Garnet Growth during Amphibolite Anatexis: Implications of a Garnetiferous Restite

Abstract

Formation of a garnet clinopyroxenite restite may have important physicochemical consequences for deep crustal and mantle processes. We have experimentally simulated the dehydration-melting of amphibolite, and report the modal and compositional evoltuion of the mafic granulitic mineral assemblage (the restite), with an emphasis on garnet. A natural, low-K, calcic amphibolite was powdered and run in sealed, gold capsules in a piston-cylinder apparatus at 10 kbar, 750-1000°C, 1 to 9-day runs, and fO_2~ Ni-NiO. Garnet first appears in 8-day runs at 825±25°C. Above 850°C, clinopyroxene and poikilitic garnet comprise most of the restite. Garnet peaks at 950°C (at ~32 vol.% of the material within the capsule, or 41 vol.% of the restite assemblage) and disappears between 975-1000°C. There is no cross-over of Fe-Mg partitioning between coexisting liquids and garnets. Liquids remain Mg-poorer than garnets throughout the melting interval but do increase in Mg# with increasing temperature; thus the hotter liquids may be less likely to react with mantle wedge peridotite. The modes are consistent with REE patterns of Archean tonalitic-trondhjemitic rocks generated by the partial melting of garnet-bearing mafic crust. A garnet clinopyroxenite with 41 vol.% garnet has a density of 3.5g/cm^3, which may induce lower crustal delamination in hot, nonthickened, amphibolitic crust. This crust is seismically indistinguishable from lithosphere or mantle. Because garnet growth traps water-bearing hornblende inclusions, the restite may contain up to 0.3 wt.% water. Thus, delivery of poikilitic garnet to the mantle, possibly by delamination or by subduction, may be an important mechanism for adding water to the mantle.