[S-IT22] [EE] Interaction and Coevolution of the Core and Mantle in the Earth and Planets

[SIT22-18] Light element isotope fractionation processes in the deep Earth

*Madhusoodhan Satish-Kumar¹, Miyako Abe² (1.Department of Geology, Faculty of Science, Niigata University, 2.Graduate School of Science and Technology, Niigata University)

Keywords:Isotope fractionation, Deep Earth, Metallic melt-silicate fractionation

Partitioning of light elements and the distribution of isotopes between silicates in the mantle and metallic melts in the core during the early Earth differentiation and accretion process have governed the present day elemental and isotopic composition of the bulk silicate Earth. However, little is known till date about the factors that controll these processes in the deep Earth, especially in the core-mantle boundary, since we are unable to gather informaion from natural samples and yet to clearly reproduce the equillibrium conditions at high-pressure and high-temperature experiments. Especially, the role of light elements in the melting phase relations of mantle rocks, metal-silicate partitioning, and mass transfer between core and mantle are key in understanding the processes going on in deep Earth. Stable isotopic compositon is widely and efficiently used tool to understand the mobility of light elements in the deep Earth environments. Here I present a review of experimental determination of partitioning of light element isotopes at high-pressure and high-temperature conditions, in systems analogous to magma ocean environment where aggregation of core has happened and in the mantle conditions where recycling occurred thereafter.
A review of the previous experimental studies in the Fe-C and Mg-Si-C-O systems suggest that large carbon isotope fractionation occur between graphite/diamond and iron carbide melt. The results indicate that the iron carbide melt will preferentially gather ¹²C than ¹³C, and has a strong temperature dependence. Factionation is also observed between graphite/diamond and carbonate melt at temperatures and pressures corresponding to upper mantle conditions. The pressure dependence on carbon isotope fractionation is also being tested at higher pressure conditions. Preliminary results indicate that carbon isotopes also fractionate at high pressures corresponding to the deep Earth. Recent results in the sulfur, nitorgen and hydrogen sytem are consistent with the carbon-bearing system, that lighter isotopes generally fractionate to the metallic melt and heavier isotopes to the silicate melt.
In order to understand the factionation process in detail, it is essential to have accurate measurement of isotopic composition for the run products at high-pressures. The difficulty arises from the smaller volume of samples, separation of phases and confirmation of equilibration between the phases. Ongoing studies on microvolume isotope measurements using laser ablation and curie point pyrolyser gave encouraging results with good accuracy.
It is anticipated that the combined high-pressure and high-temperature dependent fractionation of light element isotopes in the deep Earth is an effective mechanism that can create a “lighter core” with large scale differences in the distribution of the isotopes between the metallic core and bulk silicate Earth during the accretion and differentiation of early Earth. Our findings also have implications on the light element cycling at the core-mantle interface.