Shigeaki Ono

Mineralogy of subducted basaltic crust (MORB) from 25 to 37 GPa, and chemical heterogeneity of the lower mantle

Abstract Experimental multi-anvil study of the phase relations in subducted oceanic crust (mid-ocean ridge basalt (MORB)) composition over a pressure range from 25 to 37 GPa, equivalent to depths of 700–950 km in the lower mantle, reveals that at pressures between 25 and 30 GPa majoritic garnet disproportionates into the Mg- and Ca-perovskites and an aluminous calcium ferrite type phase, but no further change in mineralogy takes place at higher pressures up to 37 GPa. The intermittent seismic discontinuity, at least that at 920 km depth reported by Kawakatsu and Niu [H. Kawakatsu, F. Niu, Nature 371 (1994) 301–305], might be attributable to these mineralogical changes in the former basaltic crust of subducted slabs. Moreover, we estimate that a density profile of MORB intersects an average mantle density at around 1500–2000 km depth in the lower mantle. The neutral buoyancy may contribute to the observed transition in seismological heterogeneity at a depth around 1600 km.

Stability limits of hydrous minerals in sediment and mid-ocean ridge basalt compositions: Implications for water transport in subduction zones

Phase relationships in hydrated natural sediment and mid-ocean ridge basalt (MORB) compositions were investigated experimentally from 6 to 15 GPa and 700°C to 1400°C in order to determine the stability of hydrous phases in subducting slabs and to constrain reactions resulting in the release of water from subduction zones to the mantle. Phengite and hydrous aluminum silicates (topaz-OH and phase egg) are stable in the sediment composition at depths greater than 150 km and constitute H2O reservoirs. Similarly, lawsonite is an H2O reservoir in MORB composition at depths greater than 150 km. According to mass balance calculations based on experimental run products, the weight proportions of phengite and H2O at 6 GPa in the sediment are about 50% and 2%, respectively. At pressures from 8 to 12 GPa, topaz-OH (Al2SiO4(OH)2) appears as the hydrous mineral, and the sediment retains about 0.5–0.7 wt % H2O. At higher pressures, phase egg (AlSiO3(OH)) appears as the hydrous phase with a large stability field. The H2O content in the sediment is about 0.5 wt% at pressures greater than 12 GPa. In natural MORB compositions, lawsonite is the hydrous phase at pressures lower than 10 GPa. About 1 wt%, H2O in the MORB composition is retained by lawsonite at 6 GPa and 800°C. Above 6 GPa, a lawsonite-out reaction is controlled by compositional changes of garnet and clinopyroxene. If the oceanic crust descends with an overlying sediment layer, water can be carried down into the transition zone by the sediment. The amount of transported water is controlled by the chemical compositions of the subducting sediments. Water in the MORB layer can be stored to 300 km in lawsonite for a cold subduction zone geotherm. Water release at depths greater than 200 km through progressive lawsonite breakdown can hydrate the overlying mantle, causing the generation of a peridotite region including hydrous phases (chondrodite, clinohumite, phase A, and phase E). If this hydrous peridotite is dragged down by the descending oceanic crust, water can be transported into the transition zone by the subduction process. The amount of transported water is controlled by the thermal state of the slab and the chemical composition of MORB.

Post-spinel transition in Mg2SiO4 determined by high P - T in situ X-ray diffractometry

Abstract The phase boundary of the post-spinel transition in Mg2SiO4 was re-investigated by means of high P–T in situ X-ray diffractometry with a gold pressure marker in a Kawai-type apparatus. Rapid and continuous temperature changes were conducted to initiate dissociation of spinel, which tends to be inert after long annealing. Isothermal decompression at high temperature was conducted to form spinel from perovskite plus periclase. The phase boundary is located at ca. 22xa0GPa in the temperature range from 1550 to 2100xa0K, which is 1–1.5xa0GPa lower than the 660xa0km discontinuity. This discrepancy might be explained in terms of the pressure effect of thermocouple emf and inaccurate equation of state (EOS) for the pressure maker. The transition is found to be less sensitive to temperature than reported previously, with a Clapeyron slope ranging from −2 to −0.4xa0MPa/K. This small Clapeyron slope implies that the post-spinel transition would not be an effective barrier to mantle convection.

In situ measurements of the phase transition boundary in Mg3Al2Si3O12: implications for the nature of the seismic discontinuities in the Earth’s mantle

Abstract Here we report the phase boundary of pyrope garnet (Mg3Al2Si3O12) to Al-bearing silicate perovskite plus corundum, with the highest transition pressure determined by in situ measurements in a multi-anvil apparatus at high temperature. The consistency of the pressure scales by different standards of Au, NaCl, Pt, W, and Mo at high temperature was also evaluated by in situ X-ray measurements. Our results, together with recent in situ measurements of the post-spinel transition in Mg2SiO4 [Irifune et al., Science 279 (1998) 1698–1700] and the ilmenite–perovskite transition in MgSiO3 [Ono et al., Geophys. Res. Lett. (2000) submitted], show that pressures determined in conventional quench experiments [Ito and Takahashi, J. Geophys. Res. 94 (1989) 10637–10646] could have been overestimated by more than 2 GPa at pressures corresponding to the bottom of the transition zone. On the basis of the in situ measurements, the post-spinel transition occurs at a depth (∼600 km) that is too shallow to match with the 660-km seismic discontinuity in the Earth’s mantle. Therefore, an olivine dominant mantle compositional model may be inconsistent with the seismic observations. Alternatively, we propose a pyroxene–garnet dominant transition zone with an appropriate Al2O3 content (ca. 6–8 mol%), in which majorite garnet transforms to perovskite at the depth of the 660-km discontinuity. Any alternative models would have to consider chemical stratification in the mantle.

Compositional change of majoritic garnet in a MORB composition from 7 to 17 GPa and 1400 to 1600°C

Experimental phase relations of an average normal mid-ocean ridge basalt (N-MORB) composition from 7 GPa to 17 GPa and 1400 to 1600°C reveal the compositional variation of garnet in equilibrium with pyroxene. The Na2O content of majoritic garnet in equilibrium with pyroxene and stishovite increases up to 2.5 wt.% with increasing pressure and temperature. Na enters the garnet structure via the substitution (Mg,Fe,Ca) + Al = Na + (Si,Ti). The pyroxene-garnet partition coefficient (Dcp-ga) for na decreases with increasing pressure. However, clinopyroxene coexisting with garnet becomes more Na rich as pressure increases. These relationships are independent of the FeO content in garnet. Our experiments confirm that rare majoritic and Na-rich garnet inclusions in diamonds have formed at transition zone pressure, in lithologies of basaltic bulk composition.

In situ Observation of ilmenite‐perovskite phase transition in MgSiO3 using synchrotron radiation

In situ observations of the ilmenite-perovskite transition in MgSiO3 were carried out in a multianvil high-pressure apparatus interfaced with synchrotron radiation. The phase boundary between ilmenite and perovskite in the temperature range of 1300–1600 K was determined to be P (GPa) = 28.4(±0.4) - 0.0029(± 0.0020)T (K) based on Jamiesons equation of state of gold [Jamieson et al., 1982] and P (GPa) = 27.3(±0.4) - 0.0035(±0.0024)T (K) based on Andersons equation of state of gold [Anderson et al., 1989]. The consistency of our results, using Jamiesons equation of state, with previous studies obtained by quench methods leads us to conclude that the 660 km seismic discontinuity in the mantle can be attributed a phase transition to perovskite phase. However, the phase boundary based on the Andersons equation of state implies that the depth of the 660-km seismic discontinuity does not match the pressure of this transition.

Phase boundary between rutile-type and CaCl2-type germanium dioxide determined by in situ X-ray observations

Abstract In situ synchrotron X-ray experiments of the GeO2 system were made at pressures of 28-45 GPa and temperatures of 300-2300 K, using a diamond anvil cell combined with a laser heating and a 6- 8 type multianvil high-pressure apparatus. We observed a second-order phase transition between tetragonal rutile-type (P42/mnm) and orthorhombic CaCl2-type (Pnnm) phases under high pressure and temperature. The transition kinetics seem to have little effect on the second-order phase transition because the cell constants exhibit no discontinuities between the phases. Therefore, the phase transitions could be observed at low temperature conditions in this study. The phase boundary was determined to be P (GPa) = (34.9 ± 1.2) + (0.0086 ± 0.0024) × (T - 1300) (K).

High-pressure magnetic transition in hcp-Fe

Abstract High-pressure experiments and ab initio calculations on the hexagonal close-packed (hcp) structure of pure iron were performed to investigate a pressure-induced magnetic transition and the equation of state in the pressure range of 0-107 and 0-400 GPa, respectively. The experimental data at room temperature showed a significant change in the cell parameter ratio at 55 GPa without any major structural changes occurring. Ab initio calculations at 0 K indicate that the change in the cell parameter ratio observed in the high-pressure experiments corresponds to a magnetic transition from an antiferromagnetic state to a nonmagnetic state. If the hcp-Fe is stable under inner core conditions, then the density of nonmagnetic hcp-Fe is ~6% denser than that of the inner core, as determined using the PREM model. This supports the view that the composition of the inner core should be composed of iron and a significant amount of lighter elements.

Electrical conductivity of aragonite in the subducted slab

The electrical conductivity of polycrystalline aragonite (CaCO 3 ) was investigated using a multianvil press in the pressure range 3–6 GPa. The electrical conductivity of the samples was measured using a complex impedance analyzer in the frequency range 0.05–10 –6 Hz. A decrease in the electrical conductivity with increasing pressure was observed. The calculated activation enthalpy in the temperature range 800–1000 K increased with increasing pressure. The effect of pressure and temperature were included in the Arrhenius equation, and the fitted data gave an activation energy and volume of 0.40 eV (38.6 kJ/mol) and 9.28 cm 3 /mole, respectively. The positive activation volume observed in this study suggests that ionic conduction is the dominant mechanism for the electrical conductivity over the pressure and temperature range investigated. The electrical conductivity of aragonite is much higher than that of olivine, which is a major mineral in the upper mantle. Therefore, the subducted slab, which contains a significant amount of calcium carbonate, has a higher electrical conductivity compared with the surrounding mantle.