Major and Trace-Element Chemistry of Minerals in Lithologies A and B in Martian Meteorite EETA79001: Petrogenesis Revisited

EETA79001 is a unique shergottite composed of two mafic lithologies (termed LithA and LithB) that are separated by an igneous contact. Both lithologies have basaltic compositions; however, LithA contains megacrysts of olivine, orthopyroxenes, and chromite whereas LithB does not; also, LithA is finer-grained than LithB. Currently, the literature is in disagreement regarding the formation of this unique meteorite, especially regarding LithA. Different formational theories (e.g. fractional crystallization, magma mixing, assimilation, and impact melting) have their own constraints (chemical, thermal, or petrographic). This study uses petrographic observations combined with major- and trace- element compositions within minerals to investigate the petrogenesis of LithA. Previous formational theories are addressed and a new model is proposed.

The groundmass composition of LithA is important in explaining the relationship between megacrysts and the groundmass. Previous estimates do not consider weighted compositional averages or overgrowths on olivine megacrysts. In this study, a new estimate of the LithA groundmass composition is obtained using weighted averages of zoned minerals (major-element), and includes the overgrowth rims on megacrysts.

Here, I introduce a new LithA formational model that involves the mixing of cold megacrysts with magma. This hybrid model suggests that the interaction of the megacrysts and magma altered the heat balance and changed the crystallization sequence, as evidenced by the major-element trend in pyroxenes and the finer grain size of the LithA groundmass. The megacryst overgrowths and groundmass then crystallized, and was later followed by the removal of the late-stage fractionated melt (liquid ~Mg# 20). This new model would explain the formation of the overgrowths and avoids the heat constraints associated with magma mixing and assimilation. However, a short-coming of this theory lies in the necessity for the late-stage removal of the last ~10% of the melt, necessary to modify the original magma composition to that of the observed LithA groundmass composition.