Reaction infiltration instability(RII)can cause the formation of melt channels and potentially facilitate the physical transport of sulfide liquid,which contributes to the geochemical evolution of chalcophile elements in the lithospheric mantle.This study conducted some two-layer reaction experiments to explore the feasibility of reaction-driven sulfide migration along high-velocity silicate-melt channels.With increasing duration,the formation of more silicate-melt channels and the transport of more sulfide droplets into a depleted peridotite were observed due to the increase of the local permeability.However,at a longer duration,the presence of some melt-channel relics implies that melt channels are temporary and ultimately closed when the reaction infiltration of silicate melt reached equilibrium in the depleted peridotite.Furthermore,theoretical calculations indicate that the RII of the system is suppressed,which impedes the formation of melt channels.The homogeneous distribution of silicate melt in a sulfide-free experiment implies that the Zener pinning of sulfide probably enhances the RII,thereby facilitating the formation of temporary melt channels.Therefore,this study demonstrates that sufficient silicate melt disequilibrium with solid phases in a liquid source potentially promotes the mechanical extraction of sulfides during reaction infiltration of silicate melt.
Eclogites from orogens often show strong plastic deformation and high hydroxyl content. We have studied the correlation between crystallographic preferred orientations of garnet and omphacite from natural eclogites with their hydroxyl contents using the electron back-scat- tered diffraction technique. The results show: 1) Omphacite has typical L-type or SL-type crystrallographic preferred orientations, that is, [001] is distributed in a girdle in the foliation plane with a maximum parallel to lineation; (010) is distributed in a girdle normal to the lineation with a maxi- mum parallel to the foliation plane, suggesting a shear dominant deformation regime. Omphacite fabrics do not vary significantly with hydroxyl content, although the hy- drous component may cause lower flow strength. 2) Hy- droxyl can influence significantly flow properties of garnet in eclogite. Garnets behave as rigid bodies under low tempera- ture and dry conditions. Grain boundary processes will dominate the deformation and lower the flow strength of garnet under high water fugacity conditions. Garnets show no crystallographic preferred orientation in both cases. These results may have important implications for a better understanding of deformation mechanisms and associated fluid activities during deep subduction and exhumation processes.
