Basalt–atmosphere Interaction on Venus: Preliminary Results on Weathering of Minerals and Bulk Rock

Introduction: Chemical interactions between Venus’ atmosphere and surface rocks may be crucial to understanding the atmosphere’s composition [1-3], and to interpreting surface geology and physical properties [2,4,5]. Most studies have emphasized the atmosphere – its composition and ‘chemical sediments,’ and few have focused on what happens to basalts and their minerals in reaction with the atmosphere. Our preliminary re-investigation of the mineralogy of altered basaltic minerals and rocks shows that: the anorthite component of plagioclase reacts to anhydrite + andalusite + quartz; diopside pyroxene reacts to anhydrite + orthopyroxene + quartz; and bulk basalt reacts to anhydrite + cordierite + orthopyroxene + quartz (± iron oxide, depending on ƒ(O2)). Venus: Venus’ surface is at ~ 740K and ~ 96 bars pressure [5] – a metamorphic environment of ‘amphibolite grade,’ ‘hornfels facies [6];’ the paucity of H2O in Venus’ atmosphere (30±15 ppm) prevents formation of amphibole. Venus’ atmosphere, at its surface, is mostly CO2 (96.5 %), with significant proportions of S gases (SO2, 150±30 ppm, COS 4.4±1 ppm, H2S 3±2 ppm and S, 20 ppb [5,7]). The oxidation state at the Venus surface is not entirely clear, but it probably is relatively oxidizing, above the magnetite-hematite oxygen buffer, logƒ(O2) ~ -21.3 [8]. Venus’ surface materials are known only through the Soviet Venera and VEGA landers, which all landed on lava flows and volcanic constructs [9]. All analyzed materials were of basaltic composition. Here, we work from the V14 basalt, which (within its large imprecision) is similar to Earthly MORB (Table 1), and investigate how a MORB model of the V14 basalt would react at the Venus surface. Mineral Reactions: Reactions between individual minerals and Venus atmosphere were investigated via the SUPCRT code (and self-consistent database)[11], which accesses a large database of minerals, but is designed for aqueous solution chemistry. So far, computations have been done up to 623K with manual extrapolation to higher T with data from [12]. Neither database has values for relevant sulfates besides anhydrite, nor for carbonate or sulfate liquids [13]. The anorthite component of plagioclase can react with the Venus atmosphere: CaAl2Si2O8 + SO3 CaSO4 + Al2SiO5 + SiO2 (anhydrite + andalusite + quartz; corundum + quartz is always unstable [14]). This reaction should proceed [2,6], although its equilibrium SO3 is close to that of the Venus atmosphere [7,15]; our calculations imply that plagioclase with anorthite activity of ~0.3 should be in equilibrium with anhydrite, andalusite, quartz, and the atmosphere. The albite component of the plagioclase remains in feldspar (jadeite is unstable at Venus surface P [6]). Among mafic minerals, diopside is calculated to react via CaMgSi2O6 + SO3 CaSO4 + MgSiO3 + SiO2. This reaction should proceed to near completion in the Venus atmosphere [7,15,16], leaving pyroxene with diopside activity ~0.05. Enstatite and forsterite remain stable, while ferrosilite and fayalite may oxidize/sulfidize to iron oxides/sulfides and quartz. Bulk Rock Reaction: Our second approach is to model the reaction of a bulk basalt (Table 1) with the Venus atmosphere. This model would correspond to alteration of basalt glass, or of a crystalline basalt with easy element diffusion between crystals. We determine the equilibrium mineralogy of that basalt at Venus surface p-T-X, using analog systems, chemography, and thermochemical calculations [17]. Analog and synthetic systems and their chemography are summarized in [6], but few such systems take into account the high ƒ(SO2) of the Venus surface. Thermochemical calculations are done (so far as possible) with Thermocalc ®