Most Nakhlite Martian Meteorites Were Magmas, not Crystal Cumulates

Abstract The nakhlite martian meteorites, basaltic rocks with abundant crystals of augite pyroxene, have been interpreted as cumulates – formed as crystals of augite (and olivine) settled through basalt magma to accumulate at its bottom. Here, I show that the chemical compositions of most nakhlites are best explained if they represent magmas, and not crystal accumulates. The nakhlites Nakhla, Lafayette, and Y000593 are identified as cumulates; the rest are magmas. Among the magma nakhlites, abundances of all igneous incompatible elements are strongly correlated; abundances of highly incompatible elements are linearly correlated with the La/Yb ratio with R2 values from 0.7–0.95. These close correlations are not consistent with the group being cumulates, where the proportion of cumulus augite should be mostly independent of magma composition. Instead, the correlations are consistent with the nakhlites representing magmas, with the range of compositions reflecting processes including: variable enrichment in incompatible elements (i.e., metasomatism) in the source mantle; variable degrees of partial melting of the source mantle; and mixing among such magmas. Significant crustal assimilation (for the tested elements) is excluded by the correlation of Al with La/Yb. The most Al-rich nakhlite would require assimilation of 25% mass crust into the Al-poorest, which is inconsistent with thermochemical constraints. Recognition of such augite rich, pyroxenitic, magmas requires that their mantle source be similarly pyroxenitic, which implies that the Martian mantle is more heterogeneous than previously appreciated.