Experimental constraints on the origin of the 1991 Pinatubo dacite

Crystallization (dacite) and interaction (dacite-peridotite) experiments have been performed on the 1991 Pinatubo dacite (Luzon Island, Philippines) to constrain its petrogenesis. In the dacile-H2O system at 960 MPa, magnetite and either clinopyroxene (low H2O) or amphlbole (high H2O) are the liquidus phases. No garnet is observed at this pressure. Dacite-peridotite interaction at 920 MPa produces massive orthopyroxene oystallization, in addition to amphlbole +/- phlogopite. Amphibole crystallizing in dacite at 960 MPa has the same composition as the aluminium-rich hornblende preserved in the cores of amphlbole phenocrysts in the 1991 dacite, suggesting a high-pressure stage of dacite oystallization with high melt H2O contents ( >10 wt %) at relatively low temperature (< 950 degrees C). The compositions of plagioclase, amphibole and melt inclusion suggest that the Pinatubo dacite was water-rich, oxidized and not much hotter than 900 degrees C, when emplaced into the shallow magma reservoir in which most phenocrysts precipitated before the onset of the 1991 eruption. The LREE-enriched REE pattern of the whole-rock dacite demands garnet somewhere during its petrogenesis, which in turn suggests high-pressure derivation. Partial melting of subducted oceanic crust yields melts unlike the Pinatubo dacite. Interaction of these slab melts with sub-arc peridotite is unable to produce a Pinatubo type of docile, nor is a direct mantle origin conceivable on the basis of our peridotite-dacite interaction experimental results. Dehydration melting of underplated basalts requires unrealistically high temperatures and does not yield dacite with the low FeO/MgO, and high H2O, Ni and Cr contents typical of the Pinatubo docile. The most plausible origin of the Pinatubo dacite is via high-pressure fractionation of a hydrous, oxidized, primitive basalt that crystallized amphlbole and garnet upon cooling. Dacite melts produced in this way were directly expelled from the uppermost mantle or lower crust to shallow-level reservoirs from which they erupted occasionally. Magmas such as the Pinatubo dacite may provide evidence for the existence of particularly H2O-rich conditions in the sub-arc mantle wedge rather than the melting of the young, hot subducting oceanic plate.