Maiko Isshiki

Stability of magnesite and its high-pressure form in the lowermost mantle.

Carbonates are important constituents of marine sediments and play a fundamental role in the recycling of carbon into the Earths deep interior via subduction of oceanic crust and sediments. Study of the stability of carbonates under high pressure and temperature is thus important for modelling the carbon budget in the entire Earth system. Such studies, however, have rarely been performed under appropriate lower-mantle conditions and no experimental data exist at pressures greater than 80 GPa (refs 3–6). Here we report an in situ X-ray diffraction study of the stability of magnesite (MgCO3), which is the major component of subducted carbonates, at pressure and temperature conditions approaching those of the core–mantle boundary. We found that magnesite transforms to an unknown form at pressures above ∼115 GPa and temperatures of 2,100–2,200 K (depths of ∼2,600 km) without any dissociation, suggesting that magnesite and its high-pressure form may be the major hosts for carbon throughout most parts of the Earths lower mantle.

Iron partitioning in a pyrolite mantle and the nature of the 410-km seismic discontinuity

Pyrolite is a hypothetical mixture of distinct minerals which iswidely believed to represent the composition of the Earths mantle. The main pressure-induced phase transformations of the olivine component of pyrolite occur at about 13.5 GPa (α to β) and 24 GPa (γ to MgSiO3-rich perovskite + magnesiowüstite),, which are thought to be responsible for the seismic discontinuities at 410 and 660 km depths in the mantle. Recent seismological studies, however, have demonstrated that the 410-km seismic discontinuity is sharper in some areas than that expected from the α to β transformation in mantle olivine with a fixed composition. Moreover, some mineral physics studies suggest that the seismic velocity jump at the 410-km discontinuity is inconsistent with that associated with the α to β transformation in olivine,. Here we present a phase equilibria study of a material having pyrolite composition at pressures of 6–16 GPa. We found that the iron content in olivine changes significantly with increasing pressure, as a result of the formation of a relatively iron-rich majorite phase at these pressures. This variation in iron content can overcome, or at least reduce, both of the above difficulties encountered with the pyrolite model of mantle composition, by showing that the component mineral systems cannot be treated as separate.

Phase transformations in serpentine and transportation of water into the lower mantle

Phase transformations in serpentine, the major hydrous mineral in the upper part of the subducting slabs, have been studied at pressures to 26 GPa and at temperatures between 500 and 1400°C. While serpentine completely dehydrates at temperatures higher than 600 - 800°C and at pressures to 12 GPa, phase assemblages including some dense hydrous magnesium silicates (DHMS) such as phases A, E, superhydrous B and D were formed at higher pressures. Phase D was found to possess a stability field of particularly wide pressure and temperature regime, suggesting that this phase may play an important role in transportation of water into the lower mantle via subduction of cold slabs.

Post-stishovite phase boundary in SiO2 determined by in situ X-ray observations

A laser heating diamond anvil cell experiment, with an angle-dispersive X-ray diffraction using synchrotron radiation source at the SPring-8, has been developed to observe the phase transition in silica (SiO2) between the P42/mnm (rutile-type) and Pnnm (CaCl2-type) up to pressures of 100 GPa and at temperatures up to 2200 K. The transition was observed in the vicinity of 55 GPa at room temperature, and showed a positive temperature dependence of the transition pressure. The phase boundary was determined to follow the equation P (GPa)=(51±2)+(0.012±0.005)×T (K). Our result gives a transition pressure of near 80 GPa and a depth of 1900 km at an expected lower mantle temperature of 2000–2500 K. Therefore, this SiO2 transition is not the cause of recent observations of seismic anomalies between 800 and 1600 km depth in the mid–lower mantle.

Construction of laser-heated diamond anvil cell system for in situ x-ray diffraction study at SPring-8

A double-sided laser heated diamond anvil cell (DAC) system was constructed at a high brilliance, undulator beamline (BL10XU) at SPring-8 a third generation synchrotron radiation facility, for performing in situ angle-dispersive x-ray diffraction experiments under high temperature and high pressure. The design of this system puts emphasis on reliable data collection for the structural analysis. With this system, the adjustment of the optical systems for both x-ray and laser beams can be done easily, and high quality diffraction data can be obtained typically within several minutes. A system for temperature measurement of ten points in a sample area at the same time was also developed. The performance of the laser heated DAC system was tested by observing phase transitions of natural olivine.

Mg/Si ratios of aqueous fluids coexisting with forsterite and enstatite based on the phase relations in the Mg2SiO4-SiO2-H2O system

Abstract Direct observation of aqueous fluids coexisting with MgSiO3 (enstatite) and/or Mg2SiO4 (forsterite) was performed at 0.5-5.8 GPa and 800-1000 °C with an externally heated diamond-anvil cell and synchrotron X-rays. At 1000 °C in the MgSiO3 -H2O system, forsterite crystallizes below 3 GPa but not above that pressure. At 1000 °C in the Mg2SiO4 -H2O system, forsterite congruently dissolves into the aqueous fluids up to 5 GPa. These observations suggest that the aqueous fluids coexisting with enstatite and forsterite have Mg/Si < 1 below 3 GPa and 1 < Mg/Si < 2 above that pressure. Comparison with the previous studies reporting Mg/Si ratios of the aqueous fluid coexisting with enstatite and forsterite indicates that the Mg/Si ratios change rapidly from SiO2-rich to MgO-rich at around 3 GPa and 1000 °C. This change can be related to possible structural changes of liquid water under these conditions. The aqueous fluids coexisting with enstatite and forsterite do have Mg/Si ratios similar to those found in the partial melts of H2O-saturated peridotite. Somewhere within the upper mantle, these two fluids unite to form a single regime and cannot be distinguished from each other.

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).