Based on sound velocity measurements, upper mantle seismic anomalies could be explained by a melt fraction as low as 0.2%.

.[1] Ultrasonic interferometry measurements in conjunction with in situ X-ray techniques have been used to measure compressional and shear wave velocities and densities of MgSiO 3 perovskite (Mg-Pv) and Mg 0.95 Fe 0.04 2+ Fe 0.01 3+ SiO 3 perovskite ((Mg, Fe)-Pv) in the multianvil at pressures up to 25 GPa and temperatures to 1200 K. Data for Mg-Pv are consistent with previous studies and the (Mg, Fe)-Pv sample has almost identical shear properties to Mg-Pv. The adiabatic bulk modulus, Ks, for (Mg, Fe)-Pv, however, is found to be substantially lower than Mg-Pv, with a refined value of 236 GPa and a pressure derivative of 4.7. It is proposed that this low K S value result from a change in the elasticity of Fe-bearing perovskite at low pressures <30 GPa. High temperature data are consistent with recent models and it is shown that the obtained elastic properties of (Mg, Fe)-Pv are not inconsistent with a lower mantle of bulk silicate Earth composition.

The low-velocity layer (LVL) atop the 410-km discontinuity has been widely attributed to dehydration melting. In this study, we experimentally reproduced the wadsleyite-to-olivine phase transformation in the upwelling mantle across the 410-km discontinuity and investigated in situ the sound wave velocity during partial melting of hydrous peridotite. Our seismic velocity model indicates that the globally observed negative Vs anomaly (−4%) can be explained by a 0.7% melt fraction in peridotite at the base of the upper mantle. The produced melt is richer in FeO (~33 wt.%) and H2O (~16.5 wt.%) and its density is determined to be 3.56–3.74 g cm−3. The water content of this gravitationally stable melt in the LVL corresponds to a total water content in the mantle transition zone of 0.22 ± 0.02 wt.%. Such values agree with estimations based on magneto-telluric observations.

New experimental data are reported on high-pressure polymorphism of CaCO 3 . The CaCO 3 -III phase was stabilized using a large-volume press device and high-resolution X-ray powder diffraction (XRPD) patterns were collected from a few mm 3 of powder sample. The interpretation of XRPD indicates that CaCO 3 -III and CaCO 3 -IIIb structures are present simultaneously and are in similar proportions. The lack of any unindexed peaks demonstrates that these two polymorphs are the only phases in this experiment, indicating that CaCO 3 -III and CaCO 3 -IIIb are the structures most likely to occur above 2.5 GPa. Relevant co-axial crystallographic matrix transformations from lower-pressure polymorphs to both CaCO 3 -III and CaCO 3 -IIIb are discussed to illustrate a further possible occurrence of co-existing and interspersed stable polymorphs in carbonate systems.

The MESSENGER mission revealed that Mercury's magnetic field might have operated since 3.7–3.9 Ga. While the intrinsic magnetism suggests an active dynamo within Mercury's core, the mechanism that is responsible for sustaining the dynamo for prolonged period of time remains unknown. Here we investigated the electrical conductivity of Fe‐S alloys at pressure of 8 GPa and temperatures up to 1,700 K. We show that the electrical conductivity of Fe‐S alloys at 1,500 K is about 103 S/m, 2 orders of magnitude lower than the previously assumed value for dynamo calculations. The thermal conductivity was estimated using the Wiedemann‐Franz law. The total thermal conductivity of FeS is estimated to be ~4 Wm/K at the Mercurian core‐mantle boundary conditions. The low thermal conductivity suggests that a thermally driven dynamo operating on Mercury is more likely than expected. If coupled with chemical buoyancy sources, it is possible to sustain an intrinsic dynamo during time scales compatible with the MESSENGER observations.

Sound velocities of diopside liquid were determined at high pressures and temperatures up to 3.8 GPa and 2375 K, using the ultrasonic technique combined with synchrotron X‐ray diffraction and imaging in a multianvil apparatus. Our results show that the sound velocity increases with pressure but is nearly independent of temperature. Using a Monte Carlo approach, the measured high‐pressure sound velocities combined with ambient‐pressure density provide tight constraints for the equation of state of diopside liquid, with a best‐fit adiabatic bulk modulus (KS) of 23.8 ± 0.4 GPa and its pressure derivative (KS') of 7.5 ± 0.5. The calculated adiabatic temperature and density profile of diopside liquid suggest that a melt layer with diopside composition in the upper mantle would be gravitationally unstable and start to crystallize from the bottom of the layer during cooling. By comparing our results with previous acoustic measurements on silicate glasses, we demonstrate the important differences in sound velocities between silicate liquids and glasses and conclude that silicate glasses may not work as a good analog material for studying the acoustic properties of silicate liquids, as measurements on unrelaxed glasses do not capture the entropic contribution to the compressional properties of liquids. We modeled velocity reductions due to partial melts in the upper mantle using our results and found that for a given velocity reduction, the deeper the low‐velocity region, the larger the melt fraction is required. Using silicate glass data for such estimation would result in a significant underestimation of melt fractions at high pressures.

No abstract

No abstract