Carbon Dissolution in Reduced Silicate and Alloy Melts – a Frontier for Understanding Evolution of Terrestrial Planets

Abstract:

C-O-H-S-N volatile elements sequestration in and release from silicate and metallic melts are key in controlling the planet scale distribution of these elements and thus are critical for thermo-chemical and dynamic evolution of terrestrial planets. To understand the distribution of carbon during accretion, core formation, magma ocean crystallization, and ongoing evolution of reduced mantles domains such as those of Mars, the Moon, and deep Earth, carbon chemistry in reduced silicates and alloys must be constrained, but currently are poorly known.

Here we summarize recent experimental efforts to (1) constrain the speciation and solubility of carbon in graphite-saturated natural mafic melts at fO₂ below IW buffer and (2) C solubility in Fe-Ni-rich alloy melts. Quenched glasses and alloys generated to 8 GPa and 2200 °C are analyzed using a combination of electron and ion microprobe, FTIR, and Raman spectroscopy.

Dissolved carbonates and bulk C solubility (<200 ppm) of graphite-saturated silicate glasses are observed to increase with decreasing pressure and increasing temperature, melt depolymerization index (e.g., NBO/T), and fO₂ from ~IW-1.5 to IW [1,2]. Over the range in fO₂, melt H₂O content also mildly enhances carbonate dissolution, likely in part owing to the depolymerizing effect of H₂O in the melt. At fO₂>IW-1.5, dissolved C is a mixture of carbonates and other species, but below ~IW-1.5 the carbonate doublet is undetectable in IR spectra, with Raman spectra indicating the dominant species being methane or other methyl groups. In addition, with diminishing fO₂ from IW-2 to IW-5, C solubility trend reverses, i.e., it increases with decreasing fO₂ and also shows a much stronger dependence on melt H content [3], consistent with the enhanced solubility of methane in silicate melt with decreasing fO₂ and increasing with square of fH₂. However, Fe-Ni-rich alloy melts have high C solubility (~0.7-7 wt.%) that diminishes mostly with increasing Ni, Si, and S contents.

These results have important implications for origin, distribution, and storage of carbon in the planetary interiors as well as chemistry of early atmosphere and will be discussed.

[1] Dasgupta et al. (2013). GCA 102, 191-202. [2] Chi et al. (2014). GCA 139, 447-471. [3] Li et al. (submitted).