Thermal equation of state of akimotoite MgSiO3 and effects of the akimotoite?garnet transformation on seismic structure near the 660 km discontinuity

Abstract

Pressure (P)–volume (V)–temperature (T) relations of akimotoite (ilmenite structured MgSiO3) have been measured up to 19 GPa and 1373 K in a T-Cup multi-anvil apparatus. Polycrystalline specimens synthesized at 21 GPa and 1723 K were loaded in a specially designed cell assembly, together with Au or NaCl pressure standard, for synchrotron X-ray diffraction studies. Two runs were carried out with multiple temperature excursions at various pressures both inside and outside the akimotoite stability field. At conditions outside the stability field, temperature was kept below 873 K, as previous test runs indicated that akimotoite back-transformed to enstatite at about 900 K. In order to minimize effects of non-hydrostatic stress on volume and pressure determination, P–V–T data were collected on cooling cycles from the peak temperature. A total of 134 volume measurements were obtained. Adopting an ambient isothermal bulk modulus (210 GPa) obtained previously using the Brillouin scattering technique, analysis using the third-order Birch–Murnaghan equation of state yielded pressure derivative KT0′=5.6(8), with zero-pressure volume of 263.9(2) Å3 for MgSiO3 akimotoite. The high-temperature data were fit to several thermal equations of state to cross-examine consistency among the thermoelastic parameters obtained. Using the data with NaCl as the pressure standard combined with previous zero-pressure thermal expansion data of [Phys. Chem. Min. 16 (1988) 239], we obtained (∂KT/∂T)P=−0.040(1) GPa K−1, with a zero-pressure thermal expansion α0=2.41(19)×10−5 K−1. The results were applied to model seismic profiles of the transition zone, in order to examine the effects of akimotoite-forming transitions on lateral velocity variations. In depths where no phase transformations and chemical heterogeneities are present, hotter and colder areas relative to a reference geotherm are characterized by similar dlnVS/dlnVP and dlnρ/dlnVS ratios, which are material properties. The akimotoite–garnet phase transformation, however, skews the ratios because akimotoite has much higher acoustic velocities than garnet. Thus if the mantle geotherm is close to the transition temperature, anomalous apparent dlnVS/dlnVP and dlnρ/dlnVS ratios may be observed, which are no longer material properties. Aluminum content plays an important role in defining the akimotoite–garnet phase boundary relative to the geotherm and lateral Al variations may cause the transition boundary to fluctuate with depth. Therefore one must be cautious in interpreting seismic tomography observations in the transition zone.