Diffusion of the hydrous component in pyrope

Dehydrogenation experiments have been performed on natural pyrope megacrysts (PY7oAlmI6GrI4) containing 22-112 ppm total H20 by weight (H/Si is from 0.00035 to 0.0018). The concentrations ofOH in pyrope crystals were determined by Fourier-transform infrared spectroscopy. Two methods were used to obtain diffusivities. One method involved measurement of OH concentration profiles after a heating experiment. The diffusivity of the hydrous component was found to be proportional to the OH concentration along each profile, indicating that the diffusing species is not OH and is probably a minor or trace species with a concentration proportional to the square of the OH concentration (such as H2). Hence, the diffusivities are referred to as the apparent diffusivities (D*, which equals Dtc/Co, where Co is the initial OH concentration and Dt is the D* when C = Co). The other method involved measurement of the overall decrease of OH content across a wafer as a function of cumulative heating duration. From the overall decrease of OH content, the apparent bulk diffusion-out diffusivities (~) were obtained. (~, m, and Dt are different if D* depends on concentration.) The diffusivities from the two methods are consistent for a given pyrope crystal. However, the ~ values in different pyrope crystals are roughly inversely proportional to the initial OH content in the crystal, opposite the concentration dependence of D* along a diffusion profile. This apparent paradox places strong constraints on the diffusion and incorporation mechanism of the hydrous component in pyrope and indicates a control on the concentration of the diffusing species by factors other than OH content (such as FeH IFe2+). Any proposed diffusion mechanism must be able to explain this apparent paradox (both the proportionality and the inverse proportionality). The ~ values (in squared micrometers per second) in a crystal with 82-90 ppm initial total H20 can be described by In ~ = [(28.20 i: 1.30) (30580 i: 1450)]/T (where Tis temperature in kelvins and errors are at the 20"level). The ~ values are greater by a factor of ~ 3 in a crystal with 22-23 ppm initial total H20. The Dt values can be determined by dividing the ~ values by 0.347, and D* at each concentration can be determined from D* = DtCICo. The average activation energy for hydrous component diffusion in the two crystals is 253 i: 13 (20") kllmol. The diffusion-out diffusivities of the hydrous component are seven to eight orders of magnitude greater than the Fe-Mg interdiffusivities. They are so large that the OH content of a pyrope crystal can adjust to changing environmental conditions on a time scale of hours at temperatures as low as 800°C. Pyrope crystals from the mantle may dehydrogenate during ascent. Caution should be exercised in using OH content in natural pyrope crystals to infer conditions of the source region.