Geocheminar spring-2016

Iron silicide thermal equation of state: implications for silicon in Earth’s core

Silicon is a candidate for the major alloying element in Earth’s outer core. From a mineral physics perspective, the thermal equation of state, melting temperature and thermal conductivity are key measurable quantities that can be compared with seismic observations and geophysical models to either support or rule out Si as a major alloying element. We present new measurements of the isothermal equation-of-state, thermal expansion, and the Grüneisen parameter of iron silicide (Fe5Si3) at high pressures and temperatures. We performed X-ray diffraction experiments on iron silicide in the laser-heated diamond anvil cell at the Advanced Photon Source (APS) beamline 13 ID-D. Diffraction patterns were measured in situ at pressures up to 96 GPa and temperatures up to 2500 K. Our high-pressure, high-temperature measurements provide previously unknown thermal parameters, which are essential for addressing questions of core composition. Additionally, our new isothermal equation of state is in contrast with previous measurements, and shows that iron silicide compressibility, KT,0 = 172 (6) GPa, is indistinguishable from the compressibility of pure Fe. Differences between measured compressibility of this material can be explained by non-hydrostaticity in the diamond anvil cell, a problem that is mitigated (though not solved) with noble gas loading of diamond cells (as was done in the present work). Recent work on liquid Fe-Ni-Si alloys (Williams, 2015) has placed limits of 1-2% Si in the outer core. Our iron silicide data show no such limitation and imply that Si is thermoelastically compatible up to ~15% by weight in the solid phase. Implications of the differences between liquid and solid data are discussed in terms of free energy and melting temperature.