Some products of experimental dehydration-melting of amphibolite at 10 kbar

Abstract

The dehydration-melting of a natural, low-K, calcic amphibole in a piston-cylinder apparatus at 10 kbar and 750-1000 °C, for 1-9 days, has revealed a solidus temperature of less than 750 °C - lower than values reported from other amphibole experiments. Amphibolites begin to melt at relatively high temperatures in the garnet-absent field, with the formation of augite. But in our experiments the formation of garnet from fully hydrated [hornblende + plagioclase] involves the release of H_2O which is transferred directly into H_2O-undersaturated liquid (without the intervention of a vapor phase); the solidus near 10 kbar thus bends back to follow the garnet-in phase boundary, with low positive dP/dT, and it extends down to the H_2O-saturated solidus. The lower solidus temperature provides an expanded field for liquid + garnet + hornblende, allowing for the formation of HREE-depleted liquidus during amphibolite anatexis over a wider range of P-T conditions than indicated by 9ther experiments. The liquidus are more calcic and aluminous than natural continental tonalites and oceanic plagiogranites, due to the high anorthite content of the plagioclase (Au_(90)). The experimental liquidus contain 2-8 wt. % H_2O (mass balance) and those between 875 °C and 900 °C have low calculated viscosities (~ 10 Pa·s). Liquid interconnectivity has been demonstrated in corollary work on solid amphibole, so it is possible that these HREE-depleted liquidus could begin to segregate from the amphibolite source (reaching to a granulitic restite assemblage or clinopyroxene, garnet, plagioclase, and orthopyroxene) at relatively low temperatures ( < 900 °C) and very low liquid fractions ( < 5 vol.%). The abundant formation of garnet can form dense garnet clinopyroxene restite that has implications such as delamination or deep mafic crust.