Element abundances, patterns, and mobility in Nakhlite Miller Range 03346 and implications for aqueous alteration

Abstract

Nakhlite Miller Range (MIL) 03346 contains many secondary phases resulting from aqueous processes, including formation of poorly crystalline iddingsite-like veins in olivine, the precipitation of Ca-sulfates and Fe,K-sulfates from evaporating fluids, alteration of titanomagnetite to secondary Fe-oxides, and the dissolution of magmatic Ca-phosphates and residual glass in the mesostasis. A surprising variety of alteration products occur in association with olivine in MIL 03346, including: patches of incipiently-altered olivine, large Si-enriched olivine-hosted veins (up to 10μm across) some of which are complex in morphology and are composed of several phases, small Fe,S(±K)-rich veinlets that crosscut the Si-enriched veins, Ca-sulfates filling cracks in olivine, and secondary Ca-phosphates. Elemental abundances and distributions in these alteration products are consistent with the mobilization of elements from readily dissolved phases in the mesostasis such as phosphates and residual glass. Under favorable weathering conditions, these phases dissolve more readily than pyroxenes, plagioclase, and even olivine at low pH. The occurrence (crosscut and devolatilized by the fusion crust) and composition of Si-enriched alteration veins in olivine are consistent with their formation on Mars. Si-enriched, poorly crystalline alteration products and secondary Ca-sulfates commonly occur in nakhlites, but the habit and composition of these alteration products differ between meteorites. Elemental distributions in these secondary phases suggest at least two episodes of alteration have affected MIL 03346, and subtle differences in secondary minerals and chemistry indicate that each nakhlite experienced its own unique alteration history either on Mars, Earth, or both. The variable Al content and range of morphologies of the olivine-hosted Si-enriched veins suggest variable alteration conditions consistent with a water-limited regime. If the secondary phases in MIL 03346 can be shown to have formed on Mars, their chemistry will provide important clues to the aqueous environments and processes at the time of their formation. However, elevated S and REEs, Ce anomalies, and association of secondary minerals with post-impact cracks and voids indicate that terrestrial weathering has significantly affected MIL 03346. This work highlights the difficulty in distinguishing pre-terrestrial aqueous alteration from later chemical weathering of susceptible mineral phases even in meteorites with limited terrestrial modification.