Physics
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufm.p12c..05m&link_type=abstract
American Geophysical Union, Fall Meeting 2003, abstract #P12C-05
Physics
3662 Meteorites, 5400 Planetology: Solid Surface Planets, 5415 Erosion And Weathering, 5480 Volcanism (8450), 6061 Remote Sensing
Scientific paper
The chemical compositions of the martian crust and mantle are commonly inferred from SNC meteorites. However, with one exception, SNCs sampled young volcanic centers and are not necessarily chemical proxies for the ancient crust. Compositional information on the Noachian crust is provided by Mars Pathfinder APXS analyses and chemistry calculated from mineral deconvolutions of Mars Global Surveyor TES spectra. Here we compare SNC compositions with TES global surface types 1 and 2 (ST1, ST2), previously interpreted as basalt and either andesite or partly weathered basalt. On an alkalis versus silica diagram, SNC lavas plot as basalt, whereas ST1 plots as basaltic andesite and ST2 as andesite (near the Pathfinder rock). On a FeO*/MgO versus silica diagram, SNCs plot within the tholeiite field, as appropriate for dry magmas. They are clearly resolvable from ST1 and ST2, which fall within the calc-alkaline field. If the ancient crust compositions represent igneous rocks, they imply a much wetter early martian mantle than previously envisioned. Alternatively, both ST1 and ST2 compositions and the Pathfinder rock could be partly weathered volcanic materials, as suggested by their positions on chemical weathering diagrams. In this case, weathering must produce silica enrichment, as well as depletion in soluble elements. These materials have high Al/Si, as do terrestrial weathering products of basalts. A weathering scenario is consistent with the 2 wt % water in alpha-mode APXS analyses of Pathfinder rocks. SNCs are the only source of data on trace elements, isotopes, and redox state. Key element ratios (e.g. Fe/Mn, K/La, Ni/Mg, Ga/Al) are diagnostic of SNCs and presumably would be similar in the ancient crust and the mantle. Oxidation and enrichment in LREE and radiogenic isotopes in shergottites may reflect crustal assimilation, so the admixed component would provide additional information on the ancient crust. Preservation of early-formed 182W and 142Nd anomalies and chondritic element ratios (Zr/Hf, Nb/Ta) established during accretion indicates that convective mixing in the martian mantle was limited. If ST1 and ST2 are igneous, they require hydrous melting and fractionation. The chemistry and petrogenesis of the ancient martian crust are distinct from the younger crust, which must represent remelting of a relatively dry mantle source that was previously depleted by formation of the voluminous early crust. Alternatively, if ST1 and ST2 represent weathered materials, we do not know how their volcanic protoliths compare to SNCs.
Grove Timothy L.
McSween Harry Y.
Wyatt Michael Bruce
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