HR: 09:15h
AN: V41D-06    [Abstracts]
TI: Oxygen Fugacity of Basalts From Earth and Mars: Implications for Oxidation States of Terrestrial Planet Interiors
AU: * Herd, C
EM: herd@ualberta.ca
AF: Department of Earth and Atmospheric Sciences University of Alberta, 1-26 Earth Sciences Building, Edmonton, AB T6G 2E3 Canada
AB: The oxidation state of a planetary interior plays an important role in the partitioning of elements between the planet's core and mantle, the geophysical properties of the mantle, the phase equilibria of igneous rocks, and the speciation of gases in the planet's atmosphere. Determining the oxidation state of the interior of the Moon, Mars, and differentiated asteroids is difficult, because planetary samples are dominated by basaltic igneous rocks. Direct mantle samples, such as mantle xenoliths and diamond inclusions, as benefit studies on Earth, are lacking. The oxidation state of these planets' interiors is inferred from the oxygen fugacity recorded in the basaltic samples. Basalts from Mars (martian meteorites) record oxygen fugacity ranging from near the IW buffer to 3 log units above ($\sim$QFM), by several methods. The range of igneous rocks on Earth overlaps, but ranges up to $\sim$7 log units above IW, with the most oxidized samples derived from island arcs. Studies of the relationship between the oxidation state of a basalt and that of its mantle source on the Earth provide potentially important contributions to the interpretation of martian basalt oxygen fugacity and the inferred oxidation state of the martian interior. Thermodynamic considerations of ferrous-ferric mineral equilibria in the spinel and garnet facies of the Earth's mantle dictate that the oxygen fugacity should decrease, relative to the QFM buffer, with increasing pressure. Ballhaus (1995) calculated a decrease of 0.6 log unit per GPa increase, assuming a constant bulk composition. In contrast, C-H-O equilibria have isopleths of opposing slope, such that fluid composition will be dominated by more reduced species (e.g., methane) at greater depths. Ballhaus and Frost (1994) argue that C-H-O buffering influences upwelling asthenosphere, particularly by the presence of graphite, and that the oxygen fugacity of a basalt at the surface depends on the depth at which first melting occurs. This depth is where the melt is separated from graphite, becomes buffered by ferrous-ferric equilibria, and undergoes a concomitant increase in relative oxygen fugacity with decreasing pressure. Thus they argue that OIB have higher oxygen fugacity relative to MORB because the former experience first melting at greater depth than the latter. Although the details of this model are debated, such as the relative role of C-H-O fluids and the assumption of constant bulk mantle composition, it is interesting to consider its application to the petrogenesis of basalts on Mars. Assuming a constant depth of first melting of 90 km on the Earth and Mars, at the same relative oxygen fugacity, and ferrous-ferric buffering subsequent to melting, the oxygen fugacity of each erupted basalt will be different, because of the different pressure-depth relationships on each planet. A depth of 90 km on Mars is equal to $\sim$1 GPa; therefore the expected increase in oxygen fugacity is only 0.6 log units. On Earth, the increase would be $\sim$2 log units ($\sim$3 GPa). The dominant control on martian basalt oxygen fugacity appears to be the oxidation state of mantle sources, which may be inherited from the crystallization of a martian magma ocean at 4.5 Ga (e.g., Herd 2003; Borg and Draper 2003). This difference between basalts from the Earth and those from Mars may reflect a fundamental difference in planetary evolution; specifically, the preservation of "redox reservoirs" on Mars due to a lack of vigorous mantle convection. The corollary is that the oxidation state of the Earth's interior has been fundamentally altered from its initial state, by plate tectonics or other processes.
DE: 8125 Evolution of the Earth
DE: 8147 Planetary interiors (5430, 5724)
DE: 3672 Planetary mineralogy and petrology (5410)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting