The stability and P–V–T equation of state of CaSiO₃ perovskite in the Earth's lower mantle

Abstract

Abstract.Energy dispersive X-ray diffraction measurements for polycrystalline CaSiO3 perovskite were carried out at in situ transition zone and lower mantle P-T conditions (P=18-96 GPa, T=1238-2419 K) using the diamond anvil cell and double-sided laser heating at the GeoSoilEnviro Consortium for Advanced Radiation Sources (GSECARS) sector of the Advanced Photon Source. An analysis of the temperature error sources in laser heating reveals that the axial and radial thermal gradients are the greatest error source. We have used measurements where the combined temperature error (lcy) from all sources is •150 K. By obtaining X-ray diffraction patterns at 8-22 GPa and 300-2200 K range, the high-temperature phase boundary between CaSi2Os+Ca2SiO4 and CaSiO3 perovskite was determined to be 14-16 GPa, in contrast to the results of previous large-volume press (LVP) measurements (9-11 GPa). The stability of cubic CaSiO3 perovskite was confirmed to 2300 km depth in the Earth's interior. No evidence of phase transformation or break down to oxides was observed. The proposed tetragonal distortion, and hence the phase transformation from distorted phase to cubic, was not observed. The combined data set of this study and earlier LVP measurements was fit to a Birch-Murnaghan-Debye equation. By fixing V0 -27.45 cm3/mol, KTO --236 GPa, and K•0 = 3.9 from recent static compression data and 00 = 1000 K, we obtain 70 = 1.92 4-0.5 and q = 0.6 + 0.3. Although data to 69 GPa and 2380 K were used in the fitting, this result is also consistent with measurements to 96 GPa. This result yields not only density and bulk modulus but also higher-order thermoelastic parameters, such as thermal expansivity and temperature dependence of bulk modulus, at lower mantle P-T condition in an internally consistent way. This direct measurement of the equation of state at lower mantle condition verifies that the density and bulk modulus of CaSiO3 perovskite at lower mantle P-T condition• are very close to seismic values (within 1.5 and 3.0%, respectively). These differences are sufficiently small that the abundance of CaSiO3 perovskite will have negligible effects on density and bulk modulus profiles for the mantle.

Simultaneous high-pressure and high-temperature volume measurements of ice VII and its thermal equation of state

“…In our P-V-T data set, the St value ranges from −0.0020 to 35 ͒ and its pressure dependence for Au and Pt is unknown, we cannot directly estimate the deviatoric stress in the sample. For reference, Shim et al 36 reported the ͉St͉ value of CaSiO 3 perovskite at P = 18-96 GPa and T = 1238-2419 K ranged from −0.005 to 0.005. In addition, Dubrovinsky et al 37 reported that deviatoric stress in MgO is as small as 0.25 GPa at T = 850 K. Therefore, the stress state in the present EHDAC was expected to be minimal.…”

Small shear modulus of cubic CaSiO3 perovskite

“…Since then, it had been considered to have a cubic crystal structure [e.g., Tamai and Yagi, 1989]. After theoretical studies predicted that a slightly distorted tetragonal structure is stable at static conditions [Stixrude et al, 1996], CaPv was experimentally confirmed to be tetragonal at room temperature [Shim et al 2000]. Recent experimental studies have shown that CaPv undergoes a phase transition from a tetragonal to cubic structure at 580 K at 50 GPa [Kurashina et al, 2004] and is cubic at LM P-T conditions [Komabayashi et al, 2007].…”

A primary natrocarbonatitic association in the Deep Earth

“…This potentially indicates, that the calcite rhombohedral structure can be stable, in the natural environment, under pressure conditions of up to at least 14-24 GPa (Shim et al 2000). In our other studied samples, calcite was identified by analogy on the basis of its composition only; aragonite orthorhombic or trigonal structure of some of those grains may not be excluded.…”