Thermal equation of state of (Mg 0.91Fe 0.09) 2SiO 4 ringwoodite

Abstract

In situ synchrotron X-ray diffraction (XRD) experiments were conducted using the SPEED-1500 multi-anvil press of SPring-8 on (Mg 0.91Fe 0.09) 2SiO 4 ringwoodite (Rw), whose composition is similar to that expected in the Earth’s mantle transition zone. Pressure–volume–temperature data were collected using a NaCl or MgO capsule up to 21 GPa and 1273 K. A fit to high-temperature Birch-Murnaghan (HTBM) equation of state (EOS) with fixed values of ambient cell volume V 0=527.83(7) Å 3 and isothermal bulk modulus K T0 =187 GPa yielded a pressure derivative of isothermal bulk modulus K T ′=4.41(1), a temperature derivative of bulk modulus (∂ K T /∂ T) P = −0.028(5) GPa K −1, and a volumetric thermal expansivity α= a+ bT with values of a=1.9(2)×10 −5 K −1 and b=1.2(4)×10 −8 K −2. These properties are consistent with the analysis using the thermal pressure (Th-P) EOS. The derived K T ′ and (∂ K T /∂ T) P are consistent with previous studies on Mg 2SiO 4 ringwoodite. However, consistent with measurements at 0 GPa, the present equation of state yields significantly higher thermal expansivity than derived by diamond anvil experiments on Mg 2SiO 4 ringwoodite to 30 GPa and 700 K. On the basis of the equation of state, the density jump around 660 km depth expected for pyrolitic homogeneous mantle (caused by ringwoodite → Mg-perovskite (MgPv) + magnesiowüstite (Mw) and garnet (GRT)→MgPv transitions) was estimated. The density jump calculated for pyrolite (9.2%) is significantly larger than that across the 660 km discontinuity derived by recent seismological studies (4–6%). One possible explanation for this is that the recent seismic data reflect only the sharp transition concerned with the Rw→MgPv+Mw transition.