HYDROUS SILICATES AND WATER ON VENUS

We used thermochemical equilibrium calculations to predict stabilities of pure rock-forming hydrous silicates on Venus' surface as a functio n of elevation, atmospheric H2O and SO2 concentrations, and oxygen fug acity (fo(2),), About 50 different hydrous silicates were included in our calculations. We find that many of these are unstable on Venus's s urface because of the low atmospheric H,O-2 content of 30-45 parts per million by volume (ppmv) and the high surface temperatures (660 K on Maxwell Montes to 740 K in the plains), Hydrous Fe2+-bearing silicates are unstable due to oxidation to magnetite and/or hematite at the fo( 2) Of the near-surface atmosphere. Ca-bearing hydrous silicates are un stable because of sulfatization to anhydrite, Some Fe-free micas (e.g. ,, eastonite, eastonite-phlogopite micas), and some alkali amphiboles might be stable on Venus' surface, especially in the lower temperature highlands, We discuss hydrous mineral formation in the interior and o n the surface of Venus. We review the literature on mica and amphibole thermal decomposition and find that dehydration of phlogopitic micas and fibrous amphiboles produces (metastable) dehydroxylated anhydrides that decompose to more stable minerals at temperatures hundreds of de grees higher than the onset of dehydroxylation, These observations rai se the possibility that anhydrides formed from hydrous silicates, whic h may have been present during a wetter period in Venus' history, may persist somewhere on Venus' present surface. We discuss experiments th at could be used on future spacecraft missions to detect hydroxyl in r ocks and hydrous silicates on Venus, Finally, we review estimates of t he amount of water and OH (hydroxyl) in the Earth's mantle. Based on t his review, we suggest that even if no hydrous silicates are stable on Venus, significant amounts of water are plausibly present in surface rocks as OH in nominally anhydrous minerals, (C) 1997 Academic Press.