Host: The Mineralogical Society of Japan
Subsolidus phase relation in hydrous mid-ocean ridge basalt (MORB) was determined by high pressure and temperature in situ X-ray diffraction study. SPEED1500 was used for this experiment. We used a rhenium tube heater with LaCrO3 thermal insulator, the magnesia window for the X-ray path, the W3%Re-W25%Re thermocouple for monitoring temperature of the experiments. Pressure was calculated from the lattice constants of gold. We observed that increase of the intensity of the X-ray diffraction peaks of garnet phase clearly at 20.7 GPa and 1200C (Run mb41019), whereas we observed intensity increase of the diffraction lines of Mg-perovskite in three runs made at 21.6 GPa and 1100C (Run mb51017), 23.1GPa and 1200C (Run mb43028), and 25.5 GPa and 1200C (Run mb42027). These observations imply that transformation from the garnet bearing to the Mg-perovskite bearing assemblage occurs at around 21.6 GPa and 1100C. The boundary of the perovskite appearance in the wet MORB composition has a positive slope expressed as P(GPa)=0.0056(+-0.0009)T (C)+15.4(+-1.1) based on the pressure scale by Anderson et al. (1989). The boundary is about 1 GPa lower than that determined under the dry condition by Hirose et al. (1999). There is a crossover between the post garnet phase boundary in MORB and the post spinel phase bouhdary in peridotite at around 1300 C at the around 22 GPa by Anderson gold scale. The result has a significant implication of the subduction of the crustal materials into the lower mantle. In the low temperature conditions of the slabs below 1300C, post garnet transformation occurs at lower temperature compared to the post spinel transformation of the peridotite layer of the slabs. The crustal component in the wet slab can easily descend into the lower mantle.
