Melting experiments on the Fe–C binary system up to 255 GPa: Constraints on the carbon content in the Earth's core

Abstract

Phase relations, including the eutectic liquid composition in the Fe-C binary system, remain unclear under the core pressure range, which makes estimating the carbon budget in the Earth's core difficult. To explore this issue, we have conducted melting and subsolidus experiments on Fe-C alloys in a diamond-anvil cell up to 255 GPa. Textural and compositional characterizations of quenched samples show that carbon concentration in the eutectic liquid slightly decreases with increasing pressure and is about 3 wt.% at the inner core boundary (ICB) pressure. The solubility of carbon in solid Fe is found to be almost constant at ~1.0 wt.%. In situ X-ray diffraction data indicate that Fe forms eutectic melting with Fe3C to 203 GPa and with Fe7C3 at 255 GPa. Previous studies on liquid Fe-C alloys suggested that the density of the outer core is explained by liquid Fe containing 1.8 to 4.2 wt.% C. If the liquid core includes <3 wt.% C as a single light element, hexagonal close-packed (hcp) Fe crystallizes at the ICB. However, the carbon content in such solid Fe is ≤1 wt.%, less than that required to account for the inner core density deficit from pure iron. When the outer core includes ≥3 wt.% C, it forms Fe7C3 at the ICB, whose density is too small for the inner core. Carbon is therefore not a primary light element in the core. Nevertheless, the outer core liquid can be Fe-C-Si, Fe-C-S, or Fe-C-H. Such core liquid crystallizes solid Fe with light elements including less than 1 wt.% C, which may explain the density and the sound velocities observed in the inner core.

Hydrogen Limits Carbon in Liquid Iron

“…We obtained the melting temperatures of FeH x ( x > 1) to 127 GPa, close to the pressure at the CMB (Table ), which extends the previous determinations by Sakamaki et al () up to 20 GPa. Our data show that the liquidus temperatures of FeH 1.02 and FeH 2.33 are 2260 ± 110 K at 108 GPa and <2120 ± 110 K at 127 GPa, respectively, lower than the 2500 K at 108 GPa for the eutectic melting in Fe‐Fe 3 S (Mori et al, ) and much lower than those of the other binary iron alloys, Fe‐Si (Fischer et al, ), Fe‐O (Morard et al, ), and Fe‐C (Mashino et al, ; Figure ). If we do not consider the data at 127 GPa because the liquid was enriched in hydrogen more than FeH 2 , that is, an Fe‐H intermediate compound stable above 67 GPa (Pépin et al, ), the liquidus temperature between FeH and FeH 2 is about 2380 K at the CMB.…”

“…Geophysical Research Letters respectively, lower than the 2500 K at 108 GPa for the eutectic melting in Fe-Fe 3 S (Mori et al, 2017) and much lower than those of the other binary iron alloys, Fe-Si (Fischer et al, 2013), Fe-O , and Fe-C (Mashino et al, 2019; Figure 3). If we do not consider the data at 127 GPa because the liquid was enriched in hydrogen more than FeH 2 , that is, an Fe-H intermediate compound stable above 67 GPa (Pépin et al, 2014), the liquidus temperature between FeH and FeH 2 is about 2380 K at the CMB.…”

Phase Stability and Thermal Equation of State of Iron Carbide Fe3C to 245 GPa

“…There is significant disagreement on the Fe 3 C melting temperature above 150 GPa (Liu, Lin, et al, ; Mashino et al, ). The shockwave Hugoniot would reach the solidus temperature at about 175 GPa or over 200 GPa, compared with the melting curves of Mashino et al () or Liu, Lin, et al (), respectively (Figure a). Hugoniot density data are not the best way to detect melting because of the small density change associated with melting.…”

Melting Experiments on Liquidus Phase Relations in the Fe‐S‐O Ternary System Under Core Pressures

“…We searched for possible liquid core compositions in Fe‐S‐O‐Si and Fe‐S‐O‐C, which satisfy both the constraints from the density and sound velocity in the outer core and from the ICB density jump (i.e., the inner core density). The liquidus field of Fe in these quaternary systems is estimated by considering the Fe‐FeSi (Ozawa et al, ) and Fe‐C binary eutectic compositions (Mashino et al, ). The present experiments on Fe‐S‐O as well as those by Tateno et al () on Fe‐Si‐S show that the liquidus field of Fe is approximated by a tie‐line connecting the relevant binary eutectic points.…”

“…We assume it is true for the Fe‐S‐O‐Si and Fe‐S‐O‐C systems. The S, Si, and C contents in solid Fe crystallizing at the ICB are obtained from D S = 0.8 (see above), D Si = 1.0 (Alfè et al, ), and D C = 0.33 (Mashino et al, ). The compositional range in solid Fe‐S‐Si and Fe‐S‐C that explains the observed inner core density has been demonstrated by the recent calculations by Li et al ().…”