GARNET STABILITY IN ARC MAGMAS – EXPERIMENTAL RESULTS FOR CASCADE ARC BASALT, ANDESITE, AND DACITE

Some intermediate and evolved magmas in Phanerozoic arcs, as well as widespread granitoids of the Archean, have low concentrations of heavy rare earth elements (HREE) and Y consistent with garnet in their source or evolution. Cascade arc examples include some andesites and dacites around Mt. Shasta and Lassen and dacites at Mt. St. Helens. This signature is generally attributed to three processes: 1) crystallization-differentiation in the garnet stability field, 2) partial melting of deep-crustal garnet amphibolites or granulites, or 3) partial melting of subducting eclogite and subsequent interaction of melt with mantle wedge peridotite. Modern experimental results on garnet stability in common arc compositions are sparse, making it difficult to assess the merits of each scenario. We report new melt-present phase relations and compositions at garnet saturation for a Cascades calc-alkaline basalt, Mt. Shasta andesite, and Mt. St. Helens dacite. Synthesis conditions were oxidized (Re-ReO₂ buffered), with 2 to 6 wt% H₂O, 0.9 to 1.6 GPa, and 900 to 1300°C. Garnet was not produced at 0.9 GPa in any of the compositions. At 1.2 GPa, garnet was produced as thin overgrowth rims (<5 µm) on introduced garnet seeds coexisting with dacitic liquids at temperatures 950°C. At 1.3 GPa, garnet grew readily with no seeds from 900 to 1100°C coexisting with liquids ranging from rhyodacite to peraluminous basaltic andesite, and at 1.4 GPa, garnet was stable as hot as 1150°C in metaluminous basalt liquid. Garnet was produced on the liquidus only in the Mt. St. Helens dacite composition, for which inverse experiments determined a multiple saturation point at 960°C and 1.4 GPa with 7 wt% dissolved H₂O at which dacitic melt coexists with garnet, plagioclase, orthopyroxene, and amphibole. Based on these results, garnet would not be stable near the liquidus of typical Cascade arc magmas anywhere in the crust, ruling out production of garnet-signature dacites as direct melts of garnet amphibolite or garnet granulite crustal sources. The finding that ordinary dacite can be a multiply-saturated liquid coexisting with garnet at upper mantle pressures suggests that garnet-signature evolved magmas may originate from the mantle from stalled and differentiated mafic magmas, or from mafic cumulates foundering into the mantle and partially melting.