In situ high P‐T X ray diffraction studies on three polymorphs (α, β, γ) of Mg₂SiO₄

Abstract

Unit cell volumes of the beta ([•) and spinel (7) phases (Mg2SiO4) have been measured under simultaneous high pressure and high temperature using synchrotron X ray radiation, in a cubic anvil apparatus. With volume-temperature data at constant pressure, we determine the average volume thermal expansion coefficients of the [• phase from 724 to 872 K at 7.6 GPa to be 2.28(_0.45)x10-5/K and of the 7 phase from 759 to 962 K at 9.8 GPa to be 1.71(_0.14)x10-5/K. Thermodynamic relations are used to constrain the temperature derivative of the isothermal bulk modulus (Kr) from the high-pressure thermal expansion data:/)K•/i)T)• is found to be -2.7(_0.5)x10 -2 GPa/K for the [• phase and -2.8(_0.3)x10 -2 GPa/K for the 7 phase. Unit cell volumes of the olivine (o0 phase, back-transformed from the [• and 7 phases at high temperatures, have been measured under pressure at temperatures above the Debye temperature; using thermal pressure equations, we find (/)K•/i)T)• for the ot phase to be -2.1(_0.2)x10 -2 GPa/K. These new data on the temperature derivatives of the bulk modulus for the three phases are consistent with an upper mantle containing 60 to 65% olivine and the absence of a velocity signature for the [• to 7 phase transition near 520 km depth. [1967], Anderson et al. [1990, 1992a], and Chopelas and Boehler [1989,1992], there has been relatively little experimental work in this field [e.g., Bridgman, 1935, 1940; Yagi, 1978; Yagi et al., 1987; Boehler and Kennedy, 1980a, 1980b]. In the absence of experimental P-V-T data, (3Kvt3T)•, has been calculated mostly from the ambient pressure measurement of the adiabatic bulk modulus (Ks) as a function of temperature. For high-pressure phases of mantle minerals, such elasticity measurements at high temperature are very limited due to the small crystal size that causes experimental difficulties. X ray P-V-T experiments, therefore, are needed to constrain (3Kvt3T)•,. For [•-(Mgo. 84, Feo. 16)28iO4, (3K•3T)•, has been derived by Fei et al. [1992] from unit cell volume data under simultaneous high P-T conditions in a diamond anvil cell. Since in their experiments, the pressure decreased systematically as the temperature was increased, (3Kvt3T)•, was obtained by fitting all the P-V-T data to a thermal pressure model. Our experiments, using a cubic anvil apparatus (SAM-85) with synchrotron X ray radiation, enable us to measure the lattice parameters accurately under relatively constant pressure over certain temperature ranges and, consequently, to directly determine the thermal expansion at high pressure and high temperature; our technique is complementary to the diamond anvil cell, which can achieve higher pressure. The purpose of this paper is to present for the first time the results of high P-T thermal expansion measurements on the [5 and 7 phases of Mg2SiO4. As both phases transformed back to the olivine structure at high temperatures, we also measured the unit cell volumes of the back-transformed t• phase under high pressures and above the Debye temperature up to 1569 K. With the...

P‐V‐Vp‐Vs‐T measurements on wadsleyite to 7 GPa and 873 K: Implications for the 410‐km seismic discontinuity

“…To avoid transforming wadsleyite to the olivine phase, the measurements were conducted only at temperatures up to 600øC (873 K) where the transformation is inhibited by kinetics. As suggested in a previous study using powder or polycrystalline samples, back transformation from wadsleyite to olivine occurs at 7 GPa and 973 K [Meng et al, 1993]. In Figure 3 we compare the X-ray diffraction spectra for our wadsleyite sample at ambient conditions, at 8 GPa and room temperature, and at the peak P-T conditions of 7.3 GPa and 873 K. It is clear that within the detectability of the current experiment, the wadsleyite phase was preserved up to the peak pressure and temperature conditions of the current study (Figure 2).…”

“…Recently, elasticity measurements have been extended up to transition zone pressures at room temperature using ultrasonic techniques in the multianvil apparatus (P > 12 GPa [Li et al, 1996a]) and Brillouin spectroscopy in the diamond anvil cell (DAC) [Zha et al, 1998;Sinogeikin et al, 1998]. To date, the elastic properties of wadsleyite at high temperatures have only been estimated from equation of state studies using X-ray diffraction techniques [Meng et al, 1993;Fei et al, 1992;Hazen et al, 1990]. No information about the temperature derivative of the shear modulus for wadsleyite has been published, except the brief report by Li et al [1998b].…”

High pressure crystal chemistry of hydrous ringwoodite and water in the Earth?s interior

“…The cell bulk modulus indicates a linear trend with H content. (Meng et al, 1993) for all samples. We then estimated the bulk modulus for the anhydrous sample at 1200 ºC to be about 156 GPa, and at 600 ºC to be about 170 GPa.…”

Earth Materials at High Pressure and Temperature: Recent Contributions from in situ Synchrotron X-Ray Diffraction

“…Deviatoric stresses can be determined from the diffraction of a standard material pressurised within the cell, and recent studies have shown that it can be characterised quite readily. In high-P/T experiments under constant ram load competing factors which influence the measured pressure include the deviatoric stress (which saturates at some 3 GPa), the thermal pressure due to the thermal expansion of the sample (which will act to increase P) and mechanical weakening of the anvils and cell materials on increasing T (which reduces the effective P) [17,18].…”

“…On the other hand, mineralogical studies formed only a minority of the early work done at the Photon Factory. Many [17,19] of them have been single-stage experiments on the phase relations of the olivine-wadsleyite-ringwoodite phases of the (Mg,Fe)2SiO4 system; perovskite phases such as CaSiO3 [18], neighborite [20], and (Mg,Fe)SiO3 [21][22][23][24][25] have also been investigated using both single-stage and two-stage arrangements. In the latter, one or more of the second-stage cubic anvils is constructed from sintered diamond to allow the transmission of the diffracted beam.…”