Wansheng Xiao

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.

Large volume collapse observed in the phase transition in cubic PbCrO3 perovskite

When cubic PbCrO3 perovskite (Phase I) is squeezed up to ∼1.6 GPa at room temperature, a previously undetected phase (Phase II) has been observed with a 9.8% volume collapse. Because the structure of Phase II can also be indexed into a cubic perovskite as Phase I, the transition between Phases I and II is a cubic to cubic isostructural transition. Such a transition appears independent of the raw materials and synthesizing methods used for the cubic PbCrO3 perovskite sample. In contrast to the high-pressure isostructural electronic transition that appears in Ce and SmS, this transition seems not related with any change of electronic state, but it could be possibly related on the abnormally large volume and compressibility of the PbCrO3 Phase I. The physical mechanism behind this transition and the structural and electronic/magnetic properties of the condensed phases are the interesting issues for future studies.

Pressure-induced phase transition in cubic Lu2O3

The phase transition in cubic Lu2O3 has been investigated by angle dispersive x-ray diffraction and Raman scattering in a diamond anvil cell up to 46.8 GPa and 40.2 GPa, respectively. The diffraction data indicated that a phase transition from the cubic to a monoclinic structure started at 12.7 GPa and completed at 18.2 GPa. This high pressure monoclinic phase is stable up to at least 46.8 GPa and can be quenched to ambient conditions. This irreversible cubic to monoclinic structural transformation has also been confirmed by Raman scattering measurements. A third-order Birch–Murnaghan fit based on the observed pressure-volume data yields a zero pressure bulk modulus of B0=214(6) GPa, its pressure derivative B0′=9(1) for the low-pressure cubic phase; and B0=218(13) GPa, B0′=2.3(3) for the high pressure monoclinic phase, respectively. The mode Gruneisen parameters of different Raman modes for both cubic and monoclinic phases have also been determined.

Pressure-induced phase transformations in cubic Gd2O3

High-pressure transformation and compression behavior of Gd2O3 were investigated using synchrotron radiation x-ray diffraction in a diamond anvil cell up to 44 GPa at room temperature. The structural transformation from a cubic to a monoclinic phase occurred during the sample precompression process. Phase transitions from both the cubic and the monoclinic polymorphs to a hexagonal structure were observed. The hexagonal phase was stable up to the highest pressure in this study and was not quenchable and transformed to a monoclinic phase after pressure release. The bulk moduli of Gd2O3 for the cubic, monoclinic, and hexagonal phases were obtained by fitting the compression data to the Birch–Murnaghan equation of state. Moreover, an anomaly of the hexagonal type Gd2O3 was observed.

Phase transformation of Ho2O3 at high pressure

The structural stability of cubic Ho2O3 under high pressure has been investigated by angle-dispersive x-ray diffraction (ADXD) in a diamond anvil cell up to 63.0 GPa at room temperature. The diffraction data reveal two structural transformations on compression. The structural transformation from a cubic to a monoclinic structure starts at 8.9 GPa and is complete at 16.3 GPa with a ∼8.1% volume collapse. A hexagonal phase begins to appear at ∼14.8 GPa and becomes dominant at 26.4 GPa. This high-pressure hexagonal phase with a small amount of retained monoclinic phase is stable up to the highest pressure of 63.0 GPa in this study. After release of pressure, the hexagonal phase transforms to a monoclinic structure. A third-order Birch-Murnaghan fit yields zero pressure bulk moduli (B0) of 206(3), 200(7) and 204(19) GPa and their pressure derivatives (B0’) of 4.8(4), 2.1(4), 3.8(5) for the cubic, monoclinic and hexagonal phases, respectively. Comparing with other rare-earth sesquioxides, it is suggested that ...

Phase transformation of Ho[subscript 2]O[subscript 3] at high pressure

The structural stability of cubic Ho2O3 under high pressure has been investigated by angle-dispersive x-ray diffraction (ADXD) in a diamond anvil cell up to 63.0 GPa at room temperature. The diffraction data reveal two structural transformations on compression. The structural transformation from a cubic to a monoclinic structure starts at 8.9 GPa and is complete at 16.3 GPa with a ∼8.1% volume collapse. A hexagonal phase begins to appear at ∼14.8 GPa and becomes dominant at 26.4 GPa. This high-pressure hexagonal phase with a small amount of retained monoclinic phase is stable up to the highest pressure of 63.0 GPa in this study. After release of pressure, the hexagonal phase transforms to a monoclinic structure. A third-order Birch-Murnaghan fit yields zero pressure bulk moduli (B0) of 206(3), 200(7) and 204(19) GPa and their pressure derivatives (B0’) of 4.8(4), 2.1(4), 3.8(5) for the cubic, monoclinic and hexagonal phases, respectively. Comparing with other rare-earth sesquioxides, it is suggested that ...

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.

The effects of high temperature on the high-pressure behavior of CeO2

Raman spectrum and angle-dispersive x-ray diffraction (ADXD) measurements have been performed to investigate the effects of high temperature on the high-pressure behavior of bulk CeO 2 . A phase transformation of CeO 2 from fluorite to PbCl 2 -type structure occurs at 12.0 GPa and is completed at 14.2 GPa after the sample is heated, and the phase transition pressure decreases by nearly 20 GPa compared with that at room temperature. On decompression, the high-pressure phase of CeO 2 remains down to 2.2 GPa, and it changes back to a cubic structure at ambient conditions. At a pressure of 22.1 GPa, a 6.4% lattice volume difference between the fluorite and PbCl 2 -type structures was observed. The lattice volume of fluorite phase obtained in the areas that have been heated is about 1% less than that obtained in the areas that have not been heated. Besides prompting the phase transition of CeO 2 , high temperature also anneals the sample and leads a small reduction in lattice volume of the fluorite phase. The zero-pressure bulk modulus of the fluorite phase of CeO 2 after annealing is calculated to be about 200 GPa with an assumed pressure derivative of four, which is smaller than that of former x-ray diffraction experiments at room temperature.

Pressure effect on structural and vibrational properties of Sm-substituted BiFeO3

The structural and vibrational properties of 5% Sm-substituted BiFeO3 under pressure are investigated using synchrotron X-ray diffraction and Raman scattering measurements. The results yield the pressure-induced structural phase transitions from the polar R3c phase to the orthorhombic Pnma phase commencing at 3.9 and being complete at 7.6 GPa, where there is a region of the coexistence of the R3c and Pnma phases. This structural transition is companied by the ferroelectric-paraelectric transition for the Sm-substituted BiFeO3. We find that the Sm substitution leads to lower transition pressure compared to that of the pure BiFeO3 system due to the substitution-induced chemical pressure. Our results do not suggest the pressure-induced reentrance of ferroelectricity in the model multiferroic BiFeO3 in the pressure range studied.

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.