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N.M. Johnson, B. Fegley, Jr. (Planetary Chemistry Laboratory, Wash Univ., St. Louis)
The surface of Venus is a hot, dry desert and thermodynamic models predict that hydrous minerals (with a few exceptions) are unstable on its surface. Therefore, any hydrous minerals present are probably relicts from a wetter era in Venus' history. However, hydrous mineral survival depends on dehydroxylation rate(s), which are unknown. We therefore studied the kinetics of tremolite dehydroxylation. The results were extrapolated to Venus' present-day surface temperature to calculate the maximum lifetime of tremolite.
Tremolite dehydroxylation was studied at temperatures from 1090-1238K in CO2 and N2. Tremolite, Ca2Mg5Si8O22(OH)2, decomposes to water vapor and anhydrous minerals via parabolic (diffusion-controlled) kinetics. Thus, smaller grains decompose faster. Our measured parabolic rate constant (kp) is log10kp=8.93(±0.05)-24,100(±391)/T cm2/hr. The activation energy for the dehydroxylation reaction is 461±7 kJ/mol. The results predict that once formed, tremolite would survive dehydroxylation for geological timescales at current surface temperatures on Venus.
This work was supported by grant NAG5-4565 from the NASA Planetary Atmospheres Program.
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