Complex Magmatic Processes on Mars - Inferences from the Snc Meteorites

Comparison of the major-element, incompatible-trace-element, and isotopic compositions of the parent magmas of the SNC (shergottites-nakhlites-Chassigny) meteorites with eucritic, lunar, and terrestrial basalts shows that thier parent body (Mars) has a unique magmatic style. The major-element and isotopic compositions of the Nakhla and Chassigny meteorites (1.3 b.y.) suggest extensive melting of source regions with much stronger time-integrated light rare earth element (REE) depletions than those observed on Earth. The incompatible-element patterns of these magmas, however, indicate extremely strong enrichments of the light rare earth elements, suggestive of miniscule degrees of partial melting. Such compositional patterns are commonly found in basalts from terrestrial intraplate ocean islands (OIB), but relative enrichments are much greater in the martian magmas. Also, the Nakhla incompatible-element pattern (chondrite-normalized) shows prominent relative depletions of high-field-strength elements (HFSE), such as Ta, Nb, Hf, and Zr, that are not typically observed in OIB but are characteristic of island arc basalts (IAB). Closer examination of the depletions of the HFSE relative to the REE indicates that the martian pattern (Ta-Nb > Hf-Zr) is actually more similar to that of terrestrial carbonatites than to LAB. Because of their low viscosity, carbonatitic magmas at the base of the melt zone would greatly facilitate the two-phase porous flow process proposed by McKenzie (1984, 1985) at melt fractions <1%, and thus might also account for the strong REE fractionation when mixed at higher levels with more extensive silicate melts. Furthermore, CO2, which expands the orthopyroxene liquid field at high pressure, might account for the unusually high estimates of CaO in the Nakhla parent magma. The shergottites have incompatible-element patterns-depletion of the light-REE, positive-HFSE/REE anomalies-complementary to Nakhla that suggest they were generated from a source depleted in a Nakhla-like component. This complementary relation is considered with a younger age for the sherogttites such as 180 m.y. (Jones, 1986). If this young age is correct, then the Antarctic shergottites were generated by 2 Sm/Nd-enrichment process (fractional fusion?) and came to the martian surface relatively uncontaminated The classic shergottites (Shergotty and Zagami), however, also assimilated a long-term light-REE-enriched component that presumably is ancient crust. If this crust formed at approximately 4.4 b.y. and was complementary to the Nakhla source, then it had epsilon(Nd) approximately -20 at 180 m.y. ago. Also, this crust would have Sm/Nd approximately 0.15, which is well within the range of enriched basalts, which in turn suggests that if primordial melting were volatile free or dominated by CO2, the ancient martian crust would be composed of transitional to alkali basalts, whereas if the ancient melts were H2O-rich, the ancient crust would be andesitic.