Major and Trace Element Modeling of Lree-depleted Shergottites via Fractional Crystallization from a Y980459-like Parent

Introduction: The martian basaltic meteorites display enormous variation in their trace element and isotopic compositions. A subset of the shergottites including NWA1195, SaU005/008, DaG476, Y980459, Dhofar 019, and QUE94201 have many similar geochemical features, in addition to being the oldest shergottites. Strongly light-rare-earth element (LREE)-depleted REE patterns, low initial Sr and high initial Nd isotopic ratios indicate that this group is derived from either a single source or several very similar source regions that are characterized by depletions in incompatible elements. Despite these similarities, a wide range of mineral modes and compositions, as well as major element abundances, indicates that they are derived from parental magmas of different composition. One end of the mineralogical and compositional range defined by this group is represented by Y980459. This sample is a porphyritic basalt containing pyroxene and Fo 86 olivine that is in equilibrium with the groundmass. This rock is thought to represent a liquid that is in near-equilibrium with its mantle source [1, 2]. It is not surprising, therefore, that the bulk rock is characterized by low abundances of incompatible elements relative to many other shergottites belonging to the group. In contrast, QUE94201 is a coarse grained porphyritic basalt containing pyroxene, maskelynite, and phosphates with minor abundances of oxides and impact melt glass. The composition of this sample is also thought to represent a melt [3]. Nevertheless, a low bulk rock Mg# of 38, the presence of relatively Fe-rich silicates, and high whole rock incompatible element abundances relative to the other shergottites in this group demonstrate that QUE94201 is significantly more evolved. The other samples (NWA1195, DaG476, SaU005/008, and Dhofar 019) are cumulates. Although accumulation of phases makes whole-rock compositional differences between these samples difficult to interpret, many geochemical traits not strongly affected by accumulation (e.g. mineral core compositions, parental liquid Mg#'s) are intermediate between Y980459 and QUE94201. Compositional differences observed in this group may reflect either compositional differences in their sources or elemental fractionation processes that occurred after their primary parental melts left the