Kinetics of the post-garnet transformation: Implications for density and rheology of subducting slabs

Mechanisms and kinetics of the post-garnet transformation in natural pyrope were examined at 28.1-31.3 GPa and 1320-2150 K by time-resolved X-ray diffraction measurements using synchrotron radiation combined with microstructural observations of the recovered samples. The water content of samples recovered was estimated to be approximately 400 wt. ppm H₂O from infrared spectroscopic measurements. The transformation occurs by the grain-boundary reaction and reaction rims are formed along the grain boundaries of parental garnet grains. Decomposed post-garnet assemblages consisting of Mg- and Ca-perovskites, aluminous phase, and stishovite show very fine-grained symplectic texture. Analyses of kinetic data obtained have revealed that growth rates in the post-garnet transformation are strongly time-dependent. Extrapolations of the present kinetic data suggest that the post-garnet transformation cannot complete at less than ca. 1730 K, even under geological timescales, because of the slow growth kinetics. Due to the presence of the metastable garnet, subducting slabs become buoyant in the lower mantle at less than ca. 1730 K for the basaltic crust, and at ca. 1400-1600 K for the underlying peridotite layer. Rheological weakening of the basaltic crust is expected in the lower mantle through grain-size reduction by the metastable post-garnet transformation occurring under large overpressure conditions. The kinetics of the post-garnet transformation significantly affect both the density and rheology of subducting slabs in the lower mantle, and possibly has important roles in the mixing and survival of chemically differentiated slab materials.