The Rate of Pyrite Decomposition on the Surface of Venus

Abstract

We report the results of a detailed experimental study of the kinetics and mechanism of pyrite (FeS2) chemical weathering under Venus surface conditions. Pyrite is thermodynamically unstable on the surface of Venus and will spontaneously decompose to pyrrhotite (Fe7S8) because the observed S2 partial pressure in the lower atmosphere of Venus is lower than the S2 vapor pressure over coexisting pyrite and pyrrhotite. Pyrite decomposition kinetics were studied in pure CO2 and CO2 gas mixtures (CO-CO2, Ar-CO2, H2-CO2, CO-CO2-SO2) along five isotherms in the temperature range 390-531°C. In all gas mixtures studied, pyrite thermally decomposes to pyrrhotite (Fe7S8), which on continued heating loses sulfur to form more Fe-rich pyrrhotites. During this process the pyrrhotites are also being oxidized to form magnetite (Fe3O4), which converts to maghemite (γFe2O3), and then to hematite (α-Fe2O3). This reaction sequence is supported by X-ray diffraction data, Mössbauer spectra, optical microscopy, and prior literature studies. The reaction rates for pyrite thermal decomposition to pyrrhotite were determined by measuring the weight loss. The thickness of the unreacted pyrite in the samples provided a second independent reaction rate measurement. Finally, Mössbauer spectra done on 42 of the 115 experimental samples provided a third set of independent reaction rate data. All three independent methods give the same reaction rate within experimental uncertainties. Pyrite decomposition follows zero-order kinetics and is independent of the amount of pyrite present. The rate of pyrite decomposition is apparently independent of the gas compositions used and of the CO2 number density over a range of a factor of 40. The derived activation energy of ∼150 kJ mole-1 is the same in pure CO2, two different CO-CO2 mixtures, and a ternary CO-SO2-CO2 mixture. Based on data for a CO-CO2-SO2 gas mixture with a CO number density ∼10 times higher than at the surface of Venus and a SO2 number density approximately equal to that at the surface of Venus, the rate of pyrite destruction on the surface of Venus varies from about 1225 ± 238 days/cm at the top of Maxwell Montes (∼660 K) to about 233 ± 133 days/cm in the plains of Venus (∼740 K.) These lifetimes are very short on a geological time scale and show that pyrite cannot exist on the surface of Venus for any appreciable length of time.