Mathematics – Logic
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p13d..01u&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P13D-01
Mathematics
Logic
[5455] Planetary Sciences: Solid Surface Planets / Origin And Evolution, [5480] Planetary Sciences: Solid Surface Planets / Volcanism, [6225] Planetary Sciences: Solar System Objects / Mars
Scientific paper
The Mars Exploration Rover Spirit has encountered volcanic and volcaniclastic rocks having diverse alkaline compositions in the Noachian-age (~3.8-4.5 Ga) Gusev crater. Among them, we focus on Wishstone Class tephrites which have pyroclastic textures and are unusually enriched in incompatible elements (e.g. >5 wt% P2O5) with low silica contents. The high-phosphorous tephrite signature is not attributable to secondary aqueous alteration but represents an igneous rock composition. Moreover, these high-P2O5 whole-rock compositions cannot readily be explained by fractionation of other magmas in Gusev. We show that the high-P2O5 whole-rock compositions plot above solubility curves of merrillite (Ca-phosphate) in a diagram of P2O5 versus aluminosity, suggesting that mechanical admixture of merrillite is required. A source supplying merrillite cannot be a common silicate magma; instead, it could be a carbonatitic. Considering the pyroclastic textures of Wishstone Class and their geologic context, we propose that the Wishstone Class represents an alkaline-rich igneous rock suite that has mechanically mixed xenocrystic merrillites, probably during explosive volcanic eruption; the merrillites crystallized from carbonatitic melt produced by melting of a carbon-bearing Martian mantle. It has been debated whether CO2 was the effective greenhouse gas in the early Mars. To maintain persistent liquid water on the Martian surface, several bars pressure of CO2 is required, which is approximately three orders of magnitude higher than that on present-day Mars. In contrast, other greenhouse gases (e.g. methane) have been proposed, because no large carbonate deposits or significant atmospheric loss that accounts for the early CO2-rich atmosphere have been observed. Moreover, a recent thermodynamic calculation suggests that, under the redox state of the Martian meteorite source mantle (IW to IW+1), transport of CO2 to the Martian atmosphere has been quite limited and may not be sufficient to account for the greenhouse conditions. Our study suggests that, at least beneath Gusev crater, carbon should have resided in the Martian mantle. Moreover, the Martian mantle should have been enough oxidized (~QFM) that the carbon could have existed as carbonatitic melt instead of graphite or diamond. This is consistent with the redox state of Gusev basalts (~QFM) estimated by the Mossbauer spectrometer. We conclude that the Martian mantle would have been already oxidized in the Noachian era within a billion years after the core formation that requires a reducing redox condition (~IW-2). This could provide a new constraint on the important unsolved issue regarding what timeframe mantle oxidation could have occurred during the early stage of the formation of terrestrial planets such as Mars and the Earth.
Clark Ben C.
McSween Harry Y.
Usui Takuya
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