Maoshuang Song

Olivine‐wadsleyite transition in the system (Mg,Fe)2SiO4

Phase relations of the olivine-wadsleyite transition in the system (Mg,Fe) 2 SiO 4 have been determined at 1600 and 1900 K using the quench method in a Kawai-type high-pressure apparatus. Pressure was determined at a precision better than 0.2 GPa using in situ X-ray diffraction with MgO as a pressure standard. The transition pressures of the end-member Mg 2 SiO 4 are estimated to be 14.2 and 15.4 GPa at 1600 and 1900 K, respectively. Partition coefficients for Fe and Mg between olivine and wadsleyite are 0.51 at 1600 K and 0.61 at 1900 K. By comparing the depth of the discontinuity with the transition pressure, the temperature at 410 km depth is estimated to be 1760 ± 45 K for a pyrolitic upper mantle. The mantle potential temperature is estimated to be in the range 1550-1650 K. The temperature at the bottom of the upper mantle is estimated to be 1880 ± 50 K. The thickness of the olivine-wadsleyite transition in a pyrolitic mantle is determined to be between 7 and 13 km for a pyrolitic mantle, depending on the efficiency of vertical heat transfer. Regions of rapid vertical flow (e.g., convection limbs), in which thermal diffusion is negligible, should have a larger transition interval than stagnant regions, where thermal diffusion is effective. This is in apparent contradiction to short-period seismic wave observations that indicate a maximum thickness of <5 km. An upper mantle in the region of the 410 km discontinuity with about 40% olivine and an Mg# of at least 89 can possibly explain both the transition thickness and velocity perturbation at the 410 km discontinuity.

Experimental constraints on rutile saturation during partial melting of metabasalt at the amphibolite to eclogite transition, with applications to TTG genesis

Abstract TiO2 solubility in rutile-saturated felsic melts and coexisting minerals was determined at 1.5-3.5 GPa, 750-1250 °C, and 5-30 wt% H2O. TiO2 solubility in the melt primarily increases with temperature and melt basicity; it increases slightly with water content in the melt, and it decreases with pressure. A general TiO2 solubility model was obtained and is expressed as: ln(TiO2)melt = ln(TiO2)rutile + 1.701 - (9041/T) - 0.173P + 0.348FM + 0.016H2O, where TiO2 and H2O are in wt%, T is in Kelvin, P in GPa, and FM is the melt composition parameter given by FM = (1/Si)·[Na + K + 2(Ca + Fe + Mn + Mg)]/ Al, in which the chemical symbols represent cation fractions. TiO2 solubility in amphibole, garnet, and clinopyroxene also increases with temperature and empirical equations describing this temperature dependence were derived. These data were used to assess the protolith TiO2 content required for rutile saturation during partial melting of hydrous metabasalt at the amphibolite to eclogite transition. The results show that only 0.8-1.0 wt% TiO2 is required for rutile saturation during low-degree (<20%) melting. Rutile is stable up to ~1150 °C with 1.6 wt% TiO2 in the protolith and 30-40% melting for dehydration melting and up to ~1050 °C and 50-60% melting for fluid-present melting. The data also show that 0.7-0.8 wt% TiO2 in the protolith is needed for rutile saturation during subsolidus dehydration. Therefore, nearly all basaltic protoliths in deep-crustal settings and subduction zones will be saturated with rutile during subsolidus dehydration and low-degree melting at hydrous conditions. Archean tonalites-trondhjemites-granites (TTG) are widely accepted to be the products of lowdegree melting of metabasalts at the amphibolite to eclogite transition, with rutile being present in the residue. Comparison of natural TTG compositions with our experimental rutile solubility data indicates that the dominant TTG magmas were produced at temperatures of 750-950 °C, which requires that the partial melting occurred at hydrous conditions. Models involving melting at the base of oceanic plateaus are inadequate to explain TTG genesis because the plateau root zones are likely dominated by anhydrous cumulates. A slab-melting model satisfies the requirement of a hydrous metabasalt, which during subduction would melt to produce voluminous TTG melts under high Archean geothermal gradients. The geothermal gradients responsible are estimated to be between 10 and 19 °C/km based on a pressure range of 1.5-2.5 GPa for the amphibolite to eclogite transition.

Frequency domain analysis of ultrasonic velocity: An alternative bond effect correction constraining bond properties

Ultrasonic travel time measurements have been made in the time domain by observing the arrival time difference between the selected ultrasonic pulses. In the present work, we carried out a frequency domain analysis of ultrasonic travel time based on the phase difference between the Fourier coefficients of ultrasonic pulses. The purpose of the present analysis is to develop an easier experimental procedure for making accurate ultrasonic velocity measurements. The features of the present frequency domain analysis are to use digitized oscilloscope data of simple pulse reflection measurements, and to incorporate the bond effect correction by constraining the acoustic properties of the bond material. We examined the performance of the present method through test measurements on a fine polished glass plate (bk7 grade), and confirmed an accuracy of ∼10−4. Its accuracy is critical for utilizing velocity data for constructing the equation of state.Ultrasonic travel time measurements have been made in the time domain by observing the arrival time difference between the selected ultrasonic pulses. In the present work, we carried out a frequency domain analysis of ultrasonic travel time based on the phase difference between the Fourier coefficients of ultrasonic pulses. The purpose of the present analysis is to develop an easier experimental procedure for making accurate ultrasonic velocity measurements. The features of the present frequency domain analysis are to use digitized oscilloscope data of simple pulse reflection measurements, and to incorporate the bond effect correction by constraining the acoustic properties of the bond material. We examined the performance of the present method through test measurements on a fine polished glass plate (bk7 grade), and confirmed an accuracy of ∼10−4. Its accuracy is critical for utilizing velocity data for constructing the equation of state.

Structural properties of PbVO3 perovskites under hydrostatic pressure conditions up to 10.6 GPa.

High-pressure synchrotron x-ray powder diffraction experiments were performed on PbVO(3) tetragonal perovskite in a diamond anvil cell under hydrostatic pressures of up to 10.6 GPa at room temperature. The compression behavior of the PbVO(3) tetragonal phase is highly anisotropic, with the c-axis being the soft direction. A reversible tetragonal to cubic perovskite structural phase transition was observed between 2.7 and 6.4 GPa in compression and below 2.2 GPa in decompression. This transition was accompanied by a large volume collapse of 10.6% at 2.7 GPa, which was mainly due to electronic structural changes of the V(4+) ion. The polar pyramidal coordination of the V(4+) ion in the tetragonal phase changed to an isotropic octahedral coordination in the cubic phase. Fitting the observed P-V data using the Birch-Murnaghan equation of state with a fixed [Formula: see text] of 4 yielded a bulk modulus K(0) = 61(2) GPa and a volume V(0) = 67.4(1) Å(3) for the tetragonal phase, and the values of K(0) = 155(3) GPa and V(0) = 58.67(4) Å(3) for the cubic phase. The first-principles calculated results were in good agreement with our experiments.

Conductivity of NaCl solution at 0.4-5.0 GPa and 25-500℃

NaCI-H2O is the most fundamental ternary system in geology. Until now, the measurements of electrical conductivity of NaCl solutions are still little at high pressures (> O.5 GPa) We measured the conductivity of 0.01 m NaCl solution at 0.4–5.0 GPa and 25-500°C. The results are consistent with that of Quist and Marshall (1968) at 0.4 GPa. The conductivity of NaCl solution increases with increasing temperature. The results also show that the conductivity of NaCl solution changes little with increasing pressure below 1.5 GPa and changes rapidly with increasing pressure above 1.5 GPa. The rapid increase of the conductivity of NaCl solution may play an important role in many geological processes (such as the genesis of ore deposits under hydrothermal condition) and other fields.

A new cubic perovskite in PbGeO3 at high pressures

Abstract A new cubic perovskite polymorph of PbGeO3 (Phase II) was synthesized by laser heating in the diamond-anvil cell (DAC) at the pressure of 36 GPa. Fitting the Birch-Murnaghan equation of state against its observed P-V data yields a bulk modulus K0 of 196(6) GPa and the volume V0 of 56.70(13) Å3 when K0′ is assumed being 4. After the pressure is released, the PbGeO3 Phase II changes gradually into an amorphous phase, which contains mainly fourfold-coordinated germanium. It indicates that the PbGeO3 Phase II with a GeO6 octahedron framework transforms to a GeO4 tetrahedron network during the amorphization. The existence of PbGeO3 cubic perovskite Phase II at high pressures indicates that the polarized character of the Pb2+ ion induced by its 6s2 lone pair electrons would be totally reduced in the environment of major silicate perovskites inside the lower mantle, and thus the Pb atom would substitute the Ca atom to enter the CaSiO3 perovskite.

Solid–liquid hybrid assembly for ultrasonic elasticity measurements under hydrostatic conditions of up to 8GPa in a Kawai-type multianvil apparatus

A solid–liquid hybrid assembly has been designed for ultrasonic elasticity measurements of materials under hydrostatic conditions in a Kawai-type multianvil apparatus. In the assembly, a tungsten–carbide cubic anvil served as the buffer rod for the acoustic signals. The transducer and sample were mounted on two diagonally opposite truncated corners of the buffer-rod anvil. The sample was immersed in a liquid cell filled with a liquid pressure medium, a methanol–ethanol mixture (4:1 in volume), which produced hydrostatic conditions for the sample. The pressure was monitored with a bismuth pressure calibrant inside the liquid cell. Preliminary experiments using single-crystal MgO, polycrystal alumina, and silicate glass samples were successfully conducted up to 8GPa. This assembly is especially useful for precise elasticity measurements of single-crystal samples under hydrostatic compression.

The electrical conductivity of H2O at 0.21– 4.18 GPa and 20–350°C

The electrical conductivity of H2O in solid and liquid phases has been measured at 0.21–4.18 GPa and 20–350°C. The results indicate: (I) different phases of H2O in solid have different relations between electrical conductivity and temperature and pressure. The conductivity changes continuously with temperature, but discontinuously with the pressure between 2.11 and 2.58 GPa, which corresponds to the transforming pressure between ice (VI) and ice (VII);(II) the amductivity of H2O in liquid all increases with temperature and pressure, but there are discontinuities at pressures between 0.57 and 0.9 GPa, and between 2.11 and 2.58 GPa, which are also consistent with the polyrnorph of ice (ice (V), ice (VI) and ice (VII)). This reflects that H2O in liquid at different pressures has quite different properties of electron chemistry. It is probably the important reason that causes the layers with high electrical conductivity and low velocity in the earth’s ceust and upper mantle.

P-wave velocities of alkaline olivine basalt at high pressure and temperature and its controlling factors

SEISMIC wave inversion is one of the most important means for us to recognize the compositionand structure of the Earth’s interior.Although the experimental techniques of elastic wave ve-locity measurement at in situ high pressure and temperature develop very fast in recent years

Elasticity of single‐crystal NAL phase at high pressure: A potential source of the seismic anisotropy in the lower mantle

The new hexagonal aluminous phase, named the NAL phase, is expected to be stable at depths of <1200 km in subducted slabs and believed to constitute 10~30 wt% of subducted mid-ocean ridge basalt together with the CaFe2O4-type aluminous phase. Here elasticity of the single-crystal NAL phase is investigated using Brillouin light scattering coupled with diamond anvil cells up to 20 GPa at room temperature. Analysis of the results shows that the substitution of iron lowers the shear modulus of the NAL phase by ~5% (~6 GPa) but does not significantly affect the adiabatic bulk modulus. The NAL phase exhibits high-velocity anisotropies with AVP = 14.7% and AVS = 15.12% for the Fe-bearing phase at ambient conditions. The high AVS of the NAL phase mainly results from the high anisotropy of the faster VS1 (13.9~15.8%), while the slower VS2 appears almost isotropic (0.1~2.8%) at ambient and high pressures. The AVP and AVS of the NAL phase decrease with increasing pressure but still have large values with AVP = 11.4% and AVS = 14.12% for the Fe-bearing sample at 20.4 GPa. The extrapolated AVP and AVS of the Fe-free and Fe-bearing NAL phases at 40 GPa are larger than those of bridgmanite at the same pressure. Together with its spin transition of iron and structural transition to the CF phase, the presence of the NAL phase with high-velocity anisotropies may contribute to the observed seismic anisotropy around subducted slabs in the uppermost lower mantle.