Lawsonite Veins in Eclogite and Bluschist as Recorders of Subduction Fluids

Lawsonite is a hydrous mineral (containing up to ~12 wt% H2O) that is stable at depths >45 km in all but the hottest subduction zones, and is therefore a major carrier of water into the mantle. However, lawsonite is rarely preserved unaltered during exhumation, so rocks containing fresh lawsonite are important opportunities for examining the geologic record of fluid-rock interaction in subduction zones. Of particular interest are lawsonite veins, as these may preserve conditions and pathways of high-pressure fluid flow. The Tavsanli Zone (Turkey) contains, among high-pressure marbles and micaschists, lawsonite blueschist layers and, locally, eclogite pods containing veins of lawsonite + phengite + garnet and glaucophane and/or omphacite. Some lawsonite veins in eclogite are bordered by lawsonite-glaucophane-garnet-phengite. We are conducting systematic major and trace element analyses of minerals in veins, vein margins, and host eclogite and blueschist. Results to date indicate that all major phases in and adjacent to lawsonite veins in eclogite are zoned, including lawsonite, which displays zoning in Fe, Cr, and Ti. White mica in all assemblages (veins, blueschist margin, eclogite) has Si = 3.4-3.5 pfu, and some grains in veins and at vein margins are zoned, with Fe- and Na-richer cores. Paragonite inclusions occur in garnet in lawsonite veins. Garnet and glaucophane display oscillatory zoning in and near veins, and omphacite has Fe-rich rims relative to core regions. These results will be used, in concert with trace element analyses and 3D textural analysis of veins by X-ray computed tomography, to understand the processes of fluid-rock interaction in lawsonite eclogite and blueschist during subduction.