Thermal equation of state of (Mg0.91Fe0.09)2SiO4 ringwoodite

Abstract

In situ synchrotron X-ray diffraction (XRD) experiments were conducted using the SPEED-1500 multi-anvil press of SPring-8 on (Mg0.91Fe0.09)2SiO4 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 V0=527.83(7) Å3 and isothermal bulk modulus KT0=187 GPa yielded a pressure derivative of isothermal bulk modulus KT′=4.41(1), a temperature derivative of bulk modulus (∂KT/∂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 KT′ and (∂KT/∂T)P are consistent with previous studies on Mg2SiO4 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 Mg2SiO4 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.