Deep Fluids in Subducted Continental Crust

Observations from ultrahigh-pressure metamorphic rocks combined with experimentally determined phase relations provide a framework for understanding fluid-mediated mass transfer in deeply subducted continental crust. At temperatures below 650 degrees C, aqueous fluids derived from dehydration reactions involving hydrous phases contain limited amounts of solutes. At temperatures of 700-800 degrees C, a supercritical fluid with a composition intermediate between aqueous fluid and hydrous melt might be present. The most significant mass transfer at ultrahigh-pressure conditions occurs at 800-1000 degrees C, where subducted crust undergoes partial melting related to the breakdown of the hydrous mineral phengite. Partial melting leads to a significant change in the composition and density of the rocks, and also affects the rheology of deeply subducted crust.