HR: 15:10h
AN: V23D-07    [Abstracts]
TI: Ventilation of CO₂ from a reduced mantle and the climate history of Mars
AU: * Hirschmann, M M
EM: Marc.M.Hirschmann-1@umn.edu
AF: U. Minnesota, Dept. Geol. & Geophys., Minnepolis, MN 55455, United States
AU: Withers, A C
EM: withe012@umn.edu
AF: U. Minnesota, Dept. Geol. & Geophys., Minnepolis, MN 55455, United States
AB: A fundamental question regarding Martian history is the storage of carbon in the Martian interior and the processes and fluxes leading to ventilation of carbonaceous volatiles to the Martian crust and atmosphere. Evidence for liquid water on the Martian surface during the Late Noachian and Hesperian epochs (<3.7 Ga) likely requires a substantial accumulation of greenhouse gases in the Martian atmosphere. Most commonly, it is assumed that this gas was principally volcanogenic CO₂. A key consideration regarding the extraction of C during partial melting of the Martian mantle is that the Martian mantle is thought to be reduced, with fO2 between IW and IW+1. Therefore, carbon likely resides chiefly as graphite, and extraction of CO₂ from a graphite- saturated mantle can be estimated using thermodynamic models developed for terrestrial basalts by John Holloway. For melts derived from a graphite-saturated mantle at IW and IW+1, calculated liquids will have ~100 ppm and ~1000 ppm CO₂, respectively, assuming basalt extraction at 1300 °C and 1 GPa. These concentrations do not change appreciably if temperature and pressure were both higher. Formation of the 50 km thick Martian crust at 4.5 Ga would therefore have liberated an atmosphere of 0.4-4 bars of CO₂. Only the higher value is sufficient to maintain a strong greenhouse on early Mars, suggesting either that the Martian mantle must not be more reduced than IW+1 or that gases other than volcanogenic CO2 were responsible for the earliest Martian greenhouse atmosphere. It is sometimes suggested that degassing of CO2 associated with the gigantic (3 X 10⁸ km³) Tharsis magmatic province was responsible for sustaining a significant Martian greenhouse later in Martian history (<3.7 Ga). However, at IW to IW+1, only 40-400 mbars CO₂ could have been ventilated. Thus, if these calculations are applicable to the Martian mantle, either the greenhouse gases responsible for sustaining an equable climate compatible with liquid H₂O on Mars were not derived chiefly from volcanogenic CO₂ or the Martian mantle was more oxidized than IW+1.
DE: 3619 Magma genesis and partial melting (1037)
DE: 3672 Planetary mineralogy and petrology (5410)
DE: 5480 Volcanism (6063, 8148, 8450)
DE: 8408 Volcano/climate interactions (1605, 3309)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting