Abstract
We report major element compositions and water contents in upper mantle minerals from peridotites transported by silica undersaturated, mafic alkaline lavas from three volcanoes Rockeskyllerkopf, Dreiser Weiher, and Meerfelder Maar of the Eifel Volcanic Field (West Germany). The hydrogen concentrations (expressed in ppm wt. H2O) obtained from unpolarized and polarized Fourier transform infrared (FTIR) spectroscopy give water contents for olivine, enstatite and diopside of ~ 6 ppm wt. H2O, ~ 200 ppm wt. H2O and ~ 285 ppm wt. H2O, respectively. The hydrogen concentration in individual olivine grains is strongly heterogeneous whereas that in pyroxenes is homogeneous. Profiles measured across crystallographically oriented single-crystals of olivine using polarized infrared radiation reveal hydrogen depleted rims that are interpreted to be due to partial dehydration by ionic diffusion during the ascent of the xenolith to the surface. Using experimentally obtained diffusion coefficients for hydrogen in olivine at high temperature and high pressure, we estimate that the duration of the dehydration for the spinel-bearing xenoliths is limited to a few hours yielding rates of magma ascent from 3 ms− 1 to 12 ms− 1. Our study suggests that the water contents of the upper mantle based solely on measurements of mantle-derived olivine, when concentration is not homogeneous, underestimate the true water content of the equilibrated uppermost mantle and that pyroxenes are a better proxy to constrain uppermost mantle water contents.