Phengite-based chronology of K- and Ba-rich fluid flow in two paleosubduction zones.

Subduction recycles aqueous fluids from slab and sediment to the mantle. Subduction zones are long-lived, but time scales for fluid-rock interaction within subduction complexes are uncertain. Large-ion lithophile elements (potassium and barium) were added to eclogite (subducted basalt) during high pressure/temperature metamorphism via phengite crystallization from subduction zone fluids. Phengite grains from eclogite blocks and their metasomatic selvages yielded 40Ar/39Ar ages across grains and between samples that indicate 25 and 60 million years of fluid-rock interaction in the Samana Complex, Dominican Republic, and the Franciscan Complex, California, respectively.