An impact-melt origin for lithology A of martian meteorite Elephant Moraine A79001

Mixing models using major and trace elements show that the bulk composition of lithology A (xenocryst-bearing magnesian basalt) of Elephant Moraine A7 9001 (EETA79001) can be reasonably approximated as a simple mixture of simi lar to 44% EETA79001 lithology B (ferroan basalt) and similar to 56% of All an Hills A77005 (ALHA7705) light lithology (incompatible element-poor Iherz olite). Micro-instrumental neutron activation analysis ((INAA) data on xeno cryst-free groundmass samples of lithology A show that about 20-25% of the melt phase could be dissolved Iherzolite. The bulk and groundmass samples o f lithology A have excesses in Au, which indicates either meteoritic contam ination or addition by some unknown martian geochemical process. Previous workers have suggested that lithology A was formed by either assim ilation of cumulates (like ALHA77005), by a basalt (like lithology B), or b y mixing of basaltic and Iherzolitic magmas. The former scenario is energet ically improbable and unlikely to explain the normal Fe/Mg zonation in lith ology A groundmass pyroxenes, whereas the latter scenario is unlikely to sa tisfy the constraints of the mixing model indicating the ultramafic compone nt is poor in incompatible elements. We suggest rather that EETA79001 litho logy A is an impact melt composed dominantly of basalt like lithology B and Iherzolitic cumulates like the trace-element-poor fraction of ALHA77005 or Y-793605. This model can satisfy the energetic, petrologic, and geochemica l constraints imposed by the samples. If EETA79001 lithology A is an impact melt, this would have considerable consequences for current models of mart ian petrologic evolution. It would call into question the generally accepte d age of magmatism of martian basalts and preclude the use of lithology A g roundmass as a primary martian basalt composition in experimental studies. Regardless, the latter is required because lithology A groundmass is a hybr id composition.