The Earth / Mars dichotomy in Mg / Si and Al / Si ratios : Is it real ?

In light of the new Martian rock data, TES surface compositions, and larger number of known SNC meteorites, the use of Mg/Si vs. Al/Si ratios to distinguish between the Earth’s crust and the Martian crust must be reconsidered. Magmatic fractionation and crystal accumulation strongly affect the Al/Si and Mg/Si ratios of bulk compositions, thereby obscuring information about mantle characteristics. Previously observed deviations between SNC and terrestrial igneous rocks are not robust when diverse terrestrial cumulates are considered. SNC meteorites could represent mixtures of cumulate ferromagnesian minerals and evolved terrestrial-like liquids. Introduction: It has been noted by numerous workers that there appear to be significant differences in chemistry between Martian SNC meteorites and terrestrial rocks. These differences are often shown as deviations of the SNCs from the Mg/Si and Al/Si variation along the Earth’s “geochemical fractionation line” (GFL; or “Earth’s crust line”) developed from ultramafic peridotite xenoliths [1] (Fig. 1). A GFL was formulated by [2] for Mars (i.e., the “Mars crust line”) and they used this along with other SNC chemical characteristics to infer the Martian mantle composition, one that is significantly lower in Al and higher Fe than the terrestrial mantle. As the population of identified SNC meteorites has grown it is now clear that many Martian meteorites do not actually fall on the “Mars crust line” and in fact, lie in a broad field to the low Al/Si side of the Earth’s crust line (Fig. 1). Furthermore, MER Rover Spirit data on Martian basalts [3], TES Surface Types I and II [4], and recent SNC findings (e.g. Yamato 980459 [5]) yield compositions much closer to the “Earth’s crust line” [6], [3]. Before complex petrologic models are developed to explain the occurrence of both types of chemical characteristics on Mars, it is worth taking a closer look at the effects of mineral accumulation. The effects of accumulation: The GFL can be considered a simple mixing line. As such, it can be used to evaluate accumulation processes, with bulk rock compositions displaced along the line from liquid compositions towards higher Mg contents during accumulation of mafic minerals to form cumulate rocks. As with any mixing line, however, the slope and intercepts will change depending upon the nature of the liquids and accumulated phases. This is exemplified by the mixing line between dunite and the basalt used to formulate pyrolite ([7]; Fig. 1); this line is displaced from the GRL [1] because of the use of a dunite rather than a pyroxene-bearing peridotite. The shift to lower Mg/Si contents because of the accumulation of pyroxene or Fe-rich olivine (Fig. 1) suggests that terrestrial cumulate rocks may plot off of the Earth’s crust line and into the field of SNC meteorites. That this is indeed the case for many intra-plate magmatic cumulates is exemplified in Figure 1 by cumulate xenoliths from Hawaii [8], Deccan Trap [9], and the French Massif Central [10]. This deviation of terrestrial cumulate materials from the GFL (“Earth’s crust line”) clearly calls into question the usefulness of this line as a unique indicator of terrestrial characteristics.