Carbon contents in reduced basalts at graphite saturation: Implications for the degassing of Mars, Mercury, and the Moon

摘要

Carbon contents in reduced Martian basalts at graphite saturation were experimentally studied at 1400-1550°C, 1-2 GPa, and logfO_2 of IW - 0.4 to IW + 1.5 (IW denotes the Fe-FeO buffer). The results show that carbon solubility in Martian basalts, determined by secondary ion mass spectrometry, is 20 to 1400 ppm, increasing with increasing fO_2. Raman and Fourier transform infrared spectroscopic measurements on the quenched silicate glasses show that the dominant carbon species in Martian basalts is carbonate (CO_3~(2-)). The experimental data generated here were combined with literature data on similar graphite-saturated carbon solubility for mafic-ultramafic compositions to develop an empirical model that can be used to predict carbon content of graphite-saturated reduced basalts at vapor-absent conditions: in which T is temperature in K, P is pressure in GPa, X_(H2O) is mole fraction of water in basalts, ΔIW is the oxygen fugacity relative to the IW buffer, and NBO/T = 2 total O/T-4 (T = Si + Ti + Al + Cr + P). This model was applied to predict carbon content in graphite-saturated mantle melts of the Mercury, Mars, and the Moon. The results show that graphite may be consumed during the production and extraction of some Martian basalts, and CO_2 released by volcanism on Mars cannot be an efficient greenhouse gas in the early Mars. The lunar mantle carbon may be one of the main propellant driving the fire-fountain eruption on the Moon; however, the Mercurian mantle carbon may not be an important propellant for the explosive eruption on Mercury.