core by measuring the iron isotopic composition of meteorites. Broader Impacts: Two interns and one postdoc were trained through this grant, all female. One of the female interns was a high school student at Montgomery Blair High School and became an Intel semi-finalist based on the research she did through this grant. In Summary, the following was accomplished (with reference to the goals first outlined in the proposal): Silicon isotope fractionation was found between Fe-Si alloy and silicate at 1 and 7 GPa No pressure effect was found for silicon isotope fractionation up to 7 GPa. No silicon isotope fractionation was found between silicate melt and olivine. An independent estimate of ~6 wt. % Si in Earth’s core was found. Approximately 1 wt. % carbon was estimated to be in the Earth’s core based on calculations. Iron isotope fractionation was found between metal and silicate at 1650°C and 1700°C for an Earth like composition. MgO was found to be a poor capsule material for these experiments as the Fe diffused into the capsule material, causing an open system. BN was found to be an excellent capsule material in terms of a closed system for Fe, but did add both B and N into the silicate melt. Approximately 2 hours was necessary to establish isotopic equilibrium at 1650°C and 1 GPa. Greater than two hour experiments were also not equilibrated as kinetic processes began to affect the system, such as diffusion. The three-isotope technique was again found to be a very sensitive method to proving equilibrium for both Si and Fe isotopes. High-pressure experiments (up to 25 GPa) were conducted however no experiment reached equilibrium as they were all in the solid state. This is ongoing research… Sulfur was found to play an important role in determining the iron isotope fractionation between metal and silicate.
