Mechanism of olivine and glass alteration under experimental H2O-CO2 based supercritical gas: Application to modern and ancient Venus

Abstract

Extreme conditions encountered in some geological contexts (deep serpentinization, interaction of Venus atmosphere with its basaltic surface, volcanic degassing) activate mechanisms and rates of silicate alteration that are poorly understood. In the present study, we investigate the mechanisms of mineral reactions in a natural geological system at high temperature, under conditions where the low solvation of cations by fluids likely promotes surface reactions such as surface diffusion and/or local recrystallization. We focus on vitreous glasses and olivine, reputed to be the most alterable phases in volcanic rocks, by reacting samples for one week in a Ni-based alloy experimental vessel. For the framework of our experimental study, we chose to apply the deep atmosphere conditions on Venus: 470 °C and 90 bar of reconstituted Venus-like gas. We also tested the effect of water (Early Venus or wet volcanic degassing) by adding water vapor at up to 320 bar total pressure. The mineral reactions affecting the samples were identified by a set of spectroscopic surface analyses of the altered samples: Scanning Electron Microscopy, Energy Dispersive X-ray Spectroscopy, X-ray Diffraction in grazing incidence mode, X-ray Photo electron Spectroscopy and Raman spectroscopy.Samples of obsidian and tholeiitic glasses are found to be sensitive to a threshold water pressure, depending on glass composition, below which the reaction is limited to some elemental mobility in the glass (alkali enrichment, calcium loss) leading to a possibly more stable surface layer of tens to hundreds of microns. Above this threshold water pressure (ca. 50 bar H2O for the obsidian but >250 bar H2O for the tholeiitic glass), water promotes the depolymerization of the glass and the crystallization of stable minerals. This crystalline rim is less protective that the chemically modified layer.Olivine samples react differently depending on whether the olivine is isolated or included in a basaltic rock. In the latter case only, iron coatings are formed, which are identified as hematite, suggesting that this phase is not fed by olivine itself but rather by surface diffusion from neighboring Fe-rich phases. This supports the conclusions from experimental studies and orbital observations on the short-term visibility of unaltered olivine in Venus lava flows: such a coating is enhanced when Fe-bearing minerals are in the proximity of olivine. Under high water vapor pressure, Fe-bearing talc (and not serpentine) forms by a likely topotactic reaction that also incorporates silica from the gas. This talc layer may form a protective layer, implying that serpentinization of ultramafic rocks at high temperature may not be as prevalent as one might think in a gas-dominated system like the Early Venus surface.