Rates of dehydration of olivines from San Carlos and Kilauea Iki

The results show that the apparent diffusivity of total H+ varies both over time and for olivines with different defect populations, and thus H+ does not diffuse at inherently site-specific rates. These variations are due to rapid reactions that may occur between sites as a function of changing defect concentrations. Our experiments reveal three new phenomena: (1) H+ that is associated with the Ti-clinohumite defect, [Ti-2H], can enter and exit olivine at the fastest rate measured. (2) The rate of H+ loss from specific infrared absorption peaks, and in total, may change with progressive dehydration due to the buildup of Fe3+ and redistribution of defects, particularly the rehydration of [Ti-2H]. (3) The following Arrhenius laws appear to apply to total H+ diffusion out of most natural olivines that have been studied experimentally and naturally during dehydration: Da = 10−5.4exp(−130/RT); Db = 10−6.9exp(−130/RT); and Dc = 10−6.6exp(−130/RT), where R is the gas constant 8.314 J/mol K; T is the temperature, the units of the diffusivities (D) are in m2/s; and the activation energy is 130 kJ/mol. This rate is slower than proton-polaron redox rate but faster than proton-metal vacancy diffusion and appears to characterize olivine of Fo ∼90 composition in a wide range of conditions, from self-diffusion, to melt inclusion re-equilibration, to mantle xenolith and volcanic phenocryst zonation during ascent.