Hiroshi Kojitani

Melting enthalpies of mantle peridotite: calorimetric determinations in the system CaO-MgO-Al2O3-SiO2 and application to magma generation

High-temperature drop calorimetry in the temperature range of 1398–1785 K was performed for the samples of mixtures of synthetic anorthite (An), diopside (Di), enstatite (En) and forsterite (Fo) with the same compositions as those of primary melts generated at 1.1, 3 and 4 GPa at most 10° above the solidus of anhydrous mantle peridotite in the CaO-MgO-Al2O3-SiO2 system. From the differences between the heat contents (HT-H298) of liquid and that of crystal mixture at the liquidus temperature, melting enthalpies of the samples of 1.1, 3 and 4 GPa-primary melt compositions were determined at 1 atm to be 531 ± 39 J · g−1 at 1583 K, 604 ± 21 J · g−1 at 1703 K, 646 ± 21 J · g−1 at 1753 K, respectively. These heat of fusion values suggest that mixing enthalpy of the melt in the An-Di-En-Fo system is approximately zero within the experimental errors when we use the heat of fusion of Fo by Richet et al. (P. Richet, F. Leclerc, L. Benoist, Melting of forsterite and spinel, with implications for the glass transition of Mg2SiO4 liquid, Geophys. Res. Lett. 20 (1993) 1675–1678). The measured enthalpies of melting at 1 atm were converted into those for melting reactions which occur under high pressures by correcting enthalpy changes associated with solid-state mineral reactions. Correcting the effects of pressure, temperature and FeO and Na2O components on the melting enthalpies at 1 atm, heat of fusion values of a representative mantle peridotite just above the solidus under high pressure were estimated to be 590 J at 1.1 GPa and 1523 K, 692 J at 3 GPa and 1773 K, and 807 J at 4 GPa and 1923 K for melting reactions producing liquid of 1 g, with uncertainties of 50 J. By applying these melting enthalpies to a mantle diapir model which generates present MORBs, a potential mantle temperature of 1533 K has been estimated, assuming an eruption temperature of magma of 1473 K.

High-pressure synthesis, crystal structure, and phase stability relations of a LiNbO3-type polar titanate ZnTiO3 and its reinforced polarity by the second-order Jahn-Teller effect.

A polar LiNbO3-type (LN-type) titanate ZnTiO3 has been successfully synthesized using ilmenite-type (IL-type) ZnTiO3 under high pressure and high temperature. The first principles calculation indicates that LN-type ZnTiO3 is a metastable phase obtained by the transformation in the decompression process from the perovskite-type phase, which is stable at high pressure and high temperature. The Rietveld structural refinement using synchrotron powder X-ray diffraction data reveals that LN-type ZnTiO3 crystallizes into a hexagonal structure with a polar space group R3c and exhibits greater intradistortion of the TiO6 octahedron in LN-type ZnTiO3 than that of the SnO6 octahedron in LN-type ZnSnO3. The estimated spontaneous polarization (75 μC/cm(2), 88 μC/cm(2)) using the nominal charge and the Born effective charge (BEC) derived from density functional perturbation theory, respectively, are greater than those of ZnSnO3 (59 μC/cm(2), 65 μC/cm(2)), which is strongly attributed to the great displacement of Ti from the centrosymmetric position along the c-axis and the fact that the BEC of Ti (+6.1) is greater than that of Sn (+4.1). Furthermore, the spontaneous polarization of LN-type ZnTiO3 is greater than that of LiNbO3 (62 μC/cm(2), 76 μC/cm(2)), indicating that LN-type ZnTiO3, like LiNbO3, is a candidate ferroelectric material with high performance. The second harmonic generation (SHG) response of LN-type ZnTiO3 is 24 times greater than that of LN-type ZnSnO3. The findings indicate that the intraoctahedral distortion, spontaneous polarization, and the accompanying SHG response are caused by the stabilization of the polar LiNbO3-type structure and reinforced by the second-order Jahn-Teller effect attributable to the orbital interaction between oxygen ions and d(0) ions such as Ti(4+).

High-pressure phase transitions in FeCr2O4 and structure analysis of new post-spinel FeCr2O4 and Fe2Cr2O5 phases with meteoritical and petrological implications

Abstract We determined phase relations in FeCr2O4 at 12-28 GPa and 800-1600 °C using a multi-anvil apparatus. At 12-16 GPa, FeCr2O4 spinel (chromite) first dissociates into two phases: a new Fe2Cr2O5 phase + Cr2O3 with the corundum structure. At 17-18 GPa, the two phases combine into CaFe2O4- type and CaTi2O4-type FeCr2O4 below and above 1300 °C, respectively. Structure refinements using synchrotron X-ray powder diffraction data confirmed the CaTi2O4-structured FeCr2O4 (Cmcm), and indicated that the Fe2Cr2O5 phase is isostructural to a modified ludwigite-type Mg2Al2O5 (Pbam). In situ high-pressure high-temperature X-ray diffraction experiments showed that CaFe2O4-type FeCr2O4 is unquenchable and is converted into another FeCr2O4 phase on decompression. Structural analysis based on synchrotron X-ray powder diffraction data with transmission electron microscopic observation clarified that the recovered FeCr2O4 phase has a new structure related to CaFe2O4-type. The high-pressure phase relations in FeCr2O4 reveal that natural FeCr2O4-rich phases of CaFe2O4- and CaTi2O4-type structures found in the shocked Suizhou meteorite were formed above about 18 GPa at temperature below and above 1300 °C, respectively. The phase relations also suggest that the natural chromitites in the Luobusa ophiolite previously interpreted as formed in the deep-mantle were formed at pressure below 12-16 GPa.

Measurement of heat of fusion of model basalt in the system Diopside‐forsterite‐anorthite

Compositions of natural olivine tholeiites were simplified in the system CaO-MgO-Al 2 O 3 -SiO 2 , and approximately eutectic composition in the system diopside(Di)forsterite(Fo) - anorthite(An) (Di:Fo:An =49.0:7.5:43.5 wt%) was chosen as the model basalt composition. High-temperature drop calorimetry was performed at 1405 - 1676K for the samples with the model basalt composition consisting of the mixture of synthetic diopside, forsterite and anorthite. The heat of fusion of the model basalt was obtained to be 506 ± 38 J/g from the difference between the heat content of liquid and that of the mineral mixture at 1543K, which was the approximated eutectic temperature. This heat of fusion shows almost zero enthalpy of mixing in this system. Taking account of the pressure, temperature and composition effects on enthalpy, the heat of fusion of 490-560 J/g is estimated for generation of MORB (mid-ocean ridge basalt) in the upper mantle.

High-pressure high-temperature transitions in MgCr2O4 and crystal structures of new Mg2Cr2O5 and post-spinel MgCr2O4 phases with implications for ultrahigh-pressure chromitites in ophiolites

Abstract We determined phase relations in MgCr2O4 at 12-28 GPa and 1000-1600 °C using a multi-anvil apparatus. At 12-15 GPa, spinel-type MgCr2O4 (magnesiochromite) first decomposes into a mixture of new Mg2Cr2O5 phase + corundum-type Cr2O3 at 1100-1600 °C, but it dissociates first into MgO periclase + corundum-type Cr2O3 at l000 °C. At about 17-19 GPa, the mixture of Mg2Cr2O5 phase + corundum-type Cr2O3 transforms to a single MgCr2O4 phase. Structure refinements using synchrotron X-ray powder diffraction data indicated that the high-pressure MgCr2O4 phase has a CaTi2O4-type structure (Cmcm), and that the basic structure of the Mg2Cr2O5 phase is the same as that of recently found modified ludwigite-type Mg2Al2O5 and Fe2Cr2O5 (Pbam). The phase relations in this study may suggest that natural chromitites in the Luobusa ophiolite regarded as the deep-mantle origin were derived from the mantle shallower than the depths corresponding to pressure of 12-15 GPa because of absence of the assemblage of (Mg,Fe)2Cr2O5 + Cr2O3 in the chromitites.

Synthesis and magnetic and charge-transport properties of the correlated 4d post-perovskite CaRhO3.

A high-quality polycrystalline sample of the correlated 4d post-perovskite CaRhO(3) (Rh(4+): 4d(5), S(el) = 1/2) was attained under a moderate pressure of 6 GPa. Since the post-perovskite is quenchable at ambient pressure/temperature, it can be a valuable analogue of the post-perovskite MgSiO(3) (stable higher than 120 GPa and unstable at ambient pressure), which is a significant key material in earth science. The sample was subjected for measurements of charge-transport and magnetic properties. The data clearly indicate it goes into an antiferromagnetically ordered state below approximately 90 K in an unusual way, in striking contrast to what was observed for the perovskite phase. The post-perovskite CaRhO(3) offers future opportunities for correlated electrons science as well as earth science.

High-pressure phase relations and crystal chemistry of calcium ferrite-type solid solutions in the system MgAl2O4-Mg2SiO4

Abstract To map the stability field of calcium ferrite-type MgAl2O4-Mg2SiO4 solid solutions, high-pressure phase relations in the system MgAl2O4-Mg2SiO4 were studied in the compositional range of 0 to 50 mol% Mg2SiO4. The calcium ferrite solid solutions are stable above 23 GPa at 1600 °C, and the maximum solubility of Mg2SiO4 component in MgAl2O4 calcium ferrite is 34 mol%. Lattice parameters and unit-cell volume of calcium ferrite-type MgAl2O4 (space group Pbnm) determined by Rietveld analysis are a = 9.9498(6) Å, b = 8.6468(6) Å, c = 2.7901(2) Å, and V = 240.02(2) Å3. Lattice parameters for the MgAl2O4-Mg2SiO4 solid solutions with the compositions of 14, 24, and 34 mol% Mg2SiO4 indicated the following compositional dependency of lattice parameters: a (Å) = 9.9498 + 0.1947·XMg₂SiO₄, b (Å) = 8.6468 . 0.1097·XMg₂SiO₄, and c (Å) = 2.7901 + 0.0086·XMg₂SiO₄, where XMg2SiO4 is the mole fraction of Mg₂SiO₄ component. A linear extrapolation of the composition-molar volume relationship gave an estimated volume of 36.49(2) cm3/mol for the hypothetical calcium ferrite-type Mg2SiO4. This value is larger than that of the isochemical mixture of MgSiO3 perovskite and MgO, 35.72(1) cm3/mol. This implies that the mixture of MgSiO3 perovskite and MgO is more stable than the hypothetical calcium ferrite-type Mg2SiO4 under the lower mantle conditions.

Calorimetric study of olivine solid solutions in the system Mg2SiO4-Fe2SiO4

Solution enthalpies of synthetic olivine solid solutions in the system Mg2SiO4-Fe2SiO4 have been measured in molten 2PbO·B2O3 at 979 K. The enthalpy data show that olivine solid solutions have a positive enthalpy of mixing and the deviation from ideality is approximated as symmetric with respect to composition, in contrast to the previous study. Applying the symmetric regular solution model to the present enthalpy data, the interaction parameter of ethalpy (WH) is estimated to be 5.3±1.7 kJ/mol (one cation site basis). Using this Wh and the published data on excess free energy of mixing, the nonideal parameter of entropy (Ws) of olivine solid solutions is estimated as 0.6±1.5 J/mol·K.

Pressure-induced transformation of 6H hexagonal to 3C Perovskite structure in PbMnO3.

A tetragonal perovskite PbMnO(3) was obtained by treating the 6H hexagonal perovskite phase at 15 GPa and 1273 K. Structural analysis using synchrotron X-ray diffraction suggested that PbMnO(3) crystallizes in the centrosymmetric space group P4/mmm, unlike PbTiO(3) and PbVO(3) which have a polar structure in space group P4mm. Iodometric titration revealed the presence of the oxygen deficiency of x = 0.06 for PbMnO(3-x). The hexagonal 6H and the 3C perovskite phases exhibited antiferromagnetic ordering at 155 and 20 K, respectively.

Aluminum substitution in MgSiO3 perovskite: Investigation of multiple mechanisms by 27Al NMR

Abstract In the Earth’s mantle, the mechanism(s) of solid solution of Al in MgSiO3 perovskite strongly impacts its thermodynamic and transport properties. We present 27Al NMR data for perovskite samples of nominal composition Mg(Si0.9Al0.1)O2.95, to test a mechanism by which Al3+ substitutes at the octahedral Si4+ sites, leaving a corresponding number of O-site vacancies. We find evidence for this process in a significantly greater peak area for Al at B (Si) sites vs. A (Mg) sites in the structure, and the possible identification of a small concentration of five-coordinated Al adjacent to such vacancies. However, substitution of Al3+ at the A sites remains significant. As in perovskite-type technological ceramics, O-atom vacancies may play an important role in enhancing ion mobility and the dissolution of water.