标题：Anhydrous ringwoodites in the mantle transition zone: Their bulk modulus, solid solution behavior, compositional variation, and sound velocity feature
摘要：AbstractThe isothermal bulk moduli of anhydrous Mg2SiO4-ringwoodite (Rw) and Fe2SiO4-Rw, and other 4–2 oxide spinels at ambientP-Tcondition have been evaluated, and empirically fitted to a model asKT0 = 270.8(300) + 0.343(59)*V0 + 23.04(269)*EN-total, whereKT0is the isothermal bulk modulus in GPa,V0the unit-cell volume in Å3andEN-totalthe electronegativity total of all cations in the chemical formula. This model well reproduces all data used in its calibration, and may be used to predict theKT0of other 4–2 oxide spinels. Combined with the generally linear volume–composition relationship of the Rw solid solutions along the join Mg2SiO4–Fe2SiO4, this model leads to a much smaller composition effect on theKT0:KT0 = 185.0(1) + 7.0(1)*XFe, whereXFeis the atomic ratio Fe/(Fe + Mg). Furthermore, a bulk composition-independent compositional variation withPhas been disclosed for the Rw at theP-Tconditions of the lower part of the mantle transition zone (MTZ):XFe = 0.222(41) – 0.0053(19)*P, withPin GPa. The nearly ideal mixing behavior, much smaller composition effect on the bulk modulus, and significant compositional variation of the Rw in the lower part of the MTZ substantially increase the gradients of theVs-PandVp-Pprofiles to generally match those constrained by the seismic reference models PREM and AK135. If there is any global low-Tanomaly at the depth of 660 km, its required magnitude is most likely not larger than 200 K.
