Other
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p33d1782s&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P33D-1782
Other
[5410] Planetary Sciences: Solid Surface Planets / Composition, [6225] Planetary Sciences: Solar System Objects / Mars
Scientific paper
The primary oxygen fugacity of basaltic melts reflects the mantle source oxidation state, dictates the crystallizing assemblage, and determines how the magma will evolve. On Earth, variabile fO2 among basaltic melts is linked to mass exchange between the surface and the interior by tectonic processes (subduction). Martian basalts represent samples of the planet's interior and we currently have two data sets with which to work: the Amazonian age shergottitic meteorites and early Hesperian age basaltic rocks examined in situ by the Spirit rover in Gusev Crater. The Gusev basalts are diverse and range from the K-poor Adirondack class (0.02 wt% K2O) to K-rich Backstay class (up to 1.2 wt% K2O). By combining the Fe3+/FeT of igneous minerals (olivine, pyroxene, and magnetite) determined by Mössbauer spectrometer, we estimated primary fO2 for the Gusev basalts to be -3.6 to 0.5 ΔQFM (quartz-fayalite-magnetite buffer). General similarity between the fO2 estimated for the Gusev basalts and ranges in fO2 for the shergottitic meteorites (-3.8 to 0.2 ΔQFM; Herd, 2003; Goodrich et al., 2003) suggests that the overall range of fO2 for the Martian igneous rocks and mantle is relatively restricted. Also like the shergottites (Herd et al., 2003), estimated fO2 of three Gusev classes (Adirondack, Barnhill and Irvine) correlates with a proxy for LREE enrichment (K/Ti). This suggests mixing between melts or fluids derived from reservoirs with contrasting fO2, such as reduced, LREE-depleted mantle and more oxidized, LREE-enriched crust. One exception to this trend is the high-K Backstay class, which has low fO2 like the low-K Adirondack class (-3.6 ΔQFM). Backstay and Adirondack are probably melts of similarly reduced mantle and contrasting alkali concentrations may reflect partial melting processes. If the high K2O (and K/Ti) of Backstay was obtained through partial melting of primary (i.e., never melted) mantle, then its relatively low fO2 may imply that the development of the more oxidized, LREE-enriched reservoir was not caused by melting processes alone. Instead, the more oxidized, LREE-enriched reservoir may signify high degrees of fractionation, fundamental source heterogeneities formed during planetary differentiation, and/or metasomatism by oxidizing fluids. Based on the available fO2 data for the Martian interior, tectonic processes have not led to sufficient recycling of oxidized surface material into the Martian interior to entirely affect the overall oxidation state of the mantle. However, the oxygen fugacity of well-characterized samples from Mars would allow us to continue to test this hypothesis. Furthermore, by combining fO2 with other data sets, such as radiometric ages, rare earth elements and particularly Sm and Nd isotopes, we could model and discuss the development of oxidized and reduced reservoirs in an absolute timeframe.
McCoy Timothy James
Schmidt Mariek E.
Schrader C. M.
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