Isothermal bulk modulus at ambient P T

Isothermal bulk modulus at ambient P-T condition (KT0; GPa) is one of the most important elastic properties for understanding the density and sound velocity features of Earth materials at the P-T conditions of the Earth\'s interior. Although the KT0 value of the Mg2SiO4-Rw has been reasonably well determined (e.g., Weidner et al., 1984; Hazen, 1993; Li, 2003), that of the Fe2SiO4-Rw is still in discrepancy, ranging from 187.3(17) to 212(10) GPa (Mao et al., 1969; Nestola et al., 2010). As a result, the dependence of the KT0 on the XFe of the Rw, varying from ∼15 to 36 GPa/XFe (Weidner et al., 1984; Sinogeikin et al., 1998), is currently still a topic to debate. Another potential compositional factor which may strongly affect the bulk modulus of the Rw is its water content (e.g., Inoue et al., 1998; Ye et al., 2012). Although Rw can host up to ∼2.7 wt% water (Kohlstedt et al., 1996; Bolfan-Casanova et al., 2000), it may be nearly anhydrous in the MTZ since recent studies using seismicity (Green et al., 2010), electromagnetic induction (Kelbert et al., 2009), electronic conductivity measurement (Yoshino et al., 2008), and sound velocity analysis (Mao et al., 2012) suggested that the water content in the MTZ probably does not exceed 0.1 wt%. On the other hand, the only terrestrial occurrence of Rw observed as inclusion in a diamond from Brazil indicates ∼1 wt% water in the MTZ (Pearson et al., 2014), although it remains unclear whether this phenomenon has global implication or not. Many advanced experimental methods such as direct high-P compression, ultrasonic measurement and Brillouin spectroscopy have been used to determine the isothermal bulk modulus of the Rw, and extensive and invaluable knowledge has been obtained. Direct high-P buy Torin1 is commonly carried out by using a diamond-anvil cell (DAC) with a hydrostatic (usually P < 10 GPa) or quasihydrostatic (P > 10 GPa) pressure state since most liquid pressure media solidify at pressures lower than ∼10 GPa (Klotz et al., 2009; Liu et al., 2011a). Due to the large isothermal bulk modulus of the Rw (∼200 GPa), a hydrostatic pressure of about 10 GPa can press the Rw to about 95% of its ambient volume only, leading to relatively low accuracy in the determined isothermal bulk modulus. Ultrasonic measurement is intrinsically precise when single crystal or polycrystalline sample with homogeneous grain size, random grain orientation and full density (or zero porosity) is used (Sato, 1977; Liu et al., 2011b). Further, ultrasonic measurements can now be conducted at pressures higher than 3 GPa (Li et al., 1996, 1998; Li, 2003), at which a full elimination of any residual micropores and microcracks in the studied samples has been demonstrated (Liebermann, 1975; Liebermann et al., 1977; Rigden and Jackson, 1991). Ideally, Brillouin spectroscopy measurements can fully constrain the elastic tensors of a phase (Weidner et al., 1984; Wang et al., 2003a). Other experimental methods have been used as well; for a recent review, see Li and Liebermann (2014). On the other hand, it is well known that there are some specific difficulties with the synthetic Rw samples. Ringwoodites synthesized at different P-T conditions with different experimental durations might contain different amounts and types of defects (Smyth et al., 2003), and attain different magnitudes of order-disorder phenomenon (Hazen et al., 1993; Kudoh et al., 2000; O\'Neill et al., 2003). They might have different quantities of contaminant water in their structures (Higo et al., 2006), since hydrogen can easily penetrate through most experimental capsule materials (Liu et al., 2006) and subsequently enter the Rw structure. If the dependence of the isothermal bulk modulus of the Rw on the XFe variable is relatively small, which is probably the case (Mao et al., 1969; Finger et al., 1986), all these experimental complexities then would add together to prevent a very accurate determination of this dependence and lead to controversial results (Sinogeikin et al., 1998; Higo et al., 2006).