Experimental constraints on the genesis of hydrous arc dacites

Abstract:

Shallow, pre-eruptive storage conditions are well known for dacitic magmas of the Cascade arc through melt inclusions, FeTi oxides, and experiments. Zircon ages indicate protracted (>10⁵ yrs) incubation for some Cascade dacites, but their origins in the deep crust have not been established. Accordingly, we report experimental results identifying the P–T–H₂O conditions under which such liquids could segregate from (hornblende-) gabbroic sources. Two compositions were investigated: (1) MSH-Yn dacite (SiO₂: 65 wt%) from Mount St. Helens’ voluminous Yn tephra, and (2) MSH-Yn + 10% cpx to force saturation with cpx and locate the cpx + melt = amphibole peritectic. H₂O-undersaturated (3-9 wt% H₂O) piston cylinder experiments were conducted at pressures, temperatures, and fO₂ appropriate for the lower to middle crust (400 to 900 MPa, 825 to 1100°C, and ReReO₂). Results for MSH-Yn indicate near-liquidus equilibrium with a cpx-free hornblende-gabbro residue (amphibole, plagioclase, magnetite, +/- opx and ilmeno-hematite) with 6-7 wt% H₂O, 925°C, and 700-900 MPa. Opx disappears down-temperature consistent with the reaction opx + melt = amphibole. Cpx-added phase relations are similar in that once ~10% cpx crystallizes, multiple saturation is attained with cpx, amphibole, and plagioclase, +/- opx, at 6-7 wt% H₂O, 950°C, and 700-900 MPa. Plagioclase-amphibole-cpx saturated liquids diverge from plagioclase-amphibole-opx saturated liquids, consistent with the MSH-Yn dacite composition marking a (pseudo-invariant) distributary reaction point where amphibole appears down-temperature and opx + cpx cease to coexist. Liquid compositions and temperatures stall at reaction points, irrespective of whether the source is crystallizing or melting, accounting for the compositional and thermal uniformity of voluminous dacites that feed into the shallow storage regions of arc volcanoes.