Diagnostic Geochemical and Mineralogical Fingerprints for Mars: a Critical Reassessment

Introduction: The compositions of Martian (SNC) meteorites provide geochemical parameters thought to be diagnostic for Mars [1,2]. The validity of these geochemical fingerprints is important because SNC element ratios are the basis for estimates of the planet’s bulk composition [3]. The compositions of minerals in these meteorites are distinct from terrestrial rocks and are also hypothesized to be characteristic for Mars [4]. But are these parameters really diagnostic? Volcanic rocks in Gusev crater now constitute the most thoroughly characterized igneous province on Mars, and the geochemistry [5] and mineralogy [6] of the rocks and soils analyzed by the Spirit rover provide a means of assessing these diagnostics. Although the outcrops in Meridiani Planum are too altered to preserve primary compositions, Bounce Rock analyzed by the Opportunity rover is compositionally similar to some Martian meteorites [7]. The Mars Odyssey GRS [8] also provides global measurements for Fe and for one diagnostic element ratio, K/Th. Methods: Only APXS analyses of RAT-brush or abraded Gusev rocks are used. Gusev soils are chemically similar to the rocks and their compositions are also compared. Mineral compositions are based on Mossbauer measurements and norms calculated from APXS analyses [6]. GRS data [8] were rebinned to 5x5 grid points, resulting in large spatial resolution. Geochemical Diagnostics: Martian meteorites are depleted in Al relative to terrestrial rocks, and the Mg/Si vs. Al/Si diagram (Fig. 1) has been commonly used to distinguish these materials [3]. This discriminant has been previously criticized as arising from crystal accumulation in SNCs [9], rather than from melting of an Al-depleted mantle. Gusev rocks and soils generally plot between the SNC and terrestrial trends; cumulate rocks plot on or near the SNC line, and somewhat more altered rocks plot past the Earth line. Bounce rock plots near the SNC line. These data support the view that the low Al/Si signature of SNCs results from crystal accumulation, or at least that the Al-depleted source for SNCs is not a universal characteristic of the Martian mantle. Two distinct fractionation lines for SNCs and terrestrial basaltic and ultramafic rocks are observed in the Ni vs. Mg diagram (Fig. 2) [2]. However, Gusev rocks and soils, as well as Bounce Rock, plot along the terrestrial line. Although the higher Ni abundances in Gusev data might be attributed to a meteoritic component [10], most RATed rock compositions are still high. The Cr vs. Mg diagram (Fig. 3) [2] shows a more coherent relationship between SNCs and Gusev data.