Masaki Akaogi

Pyroxene-garnet solid-solution equilibria in the systems Mg4Si4O12Mg3Al2Si3O12 and Fe4Si4O12Fe3Al2Si3O12 at high pressures and temperatures

Pyroxene-garnet solid-solution equilibria have been studied in the pressure range 41–200 kbar and over the temperature range 850–1,450°C for the system Mg4Si4O12Mg3Al2Si3O12, and in the pressure range 30–105 kbar and over the temperature range 1,000–1,300°C for the system Fe4Si4O12Fe3Al2Si3O12. At 1,000°C, the solid solubility of enstatite (MgSiO3) in pyrope (Mg3Al2Si3O12) increases gradually to 140 kbar and then increases suddenly in the pressure range 140–175 kbar, resulting in the formation of a homogeneous garnet with composition Mg3(Al0.8Mg0.6Si0.6)Si3O12. In the MgSiO3-rich field, the three-phase assemblage of β- or γ-Mg2SiO4, stishovite and a garnet solid solution is stable at pressures above 175 kbar at 1,000°C. The system Fe4Si4O12Fe3Al2Si3O12 shows a similar trend of high-pressure transformations: the maximum solubility of ferrosilite (FeSiO3) in almandine (Fe3Al2Si3O12) forming a homogeneous garnet solid solution is 40 mol% at 93 kbar and 1,000°C. If a pyrolite mantle is assumed, from the present results, the following transformation scheme is suggested for the pyroxene-garnet assemblage in the mantle. Pyroxenes begin to react with the already present pyrope-rich garnet at depths around 150 km. Although the pyroxene-garnet transformation is spread over more than 400 km in depth, the most effective transition to a complex garnet solid solution takes place at depths between 450 and 540 km. The complex garnet solid solution is expected to be stable at depths between 540 and 590 km. At greater depths, it will decompose to a mixture of modified spinel or spinel, stishovite and garnet solid solutions with smaller amounts of a pyroxene component in solution.

Molecular Dynamics Simulation of the Structural and Physical Properties of the Four Polymorphs of TiO2

Abstract The structural and physical properties of the four TiO2 polymorphs [rutile, anatase, brookite and TiO2 II (α-PbO2 structure)] have been calculated by means of molecular dynamics simulation with quantum corrections. The potential model used is composed of the pairwise additive Coulomb, dispersion and repulsion interactions. Energy parameters were determined to reproduce the observed crystal structures of rutile, anatase and brookite, and the measured elastic constants of rutile. Overall, the simulation is successful in reproducing a wide range of properties of the four polymorphs, including the crystal structures, volume compressibilities, volume thermal expansivities, and enthalpy relationships between them.

High-pressure phase equilibria in a garnet lherzolite, with special reference to Mg2+Fe2+ partitioning among constituent minerals

Abstract Phase equilibria in a natural garnet lherzolite nodule (PHN 1611) from Lesotho kimberlite and its chemical analogue have been studied in the pressure range 45–205 kbar and in the temperature range 1050–1200°C. Partition of elements, particularly Mg 2+ Fe 2+ , among coexisting minerals at varying pressures has also been examined. High-pressure transformations of olivine(α) to spinel(γ) through modified spinel(β) were confirmed in the garnet lherzolite. The transformation behavior is quite consistent with the information previously accumulated for the simple system Mg 2 SiO 4 Fe 2 SiO 4 . At pressures of 50–150 kbar, a continuous increase in the solid solubility of the pyroxene component in garnet was demonstrated in the lherzolite system by means of microprobe analyses. At 45–75 kbar and 1200°C, the Fe 2+ /(Mg + Fe 2+ ) value becomes greater in the ascending order orthopyroxene, Ca-rich clinopyroxene, olivine and garnet. At 144–146 kbar and 1200°C, garnet exhibits the highest Fe 2+ /(Mg + Fe 2+ ) value; modified spinel(β) and Ca-poor clinopyroxene follow it. When the modified spinel(β)-spinel(γ) transformation occurred, a higher concentration of Fe 2+ was found in spinel(γ) rather than in garnet. As a result of the change in the Mg 2+ Fe 2+ partition relation among coexisting minerals, an increase of about 1% in the Fe 2 SiO 4 component in (Mg,Fe) 2 SiO 4 modified spinel and spinel was observed compared with olivine. These experimental results strongly suggest that the olivine(α)-modified spinel(β) transformation is responsible for the seismic discontinuity at depths of 380–410 km in the mantle. They also support the idea that the minor seismic discontinuity around 520 km is due to the superposition effect of two types of phase transformation, i.e. the modified spinel(β)-spinel(γ) transformation and the pyroxene-garnet transformation. Mineral assemblages in the upper mantle and the upper half of the transition zone are given as a function of depth for the following regions: 100–150, 150–380, 380–410, 410–500, 500–600 and 600–650 km.

Negative Pressure-Temperature Slopes for Reactions Formign MgSiO3 Perovskite from Calorimetry

A new and sensitive differential drop solution calorimetric technique was developed for very small samples. A single experiment using one 5.18-milligram sample of perovskite, synthesized at 25 gigapascals and 1873 Kelvin, gave 110.1 � 4.1 kilojoules per mole for the enthalpy of the ilmenite-pervoskite transition in MgSiO3. The thermodynamics of the reaction of MgSiO3 (ilmenite) to MgSiO3 (perovskite) and of Mg2SiO4 (spinel) to MgSiO3 (pervoskite) and MgO (periclase) were assessed. Despite uncertainties in heat capacity and molar volume at high pressure and temperature, both reactions clearly have negative pressure-temperature slopes, -0.005 � 0.002 and -0.004 � 0.002 gigapascals per Kelvin, respectively. The latter may be insufficiently negative to preclude whole-mantle convection.

Refinement of enthalpy measurement of MgSiO3 perovskite and negative pressure‐temperature slopes for Perovskite‐forming reactions

Enthalpy for MgSiO3 orthopyroxene-perovskite transition at 298 K was measured by differential drop-solution calorimetry. Based on four measurements of MgSiO3 perovskite, the ΔH°298 of 102.2±2.5 kJ/mol was more precisely determined than that in the previous study. The enthalpies of transitions from MgSiO3 ilmenite to perovskite and from Mg2SiO4 spinel to perovskite + periclase were calculated to be 43.2±5.0 and 86.1±3.6 kJ/mol, respectively. The boundaries of these transitions were calculated using the above data together with physical properties of relevant phases, including recently determined properties of perovskite: heat capacity, entropy, and temperature dependence of bulk modulus. The calculated P-T slopes for the ilmenite-perovskite transition and the spinel dissociation are −3.4±2 and −3±1 MPa/K, respectively. The latter slope may suggest that the upper and lower mantle convect separately but intermittent mixing occurs between them.

Thermodynamic properties of α-quartz, coesite, and stishovite and equilibrium phase relations at high pressures and high temperatures

Isobaric heat capacities of α -quartz, coesite, and stishovite were measured by differential scanning calorimetry at 183–703 K. The heat capacity data of coesite represent a significant revision of the previous data. The heat capacities and entropies were also calculated using Kieffers model. By high-temperature solution calorimetry, transition enthalpy for the β-quartz-coesite at 979 K was measured to be 1.27±0.39 kJ/mol. The enthalpies for the α-quartz-coesite and coesite-stishovite transitions at 298 K were measured to be 3.40±0.56 and 33.62±1.01 kJ/mol, respectively, by differential drop-solution calorimetry. The enthalpy of the coesite-stishovite transition obtained in this study is about 10–15 kJ/mol smaller than those in the previous studies. Phase relations in SiO2 at high pressures and high temperatures were calculated using these new thermodynamic data. The calculated boundary for the α-quartz-coesite transition is consistent with those determined experimentally. For the coesite-stishovite transition, the calculated boundary having a slope of 2.5±0.3 MPa/K disagrees with that by Yagi and Akimoto but is generally consistent with that by Zhang et al.s new in situ X ray diffraction study.

High pressure transitions in the system KAlSi3O8-NaAlSi3O8

Phase relations in the system KAlSi3O8-NaAlSi 3O8 have been examined at pressures of 5–23 GPa and temperatures of 700–1200° C. KAlSi3O8 sanidine first dissociates into a mixture of wadeite-type K2Si4O9, kyanite and coesite at 6–7 GPa, which further recombines into KAlSi3O8 hollandite at 9–10 GPa. In contrast, NaAlSi3O8 hollandite is not stable at 800–1200° C near 23 GPa, where the mixture of jadeite plus stishovite directly changes into the assemblage of calcium ferrite-type NaAlSiO4 plus stishovite. Phase relations in the system KAlSi3O8-NaAlSi3O8 at 1000° C show that NaAlSi3O8 component gradually dissolves into hollandite with increasing pressure. The maximum solubility of NaAlSi3O8 in hollandite at 1000° C was about 40 mol% at 22.5 GPa, above which it decreases with pressure. Unit cell volume of the hollandite solid solution decreases with increasing NaAlSi3O8 component. The hollandite solid solution in this system may be an important candidate as a host mineral of K and Na in the uppermost lower mantle

The quartz-coesite-stishovite transformations: new calorimetric measurements and calculation of phase diagrams

Abstract High temperature solution calorimetry of synthetic quartz, coesite and stishovite provides enthalpies of transition. ΔH 975 0 for quartz-coesite and ΔH 298 0 for coesite-stishovite transition are 320 ± 70 and 11700 ± 410 cal mol −1 , respectively. The present transformation enthalpy data represent a small but significant revision of those of Holm et al. Using the published phase equilibrium data, thermal expansivity, compressibility and heat capacity data, ΔS 975 0 for the quartz-coesite and ΔS 298 0 for the coesite-stishovite transition are −1.2 ± 0.1 and −1.0 ± 0.4 cal K −1 mol −1 , respectively. These thermochemical data are used to calculate phase boundaries of the transitions. The calculated quartz-coesite transition boundary agrees well with the one determined experimentally by Bohlen and Boettcher. The calculated coesite-stishovite boundary is generally consistent with data by Yagi and Akimoto and by Suito.

Post-garnet transitions in the system Mg4Si4O12–Mg3Al2Si3O12 up to 28 GPa: phase relations of garnet, ilmenite and perovskite

Phase transitions in Mg3Al2Si3O12 garnet (pyrope) were examined at 23–28 GPa and 1600–2000°C using a 6–8-type multianvil apparatus. It was found that pyrope dissociated at 26.5–27 GPa and 1600–2000°C into MgSiO3-rich perovskite solid solution and Al2O3-rich corundum solid solution with a slightly positive dP/dT slope. Perovskite solid solution containing 10 mol% Al2O3 was stabilized at about 26.5 GPa and 1600°C, coexisting with corundum solid solution of 23 mol% MgSiO3. Powder X-ray diffraction of the single-phase aluminous perovskites in the system MgSiO3–Al2O3 showed that these were orthorhombic and that the increase in the b- and c-axes and a slight decrease in the a-axis accompanied increasing Al2O3 content. Our data and other available X-ray data indicate that the unit cell volume of perovskite in the system MgSiO3–Al2O3 is best expressed as V (A3)=162.35+6.95x, where x represents the mol fraction of Al2O3 in perovskite (0≤x≤0.25). Phase relations on the Mg4Si4O12–Mg3Al2Si3O12 join were examined at 20–27 GPa at 1600°C, and also at about 900°C. A wide two-phase field of garnet+perovskite and a relatively narrow field of ilmenite+garnet were delineated at 1600°C. However, at about 900°C at 21–26 GPa, the stability fields of single-phase ilmenite and of ilmenite+garnet were greatly expanded. Thus, in the cool interior of descending slabs near 660 km discontinuity, there may be a significant depth interval over which garnet gives way to ilmenite-bearing assemblages, before ultimate transformation to perovskite takes place.

HIGH PRESSURE TRANSITIONS IN THE SYSTEM MGAL2O4-CAAL2O4 A NEW HEXAGONAL ALUMINOUS PHASE WITH IMPLICATION FOR THE LOWER MANTLE

Abstract Phase transitions in MgAl 2 O 4 and CaAl 2 O 4 and in the binary system MgAl 2 O 4 –CaAl 2 O 4 were established at high pressures and high temperatures. MgAl 2 O 4 spinel dissociates to periclase and corundum at 15–16 GPa and 1200°C–1600°C with an equilibrium boundary of P (GPa)=12.3+0.0023 T (°C). The mixture recombines into a phase with calcium ferrite structure at about 26.5 GPa at 1600°C. At 780°C–1400°C, CaAl 2 O 4 crystallizes with the calcium ferrite structure at pressure above 8–9 GPa. At lower pressures, phases III and IV are stable. On the join MgAl 2 O 4 –CaAl 2 O 4 , solubility of MgAl 2 O 4 component into CaAl 2 O 4 -rich calcium ferrite is limited to about 10 mol% at 20–21 GPa and 1200°C, and a new phase with composition of (Mg x ,Ca 1− x )Al 2 O 4 ( x =0.66–0.8) was found to be stable at pressure above 15 GPa. Powder X-ray diffraction data show that the new phase possesses hexagonal symmetry. Moreover, it is interpreted that an Al-rich phase observed in high pressure assemblage of mid-ocean ridge basalt (MORB) above 25 GPa by Irifune and Ringwood [Irifune, T., Ringwood, A.E., 1993. Phase transformations in subducted oceanic crust and buoyancy relationships at depths of 600–800 km in the mantle. Earth Planet. Sci. Lett. 117, 101–110.] has the same structure as the hexagonal phase found in this study, rather than calcium ferrite structure. And another Al-rich phase synthesized at 24–27 GPa in high pressure experiments from natural pyrope-rich garnet starting material is also similar to the hexagonal phase. These results suggest that aluminous phase of the hexagonal structure would be one of major constituent minerals of subducted oceanic crust in the upper part of the lower mantle.