Résumé
The relation between deformation mechanisms and water contents in the supra-subduction mantle have been investigated on two series of spinel peridotite xenoliths extracted from the base of the supra-subduction plate by the calc-alkaline volcanism in southern Kamchatka (Avacha Volcano) and the alkaline volcanism near the Lihir Island, Papua New Guinea. Coupled microstructural, petrological and geochemical studies of these samples highlight a reactive percolation of hydrous Si-rich melt synchronous to a deformation event occurring under relatively high temperature and low stress conditions, consistent with the PT conditions at the base of the lithosphere or in the asthenosphere. This reactive percolation is responsible for enrichment in pyroxene, mainly orthopyroxene, which is often localized in band parallel to the high temperature foliation. The measurement of minerals crystal preferred orientation (CPO) for both series of xenolith show that the main slip system in olivine is {0kl}[100], with either (010) or (001) as the dominant slip plane. Infrared analyses show that olivine from the both collections contains less water (1 to 8.6 wt ppm H2O) than the theoretical saturation calculated for their estimated equilibrium temperature in the spinel stability field (5 to 35 wt ppm H2O). These low water content are similar to those observed in spinel peridotites from other subduction zones and probably record both the low solubility of water in olivine at relatively low pressure and dehydration during exhumation of the xenoliths. These measured water contents as well as theoretical saturation estimations are not sufficient to change the dominant slip direction in olivine from [100] to [001], but they may nevertheless lead to a change of the dominant slip plane from (010) to (001), which is consistent with the measured olivine CPO. These results show that the hydration of the shallow upper mantle, even if it has an influence on the olivine slip systems, cannot be responsible for fast S-wave polarization parallel to the trench. The water contents in orthopyroxene and clinopyroxene are strongly variable, from 25 to 506 and from 77 to 380 wt ppm H2O, respectively. These variations may reflect the compositional disequilibrium in the sample that may result from spatially and temporally heterogeneous interactions with fluids or melts.