Oxygen Isotope Compositions of Mineral Separates from Snc Meteorites: Constraints on the Petrogenesis of Martian Magmas

Introduction: Martian (SNC) meteorites are igneous rocks that range from a reduced, trace-element depleted end member to an oxidized, trace-element enriched end member [1-3] (Figure 1). One interpretation of this trend is that relatively reduced, depleted magmas are partial melts of a reduced peridotitic mantle, whereas more oxidized, enriched magmas are mantle-derived magmas contaminated by oxidized, aqueously altered crustal rocks. In this case, the compositional trend defined by SNC meteorites would be a record of crustal assimilation processes and could be used to constrain the average composition of the Martian crust [4, 5]. Alternatively, the two end members could reflect partial melting of two (or perhaps more) compositionally distinct mantle components (e.g., enriched, oxidized domains entrained in the ambient depleted mantle [6, 7]; the enriched mantle domains might be products of early differentiation of the Martian mantle). Oxygen isotope geochemistry has the potential to discriminate between these alternate hypotheses. On Earth, oxygen isotope ratios (i.e., O/O) of mantle peridotites differ systematically from crustal materials (mainly as a consequence of aqueous alteration in the shallow crust) [8 and references therein]. This contrast in oxygen isotope composition has been used to constrain crustal assimilation processes in the differentiation of terrestrial igneous rocks and to identify subducted crust in the sources of mantle-derived magmas [9]. It is conceivable that this approach can be applied to understanding similar processes for Mars. Oxygen isotope compositions of whole rock samples of the SNC’s appear to correlate with oxygen fugacity [7] (as well as various other correlated properties; Figure 1). This relationship has been interpreted as evidence that martian magmas are derived from a depleted, reduced mantle followed by variable amounts of contamination by oxidized, O-rich, aqueously altered crust. However, there are two reasons why the available oxygen isotope data may not simply reflect the δO values of Martian magmas: (1) Many SNC’s contain high-δO alteration phases from martian and/or terrestrial weathering. And, (2) many SNC’s are cumulates that would not be expected to match compositions of the magmas from which they accumulated. This study examines mineral separates subjected to various pre-treatment procedures in an effort to reconstruct the δO values of melts. Figure 1. (a) Shergottites define a trend between enriched and depleted endmembers. (b) All SNC’s define a similar trend between oxidized and depleted endmembers. Both plots are reproduced after [3].