Experimentally determined postspinel transformation boundary in Mg₂SiO₄ using MgO as an internal pressure standard and its geophysical implications

Abstract

[1] We have determined the postspinel transformation boundary in Mg 2 SiO 4 by combining quench technique with in situ pressure measurements, using multiple internal pressure standards including Au, MgO, and Pt. The experimentally determined boundary is in general agreement with previous in situ measurements in which the Au scale of Anderson et al. [1989] was used to calculate pressure: Using this pressure scale, it occurs at significantly lower pressures compared to that corresponding to the 660-km seismic discontinuity. In this study, we also report new experimental data on the transformation boundary determined using MgO as an internal standard. The results show that the transition boundary is located at pressures close to the 660-km discontinuity using the MgO pressure scale of Speziale et al. [2001] and can be represented by a linear equation, P(GPa) = 25.12 À 0.0013T(°C). The Clapeyron slope for the postspinel transition boundary is precisely determined and is significantly less negative than previous estimates. Our results, based on the MgO pressure scale, support the conventional hypothesis that the postspinel transformation is responsible for the observed 660-km seismic discontinuity.

Stability field of phase Egg, AlSiO₃OH at high pressure and high temperature: possible water reservoir in mantle transition zone

“…, both estimated using the MgO pressure scale (Fei et al, 2004). The pressure uncertainty is about 1.5 GPa based on our previous calibrations at high temperature (Litasov and Ohtani, 2005;2009).…”

“…The mantle geotherm was based on Akaogi and Ito (1993). Pressure of the postspinel phase boundary is estimated by the equation of state of MgO (Speziale et al, 2001;Fei et al, 2004). scopes installed at The University of Tokyo or Tohoku University using Ar ion lasers of 514.5 nm with 6 mW or 488 nm with 24 mW, respectively.…”

“…The difference partly results from the use of different pressure scales: Au (Anderson et al, 1989) in Sano et al (2004) and MgO (Speziale et al, 2001) in Pamato et al (2015). Here we modified the stability filed of phase Egg determined by Sano et al (2004) based on the post-spinel boundaries determined by Fei et al (2004) (Fig. 1).…”

Influence of the Ringwoodite-Perovskite transition on mantle convection in spherical geometry as a function of Clapeyron slope and Rayleigh number

“…This reaction is widely believed to correspond to the seismic discontinuity at 660 km depth, separating the upper and lower mantle (Bernal, 1936;Ringwood, 1969;Shim et al, 2001). Mineralogical work produces estimates of the Clapeyron slope between −0.5 MPa K −1 and −3.5 MPa K −1 , with recent values for a dry mantle heading towards the less negative value (Katsura et al, 2003;Fei et al, 2004;Litasov et al, 2005;Hirose, 2002;Ito and Takahashi, 1989;Irifune et al, 1998). illustrates a cold downwelling impinging on the phase change.…”

Thermodynamic properties of wadsleyite with anharmonic effect

“…The wedgeshaped zones of metastable peridotite probably persist to the bottom of transition zone or uppermost of lower mantle because of the transitions of olivine to its high pressure polymorphs kinetically hindered in cold slabs. Therefore, the basic properties of the olivine and its high-pressure phases and their phase relations are very critical to improve our understanding to upper mantle and deep earthquake and have been extensively studied (Akaogi et al 1989;Bell and Rossman 1992;Fei et al 2004;Gillet et al 1991;Irifune et al 1998;Katsura and Ito 1989;Li et al 2007;Reynard et al 1996;Shim et al 2001;Weidner et al 1984;Wu and Wentzcovitch 2007;Yu et al 2007;Yu and Wentzcovitch 2006).…”