Kinetics of the Pyroxene-Garnet Transformation: Preliminary Results

Previous experimental studies have shown that in a "pyrolite" mantle, Ca-poor pyroxene is completely dissolved into garnet at about 10 GPa, and Ca-rich pyroxene at about 16 GPa. Current mineralogical view of the Earth's upper mantle and the transition zone is mainly based on these results with the assumption that the pyroxene-garnet transformation proceeds with relative ease under the mantle conditions. In order to assess the validity of this critical assumption, knowledge about kinetics of the transformation is required. Here we report the preliminary results from an experimental study designed to investigate kinetics of the pyroxene to garnet transformation. A 2000-ton uniaxial-split-sphere high-pressure apparatus was used to achieve mantle conditions of 6 to 14.5 GPa and 1200-1400 °C. Powdered KLB-1, whose bulk composition is close to "pyrolite", was used as starting material. The powder sample was annealed at 6 GPa and 1300 °C for at least 4 hours before the target pressure and temperature were applied. Run duration ranged from 4 to 21 hours. Run products were analyzed using a micro-diffractometer and an electron microprobe. It was found that substantial amount of Ca-poor pyroxene was still present in all samples, including the sample in which olivine has completely transformed to wadsleyite. Concentrations of oxide indicators for phase transition in garnet, such as SiO₂, CaO, and Al₂O₃, were fairly unchanged from the pre-annealed material, suggesting that the transformation is extremely slow in occurring. We also observed exsolutions of Ca-rich pyroxene or garnet in Ca-poor pyroxene, but they were too small to be analyzed. These results indicate that the pyroxene-garnet phase transformation can take complicate paths, and metastable pyroxene may exist in the Earth's lower upper mantle and the transition zone.