The melting relation of the system, iron and carbon at high pressure and its bearing on the early stage of the Earth

Abstract

The melting relation of iron‐carbon system is studied to see how iron takes in or disgorges carbon at high pressure (up to 12GPa). The understanding on these points is closely related to the problem of the reservoir of carbon in the earth's interior. The results are: (1) Iron‐carbon system shows eutectic melting till at least 12GPa. (2) The eutectic composition is about 3 or 4 wt% of carbon, which does not vary very much with pressure. (3) The eutectic temperature slightly goes up as pressure increases at a rate of 7°C/GPa. This gradient is fairly lower than that of the melting temperature of silicates. Based on these results and other facts, the following scenario is inferred on the core formation during the early stage of the earth. Because carbon melts into iron forming eutectic system at low temperature, carbon within the accreted chondritic materials might be absorbed into iron melt near the surface of the magma ocean. The observation that the melting temperature of silicate goes up more rapidly as pressure increases than the eutectic temperature of iron‐carbon system, indicates that the temperature within the magma ocean is maintained higher than the melting temperature of iron‐carbon system. Therefore, the carbon‐bearing iron melt may sink into the deep interior of the earth without solidifying and disgorging carbon. After all, it is strongly suggested that carbon may settle with liquid iron forming core.

Melting diagrams of Fe-rich alloys determined from synchrotron in situ measurements in the 15–23GPa pressure range

“…It has been 427 proposed that the eutectic temperature increases with pressure linearly with a slope of ~7 428 K/GPa (Hirayama et al, 1993). The eutectic temperature of 1460 K observed in this study at 429 20.7 GPa falls slightly below a linear extrapolation of their data but still agrees with this trend 430 within the error of both studies.…”

“…This good agreement confirms that the measurements performed in this 547 study are correct. Concerning the Fe-C system the eutectic temperature of 1460 K at 20.7 GPa 548 falls slightly below a linear extrapolation of the data of Hirayama et al (1993). 549 Our results disagree considerably with previous studies in the case of the Fe-Ni-Si system 550 and the Fe-O system.…”

“…Still, the results of 436 Hirayama et al, 1993 andWood, 1993 are being challenged by recent experimental data (Fei 437 et al, 2007). 438…”

Natural moissanite (SiC) – a low temperature mineral formed from highly fractionated ultra-reducing COH-fluids

“…This is of particular relevance at oxygen fugacities just below the metalforming reaction (11) where a de facto binary FeC-alloy is almost Si-free. The Fe-C eutectic is located at 1150°C, 1 atm (Chipman 1972) and 1210°C, 10 GPa (Hirayama et al 1993;Rohrbach et al 2014), although this location is not entirely unambiguous as Lord et al (2009) report the eutectic at 10 GPa at 1420°C. In a graphite/diamondsaturated system, the ternary FeSiC-alloy would then be replaced by a metallic melt phase.…”

Experimental study and thermodynamic calculations of phase relations in the Fe–C system at high pressure

“…Shterenberg et al, 1975;Kocherzhinskii et al, 1992) were conducted to understand the conditions of iron carbide formation and carbon solubility in the melt up to 8 GPa, in an effort to refine the diamond synthesis process. Hirayama et al (1993) investigated the melting relations in the Fe-C system up to 12 GPa in a multianvil apparatus. Specifically, they examined the eutectic melting and observed a small increase of eutectic temperature with increasing pressure (∼7 • C/GPa), but no resolvable change in eutectic composition.…”