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Published May 1993 | public
Journal Article

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.

Additional Information

© 1993 University of Chicago Press. Manuscript received March 31, 1992; accepted June 28, 1992. This research was supported by the Earth Science section of the U.S. National Science Foundation, grant EAR 89-04375. Caltech Division of Geological and Planetary Sciences Contribution 5169. We thank John Armstrong for his help in obtaining and processing the EMP-ARP data, and Mark Fahnestock for his help in writing the EMP-ARP phase discrimination program. A review by David Bell improved an early version of the manuscript. The manuscript was further improved with constructive criticism by Dana Johnston, Bob Newton, and an anonymous reviewer.

Additional details

Created:
August 22, 2023
Modified:
October 18, 2023