Weathering of olivine under CO2 atmosphere: A martian perspective

Abstract

Recent analyses from the Curiosity rover at Yellowknife Bay (Gale crater, Mars) show sedimentary rocks deposited in a lacustrine environment and containing smectite clays thought to derive from the alteration of olivine. However, little is known about the weathering processes of olivine under early martian conditions, and about the stability of smectite clays in particular. Here, we present a 3-month experiment investigating the weathering of forsteritic olivine powders (Fo90) under a dense CO2 atmosphere, and under present-day terrestrial conditions for comparison. The experiment also evaluates the potential effects of hydrogen peroxide (H2O2), as a representation of the highly oxidizing compounds produced by photochemical reactions throughout martian history. The weathered samples were characterized by means of near-infrared spectroscopy (NIR), X-ray diffraction (XRD), transmission electron microscopy with energy dispersive X-ray spectrometry (TEM-EDX), Mössbauer spectroscopy and thermogravimetry. The results show that a Mg-rich smectite phase formed from the weathering of olivine under CO2 conditions, although in lower abundance than under terrestrial conditions. The main secondary phase formed under CO2 turns out to be a silica-rich phase (possibly acting as a “passivating” layer) with a non-diagnostic near-infrared spectral signature. The use of H2O2 highlights the critical importance of both the redox conditions and Fe content of the initial olivine on the nature of the secondary phases.