Amphibolite dehydration-melting: sorting out the solidus

Abstract

Amphibolite melts under vapour-absent conditions by dehydration-melting, during which H_2O released from hornblende is transferred directly into H_2O-undersaturated silicate liquid. Five independent 1991 studies on amphibolite dehydration-melting differ from each other and from earlier conceptual treatments. Five solidi between 7 and 10 kb vary from 975°C to 740°C. Two solidi above 10 kb are near-vertical, near 975°C and 850°C. Some differences are due to different amphibolite compositions and mineralogy. We use the results of earlier experiments involving garnet, supported by our own new results, to construct a closed-system phase diagram for a simple, fully-hydra ted amphibolite (hornblende+ plagioclase) showing the solidus and the melting interval for ( hornblende+ garnet + other minerals+ liquid). The amphibolite vapour-absent solidus is defined by the beginnings of two multivariant reactions: (1) a near-vertical curve (large positive d P/dT) where the formation of augite dominates; and (2) a near-horizontal curve a t higher pressures where the formation of garnet dominates. The solidus curve bends back (estimated near 900°C, 9 kb) with slope changing to low positive dP/dT. The new phase diagram expands the field for liquid generation with garnet-amphibolite residues to much lower temperatures and pressures than the other recent experimental results, increasing the range from which small amounts of hydrous granitoid melts may be segregated by compaction. The reversed, near-horizontal slope of the solidus near 10 kb has interesting petrological consequences.