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

Carbon contents in reduced Martian basalts at graphite saturation were experimentally studied at 1400-1550 degrees 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 (CO32-). 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: At IW + 1.7 >= logfO(2)>= IW-1: log(C, ppm) = -3702(+/- 534)/T - 194(+/- 49)P/T - 0.0034(+/- 0.043) logX(H2O) +0.61(+/- 0.07)NBO/T + 0.55(+/- 0.02)Delta IW +3.5(+/- 0.3) (R-2 = 0.89) At IW - 5.3 <= logfO(2) <= IW - 1: log(C, ppm) = 0.96(+/- 0.19) logX(H2O) -0.25(+/- 0.04)Delta IW + 2.83(+/- 0.34) (R-2 = 0.6) in which T is temperature in K, P is pressure in GPa, X-H2O is mole fraction of water in basalts, Delta IW 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 CO2 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.