Metastable transformations of eclogite to garnetite in subducting oceanic crust

Abstract

The eclogite-garnetite transformation experiments were conducted at 11 GPa and 1000-1550 °C using synthesized polycrystalline eclogite with MORB composition as a starting material. We found that the eclogite-garnetite transformation proceeds by two stages, (1) precipitation of majoritic garnet from clinopyroxene and (2) formation of majoritic garnet from original pyropic garnet by absorption of clinopyroxene. The 1st stage in the transformation proceeds at 1000 °C for 180 minutes, however the 2nd stage was not observed even at the high temperature of 1550 °C on the same time scale. The differences in kinetics are due to contrasting diffusivities between clinopyroxene and garnet. The kinetic effect would modify the mineralogy and rock microstructures in MORB, which provides important implications for dynamics of subducting oceanic crust by changing the density and viscosity relation.