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Rock-Atmosphere Reactions in the Context of Venus: Kinetics as Affected by the Semiconductor Condition

Session: Advancing Mineralogy and Spectroscopy Across the Solar System in Honor of MSA Roebling Medalist M. Darby Dyar

Presenting Author:

Reid Cooper

Authors:

Cooper, Reid F.¹, McCanta, Molly C.², Livi, Kenneth J. T.³, Dyar, M. Darby⁴, Rutherford, Malcolm J.⁵

(1) Earth, Environmental and Planetary Sciences, Brown University, Providence, RI, USA, (2) Earth, Environmental and Planetary Sciences, Univ of Tennessee, Knoxville, Knoxville, TN, USA, (3) Materials Science and Engineering, Johns Hopkins University, Baltimore, MD, USA, (4) Planetary Science Institute, South Hadley, MA, USA; Astronomy, Mount Holyoke College, South Hadley, MA, USA, (5) Earth, Environmental and Planetary Sciences, Brown University, Providence, RI, USA,

Abstract:

Igneous rock, created at depth under certain (arguably reducing) thermodynamic conditions and then rapidly exposed to an atmosphere of decidedly different potentials, will evolve chemically in the attempt to come into equilibrium with that atmosphere. In such reactions, the initial driving potentials (∆_rG) are often large; the initial driving forces (d∆_rG/d𝜉) are arguably infinite. The situation suggests metastable forms are likely to occur, depending on the dynamics characteristic of the material(s): the reaction texture, thus, is affected. In that orbital reflectance spectrometry of planetary surfaces perceives but sub-millimeter depths, metastable rinds on surface rocks are likely characterized, masking the bulk-rock chemistry beneath. We have pursued experimental characterization of reactions between predominantly glassy lavas—ranging from basaltic to rhyolitic—and pressurized atmospheres of CO₂ ± SO₂ to investigate reaction textures applicable to the surface of Venus. A variety of analytical techniques were employed. The reaction dynamics involved are effected by the decoupled diffusive fluxes of network-modifying cations, which stream to the free surface and react with the atmosphere to create heterophase surfaces of carbonates, sulfates and oxides; these phases reside over a resultant, silica-rich rind. The ionic flux decoupling and resultant texture results from the initial lava being a defect semiconductor, that is, the product of concentration and mobility—the transport coefficient—of electronic defects (specifically electron holes) exceeds that of all other atomic and ionic species. The relative transport coefficients of modifier alkali and alkaline earth ions, varying strongly with initial lava chemistry and temperature, affect the results, which are interpreted following diffusion-limited (parabolic) kinetics.