Trace element systematics in convergent margin metamorphic and volcanic rocks show that subducting slabs release fluids of changing composition as a function of depth. Volcanic transects across arcs record declines in H2O-soluble elements (B, Cs, As, and Sb) with increasing depth that parallel declines with increasing metamorphic grade in “subduction complex” associations. These paired, prograde declines point to decreasing inputs of H2O-rich fluids from the subducting slab. Uniform K, Ba, REE and 10Be levels across arcs suggest that slab-derived fluxes of different compositions persist to greater depths. Slabs returned to the mantle via subduction should have profoundly fractionated chemical signatures, and substantial fluid releases should occur through forearc regions. Serpentinites from the Marianas forearc show elevated B contents, and fractionated trace element signatures suggesting inputs of fluids like those released from metamorphosed slabs at low temperatures.