Exsolution kinetics of majoritic garnet from clinopyroxene in subducting oceanic crust

摘要

Mechanisms and kinetics of exsolution of majoritic garnet from Ca-Tschermak-rich clinopyroxene were examined at 14.0-15.6GPa, 1250-1590°C, and H_2O-undersaturated conditions (800-1170wt. ppm H_2O) by time-resolved in situ synchrotron X-ray diffraction measurements. Textural observations of recovered samples revealed that majoritic garnet had exsolved along the grain boundaries of parent clinopyroxene. Analyses of the kinetic data obtained indicate that exsolved majoritic garnet grew by diffusion-controlled growth, and the growth kinetics can be described by k~2 (m~2/s)=7.4×10~(-5) exp[-389(kJmol~(-1))/RT]. The exsolution of majoritic garnet from clinopyroxene would be kinetically inhibited at temperatures below ～800°C in geological time scales (10~5-10~6years) when the parent grain size is larger than 1mm. We constructed possible phase relations of subducting mid-ocean ridge basalt (MORB) taking the exsolution kinetics into consideration. When the effect of reaction kinetics is considered, the density contrast between the MORB component of a subducting slab and surrounding mantle becomes significantly reduced relative to that calculated for the equilibrium MORB assemblage.