Sho Yokoshi

Olivine‐wadsleyite transition in the system (Mg,Fe)2SiO4

Phase relations of the olivine-wadsleyite transition in the system (Mg,Fe) 2 SiO 4 have been determined at 1600 and 1900 K using the quench method in a Kawai-type high-pressure apparatus. Pressure was determined at a precision better than 0.2 GPa using in situ X-ray diffraction with MgO as a pressure standard. The transition pressures of the end-member Mg 2 SiO 4 are estimated to be 14.2 and 15.4 GPa at 1600 and 1900 K, respectively. Partition coefficients for Fe and Mg between olivine and wadsleyite are 0.51 at 1600 K and 0.61 at 1900 K. By comparing the depth of the discontinuity with the transition pressure, the temperature at 410 km depth is estimated to be 1760 ± 45 K for a pyrolitic upper mantle. The mantle potential temperature is estimated to be in the range 1550-1650 K. The temperature at the bottom of the upper mantle is estimated to be 1880 ± 50 K. The thickness of the olivine-wadsleyite transition in a pyrolitic mantle is determined to be between 7 and 13 km for a pyrolitic mantle, depending on the efficiency of vertical heat transfer. Regions of rapid vertical flow (e.g., convection limbs), in which thermal diffusion is negligible, should have a larger transition interval than stagnant regions, where thermal diffusion is effective. This is in apparent contradiction to short-period seismic wave observations that indicate a maximum thickness of <5 km. An upper mantle in the region of the 410 km discontinuity with about 40% olivine and an Mg# of at least 89 can possibly explain both the transition thickness and velocity perturbation at the 410 km discontinuity.

High-pressure generation in the Kawai-type apparatus equipped with sintered diamond anvils: application to the wurtzite–rocksalt transformation in GaN

Abstract Technical development of the Kawai-type apparatus has briefly been described from the original single-staged split-sphere to the recent 6–8 double-staged devise in which sintered diamond (SD) is equipped as the anvil material. In this article, high-pressure generation using sintered diamond cubes with 14.0 mm edge length and 1.5 mm corner truncation has been reported together with results for the wurtzite–rocksalt transformation in GaN at high pressure. Octahedral specimen assembly with 5.0 mm edge length containing GaN powder in an MgO capsule (pressure marker) was compressed in a DIA-type press SPEED MkII recently installed at SPring-8, Japan. Pressure value was determined from compression of the MgO marker and the transformation of GaN was monitored by means of in situ X-ray diffraction. The maximum pressure of 63.3 ± 0.4 GPa was reached based on the MgO scale by Jamieson et al. at 300 K. Onset of the wurtzite–rocksalt transformation in GaN was observed at 54 GPa and 300 K and at 51.4 GPa and 750 K, suggesting a negative Clapeyron slope of −170 K/GPa. The rocksalt type of GaN is judged to be quenchable to the ambient conditions because no reverse transformation was observed. Noticeable change in electric resistance may not be accompanied with the transformation. Moreover, the sluggishness also prevents the phase transformation in GaN from being used as a pressure fixed point.

Determination of high-pressure phase equilibria of Fe2O3 using the Kawai-type apparatus equipped with sintered diamond anvils

Abstract Phase equilibria of Fe2O3 have been studied up to 58 GPa and 1400 K using the Kawai-type multi anvil apparatus equipped with sintered diamond anvils. Identification of phases and pressure determination has been carried out by means of in situ X-ray observation using synchrotron radiation at SPring-8. Hematite (phase I) successively transforms to the Rh2O3(II)-type structure (phase II) and then to an orthorhombic structure (phase III) with increasing pressure. The transformations of hematite into high-pressure phases have been observed only at temperatures higher than 500 K, which is not concordant with previous results obtained by using the diamond anvil cell. Volume changes accompanied by the I-II and II-III transformations are calculated to be -2.8 and -5.0%, respectively. The phase boundary between I and II phases and that between II and III have been proposed to be P (GPa) = -0.015 T (K) + 44.2 and P (GPa) = -0.005 T (K) + 48.7, respectively. Possible correlation between a Mott transition and the phase stabilities may be concealed at room temperature due to slow reaction kinetics of the structural transformations.

Equation of state of (Mg0.8,Fe0.2)2SiO4 ringwoodite from synchrotron X-ray diffraction up to 20 GPa and 1700 K

We present a temperature-pressure-volume ( T-P-V ) equation-of-state (EOS) of (Mg 0.8 ,Fe 0.2 ) 2 SiO 4 ringwoodite based on in situ high- T and high- P synchrotron X-ray diffraction experiments up to 1700 K and 20 GPa with a multi-anvil apparatus at SPring-8. The third-order Birch-Murnaghan equation was applied to the data between 300 and 900 K, while a constant ( ∂P/∂T) V fitting at temperatures higher than 900 K. By fixing previously measured volume thermal expansivities at 0 GPa and the isothermal bulk modulus at 300 K and 0 GPa of K 0,300k = 189.7 GPa, we derived the T-P-V EOS parameters of (Mg 0.8 ,Fe 0.2 ) 2 SiO 4 ringwoodite using least squares to be ( ∂K 0 /∂ P ) T = 4.57(7) and ( ∂K T /∂T) P = −0.0283(13) GPa/K between 300 and 900 K, and ( ∂P/∂T) V = 0.00535(11) GPa/K at temperatures above 900 K. These values compare very well with previously measured EOS data for Mg 2 SiO 4 ringwoodite of ( ∂K 0 /∂P) T = 4.6(2), ( ∂K T /∂T) P = −0.029(1) GPa/K, and ( ∂P/∂T) V = 0.0052 − 0.0055 GPa/K at high temperatures. At P = 20 GPa and T = 1800 K, as representative conditions in the lower part of the mantle transition zone, the relative V and K T values of (Mg 0.8 ,Fe 0.2 ) 2 SiO 4 ringwoodite with respect to the values at 300 K and 0 GPa are found to be V/V 0 = 0.9424, K T / K 0,300K = 1.263, based on the present EOS. These results for (Mg 0.8 ,Fe 0.2 ) 2 SiO 4 ringwoodite, combined with the corresponding data for Mg 2 SiO 4 ringwoodite, describe that the effects of Fe substitution for Mg on the T-P-V EOS of ringwoodite with (Mg 0.9 ,Fe 0.1 ) 2 SiO 4 , thought to be the composition in the mantle transition zone, are virtually negligible for V/V 0 , and less than 1% for K T /K 0,300k .