High-pressure high temperature generation over 40 GPa using Kwai-type multianvil apparatus with carbide anvils

Abstract:

Kawai-type multianvil apparatus are widely used as high-pressure devices to generate pressures to the uppermost lower mantle using tungsten carbide (WC) anvils. Combination with sintered diamond (SD) anvils allows generating pressure above 30 to usually 60 GPa. However, it is difficult to use SD anvils practically, because they are much expensive than WC anvils. We therefore developed experimental techniques to generate pressures over 40 GPa using a Kawai-type apparatus with WC anvils particularly for studying phase relations in multi-component systems in the lower mantle down to 1000 km depth. We used a 15-MN Kawai-type multianvil apparatus with DIA-type guide blocks with careful optimization of a cubic compression space formed by first-stage anvils. Pressures were generated using hard WC anvils (TF05, Fujilloy Co., Ltd) with truncation of 1.5 mm and taper of 1.0ᴼ with a semi-sintered MgO + 5wt.%Cr₂O₃ octahedron as a pressure medium. Pressure was calibrated by detecting resistance changes of Bi, ZnS, GaP and Zr at room temperature. The maximum achievable pressure at 2000 K, which was generated using a Re heater in a LaCrO₃ thermal insulator, was examined by the Al₂O₃ content in aluminous bridgemanite. Al₂O₃ rods were placed at the both side of the sample in a furnace. The starting material was sintered ilmenite-type Mg₃Al₂Si₃O₁₂ synthesized at 1200 K and 26 GPa (Kubo and Akaogi, 2000). We observed the ω-β transition in Zr (34 GPa) (Ono and Kikegawa, 2015) at a press load of 8 MN at room temperature. The sample synthesized at 2000 K and 15 MN has a composition nearly equal to pyrope and the LiNbO₃-type structure. The LiNbO₃ structure of the recovered sample with pyrope composition suggests that it had the perovskite structure before recovery. The composition of this sample suggests generation of 44 GPa at a high temperature of 2000 K, according to the phase relations in the MgSiO₃-Al₂O₃ system (Liu, private communication).