The thermal equation of state and melting curve of Fe3C to 120 GPa
Abstract
The Earth's core is known to be less dense than iron from seismic body wave and normal mode constraints. The presence of some combination of light elements Si, S, O, C and H in the core can explain the density deficit of ~7 wt% and 4 wt% in the liquid outer core and solid inner core, respectively. Identifying the predominate light elements present in the core provides a window into planetary formation processes, and serves to benchmark models of Earth's accretion, differentiation and giant impact history. Carbon is a compelling candidate to alloy with iron in the core, as it is abundant in carbonaceous chondrites (~3 wt%) and readily forms iron carbides, such as Fe3C, the mineral cohenite. However, the Fe-C binary system at core pressure and temperature conditions has not been completely characterized, due to uncertainty about phase stability, the melting curve and the thermal equation of state of Fe3C above 30 GPa.
In this study, we present new laser-heated diamond anvil cell P-V-T data for Fe3C to 120 GPa and 2500 K and assess thermal equation of state parameters in the Mie-Grüneisen-Debye formalism. To address technical challenges with measuring molar volumes of iron carbides at high temperatures in the diamond anvil cell, we employ a combination of burst laser heating implemented at GSECARS 13-ID-D at the Advanced Photon Source, and ex-situ sample characterization by FE-SEM analysis to confirm sample textural and chemical homogeneity. In a separate set of experiments, we determine the melting temperature of Fe3C up to 60 GPa at BL10 XU, SPring-8 and at GSECARS. Solid-melt texture and carbon concentration were determined by ex-situ FE-SEM analysis. The experimental results for the melting curve and thermal equation of state of Fe3C are placed in context of the binary Fe-C system, and implications for the carbon concentration of the Earth's core are discussed.- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMMR41B0062M
- Keywords:
-
- 1042 Mineral and crystal chemistry;
- GEOCHEMISTRY;
- 3924 High-pressure behavior;
- MINERAL PHYSICS;
- 3999 General or miscellaneous;
- MINERAL PHYSICS;
- 3620 Mineral and crystal chemistry;
- MINERALOGY AND PETROLOGY;
- 3672 Planetary mineralogy and petrology;
- MINERALOGY AND PETROLOGY;
- 5430 Interiors;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS;
- 7208 Mantle;
- SEISMOLOGY