Post-garnet transition in a natural pyrope: a multi-anvil study based on in situ X-ray diffraction and transmission electron microscopy

Post-garnet transition in a natural pyrope under the conditions close to 670 km discontinuity was studied by in situ X-ray diffraction (XRD) using an MA8-type multi-anvil apparatus and analytical transmission electron microscopy (ATEM) on recovered samples. The pyrope with a composition of (Mg0.72Fe0.17Ca0.11)(3)Al2Si3O12 directly breaks down into (Mg, Fe)SiO3-Al2O3 perovskite, CaSiO3 perovskite, stishovite, and the 'NAL' phase with increase in density of about 10%. The presence of the 'NAL' phase that was first discovered in the diamond-anvil cell (DAC) experiments was confirmed by multi-anvil experiments. The transition boundary can be expressed as P (GPa) = 14.4 + 0.0065T (K) between 1200 and 2000 K, suggesting that, below 1400 K, the transition pressure may be lower than 24 GPa, the pressure corresponds to the 670 km discontinuity. These results, together with available information on garnets with various compositions, suggest that chemical compositions of garnets strongly affect both transition conditions and post-garnet high-pressure mineral assemblages. (C) 2000 Elsevier Science B.V. All rights reserved.