Experimental exploration of volcanic rocks-atmosphere interaction under Venus surface conditions

Abstract

Abstract This study presents an inventory of possible chemical reactions affecting, or having affected, the Venus surface. Fluid rock reactions are simulated using experiments under conditions close to the present surface. Slabs or powder of several natural and synthetic silicate material (crystalline fresh basalt, altered basalt, obsidian, pumice and basalt glass) were reacted at 475\u202f°C in CO2-H2O-H2S-SO2-CO gas mixture. Most of the runs were carried out at roughly 90\u202fbars with a duration of one week, some experiments having longer (one month) or shorter (one day) durations. The role of H2O content was explored through a wide range of water pressure: from dry gas for the current Venus conditions up to 590 bars (86%H2O) for early Venus (or other early terrestrial planets). The gas phase was sampled before the completion of the runs for chemical analysis of major gas components (CO, H2S, SO2) as well as trace elements possibly released by the rocks. The altered samples were examined by a suite of mineralogical and chemical techniques (scanning and transmission electron microscopy, X-ray diffraction and spectroscopy). In dry atmosphere, the redox potential of the gas was close to the Ni/NiO buffer (−21.3 to −27.3 log fO2), thus close to the current Venus conditions. The sample alteration is tenuous and limited to surface oxidation of glasses and coating of olivine by iron oxides, as well as the general deposition of (Ca,Na)SO4 at the sample surface. The oxidation of glass is reflected in the formation of magnesioferrite under the surface and is accompanied by the release of Ca, Mg and Na into the gas phase or mineralized as sulfate at the surface. In wet atmosphere, obsidian recrystallizes into a mixture of plagioclase and amphibole while basaltic glass produced non-expandable clays minerals: chlorite-type (2:1:1) at the surface and likely celadonite (2:1) below the surface. Olivine is preserved. Using obsidian (the most alterable material) as a proxy of aluminosilicates, we discuss the surface reactions operating under supercritical conditions, and we used a shrinking-core equation for modeling the long-term reactions. These parametric exploration offer new insights into processes having affected the surface of Venus and contribute to the discussion of open questions such as the fate of water or the lifetime of vitreous dust or fine grain material if present in the current or past Venusian environment. Longer duration experiments will provide more kinetic parameters that can be extrapolated to the geologic history of Venus.