MSA AWARD LECTURE: AN ISOTOPIC PERSPECTIVE ON HIGH-TEMPERATURE AND HIGH-PRESSURE CRYSTAL CHEMISTRY

Isotopes have the potential to provide information on high pressure and temperature processes that occurred on Earth and other planetary bodies. In the past two decades, natural samples have been analysed for their stable isotopic ratios, along with extra-terrestrial samples and biological samples, and an ever-increasing database has emerged. However, what is missing is a more methodical understanding of how and why these seen fractionations are formed. Experiments are key to understanding the mechansims behind the fractionations seen in nature and calculated in models as the pressure, temperature and compositional space can be interrogated systematically.

In this talk we present how pressure, temperature and composition affect the partitioning of stable isotopes between phases. We report high presure ⁵⁷Fe NRIXS spectra obtained at beamline 16 ID-D (HPCAT) of the Advanced Photon Source, in order to better understand the effect that pressure has on iron isotope fractionation. We conducted experiments in a panaromic-type diamond anvil cell using 100% ⁵⁷Fe enriched samples. We will present the force constants and calculated beta factors for Fe in FeO, Fe₃C and FeH_x. We find that extreme pressure conditions do in fact have an effect on the iron isotope fractionation factors and should be considered in models used to understand planetary scale iron isotope ratios. We will also present work conducted in the piston cylinder apparatus at 1 GPa focusing on how temperature and composition affect iron isotopic ratios. Our new data demonstrate that the composition of the metallic alloy influences the iron isotope fractionation between metal and silicate in the Fe - C, Si or S system. This result implies that the amount of light elements in a core should influence the extent of equilibrium iron isotope fractionation measured in samples of the mantle.

Our goal is to build up enough of a database so as to be able to paramaterize how stable isotope partitioning occurs in nature under varying temperature, pressure and compositional conditions.