Tadashi Kondo

In situ determination of the phase boundary between Wadsleyite and Ringwoodite in Mg2SiO4

The phase boundary between wadsleyite and ringwoodite in Mg2SiO4 composition was determined by in situ observation using synchrotron X-ray and multi anvil apparatus in KEK, Tsukuba, Japan. An energy dispersive method was employed using the Ge solid state detector and the white X-ray beam from the synchrotron radiation source. The pressure was determined by the equation of state of NaCl. The stability field was identified by the change in intensities of diffraction lines of each phases. As a result, the phase boundary is expressed as a linear equation P=10.32(28)+0.00691(9)×T, where P is pressure in gigapascals and T is temperature in degrees Celsius.

Water solubility in Mg-perovskites and water storage capacity in the lower mantle

The water storage capacity of the major constituent of the lower mantle, Mg-perovskite, is a matter of debate. Here we report water solubility of Mg-perovskites with different compositions observed in peridotite and MORB systems. IR spectra of pure MgSiO3-perovskite show bands at 3397, 3423, 3448, and 3482 cm−1 and suggest about 100 ppm H2O. The H2O content in Al-Mg-perovskite (4–7 wt% Al2O3; Mg#=100) is 1000–1500 ppm (major band at 3448 cm−1), whereas Al-Fe-Mg-perovskite in MORB (Al2O3=13–17 wt%; Mg#=58–61) contains 40–110 ppm H2O (major band at 3397 cm−1). The H2O content in Al-Fe-Mg-perovskite observed in peridotite (Al2O3=5–6 wt%; Mg#=88–90) is 1400–1800 ppm (major band at 3397 cm−1). Al-Fe-Mg-perovskite from the MORB system has a high Fe3+ content, Fe3+/∑Fe=0.6, determined by electron energy loss spectroscopy measurements. Water can enter into the perovskite structure with oxygen vacancies originating from the substitution of Si by Al and Fe3+. Oxygen vacancy incorporation is favored for aluminous perovskite synthesized from the MgO-rich peridotite system. The substitution of Si4++Mg2+=2(Al,Fe)3+ prevails however in the Al-Fe-Mg-perovskite from the MORB system (MgO-poor, Al- and Fe-rich), explaining its restricted water solubility. The maximum amount of water stored in the lower mantle is estimated to be 3.42×1021 kg, which is 2.5 times the present ocean mass. Comparison of the phase relations in hydrous pyrolite and hydrous MORB indicates that pyrolite is more important as water container and water carrier in the mantle. Pyrolite contains: (1) dense hydrous magnesium silicates, existing under conditions of subducting slabs, and (2) hydrous wadsleyite, hydrous ringwoodite and water-bearing perovskite under the normal mantle and hotter conditions. Distribution of water to the MORB is restricted at the conditions of the transition zone and lower mantle.

Fe‐Mg partitioning between (Mg, Fe)SiO3 post‐perovskite, perovskite, and magnesiowüstite in the Earth's lower mantle

[1]xa0We report here new data on pressure dependence of Fe-Mg partitioning between (Mg, Fe)SiO3 perovskite (Pv) and magnesiowustite (Mw), KPv/Mw, and (Mg, Fe)SiO3 post-perovskite (PPv) and Mw, KPPv/Mw, up to 123.6 GPa at 1600 K measured by synchrotron X-ray diffraction method and analytical transmission electron microscopy (ATEM). We observed a high FeO content in PPv coexisting with Mw [KPPv/Mw = (FeO/MgO)PPv/(FeO/MgO)Mw = 0.30] compared to that in Pv [KPv/Mw = (FeO/MgO)Pv/(FeO/MgO)Mw = 0.12] observed from 23.0 to 95.4 GPa. KPv/Mw keeps a constant value of 0.12 up to the PPv phase boundary. Our results also support the possibility that a metallic phase may form in the lower mantle. The assemblage of PPv and Mw is 1.5–1.7% denser than the Pv bearing assemblage, which results in a gravitational stabilization of the lowermost mantle.

Interaction between iron and post‐perovskite at core‐mantle boundary and core signature in plume source region

[1]xa0Interaction between the lower mantle and core is essential for understanding the nature of D″ layer at the core-mantle boundary (CMB). Here, we report the reaction between post-perovskite (PPv) and metallic iron under the condition of the CMB, for example, 139 GPa and 3000 Kelvin. Analytical transmission electron microscope (ATEM) analysis revealed that significant amount of oxygen up to 6.3 weight percent (wt.%) and silicon up to 4.0 wt.% can be dissolved into molten iron. The dihedral angle between PPv and molten iron is 67 degrees. Thus, a small amount of core metal of about 2 volume percent (vol.%) can be trapped without separation in the PPv region at the CMB. The amount of core metal trapped by this mechanism can produce the isotopic signature of the outer core in the plume source at the base of the lower mantle.

Metastable garnet in oceanic crust at the top of the lower mantle

As oceanic tectonic plates descend into the Earths lower mantle, garnet (in the basaltic crust) and silicate spinel (in the underlying peridotite layer) each decompose to form silicate perovskite—the ‘post-garnet’ and ‘post-spinel’ transformations, respectively. Recent phase equilibrium studies have shown that the post-garnet transformation occurs in the shallow lower mantle in a cold slab, rather than at ∼800u2009km depth as earlier studies indicated, with the implication that the subducted basaltic crust is unlikely to become buoyant enough to delaminate as it enters the lower mantle. But here we report results of a kinetic study of the post-garnet transformation, obtained from in situ X-ray observations using sintered diamond anvils, which show that the kinetics of the post-garnet transformation are significantly slower than for the post-spinel transformation. Although metastable spinel quickly breaks down at a temperature of 1,000u2009K, we estimate that metastable garnet should survive of the order of 10u2009Myr even at 1,600u2009K. Accordingly, the expectation of where the subducted oceanic crust would be buoyant spans a much wider depth range at the top of the lower mantle, when transformation kinetics are taken into account.

Compression of iron hydride to 80 GPa and hydrogen in the Earth's inner core

[1]xa0Here we report a new data on the equation of state of iron hydride up to 80 GPa measured by an in situ X-ray diffraction method. We observed anomalous compression behavior at 30 to 50 GPa, and found that iron hydride might be less compressible above 50 GPa than at lower pressures. The bulk modulus above 50 GPa is larger than that of pure hcp-iron. Our data support the ab initio calculation suggesting a magnetic transition of iron hydride at around 60 GPa. The density deficit of the Earths inner core could be explained by dissolution of smaller amount of hydrogen than previously suggested.

In situ X‐ray observation of decomposition of superhydrous phase B at high pressure and temperature

[1]xa0High pressure and high temperature in situ X-ray diffraction experiments on the stability of superhydrous phase B have been conducted to clarify the host of water in the slabs descending into the lower mantle. We observed the decomposition reaction of superhydrous phase B into phase G (= phase D and F), periclase, and MgSiO3 perovskite at around 700 ∼1100°C and above 30 GPa, and the boundary of the decomposition reaction can be expressed as P(GPa) = −0.0037(±0.0017)T(°C) + 32.2(±0.5). This reaction can occur in slabs descending into the uppermost part of the lower mantle.

Equation of state of the hydrous phase δ-AlOOH at room temperature up to 22.5 GPa

[1]xa0The equation of state of δAlOOH has been determined at room temperature up to 22.5 GPa by angle-dispersive x-ray diffraction using synchrotron radiation. The isothermal bulk modulus K0T was found to be 252 (±3) GPa, assuming that its pressure derivative K′0T is 4, by fitting a third order Birch-Murnaghan equation of state. δAlOOH is thus the least compressible hydrous phase known with compressibility comparable to that of corundum and stishovite, confirming that this phase can be stable at the lower mantle conditions.

Formation of metastable cubic-perovskite in high-pressure phase transformation of Ca(Mg, Fe, Al)Si2O6

Abstract We have carried out in-situ X-ray diffraction experiments on high-pressure transformations of a Ca- and Fe- rich pyroxene (Ca1.03Mg0.61Fe0.23Al0.14Si2O6) to investigate the stability of Ca0.5(Mg, Fe, Al)0.5SiO3 perovskite (CM-perovskite) in a multi component system at about 32 GPa and up to 1900 °C. We observed that cubic CM-perovskite was formed at about 1300 °C and decomposed into cubic Ca-perovskites and orthorhombic Mg-perovskites and stishovite at 1800 °C when using a glass starting material. In another experiment using a crystalline pyroxene starting material, two cubic perovskites; Ca-perovskite and CM-perovskite, and orthorhombic Mg-perovskite formed simultaneously during the initial stage of the transformation. However, the cubic CM-perovskite subsequently decomposed into Mg- and Ca-perovskites and stishovite at 1200 °C. These results indicate that the assembly of cubic Ca-perovskite, orthorhombic Mg-perovskite and stishovite is stable and cubic CM-perovskite is a metastable phase at around 32 GPa and temperatures over 1000 °C in this system. Chemical analyses of product phases showed that Mg, Fe, and Al were preferentially partitioned into Mg-perovskite and the compositions of Ca-perovskite were close to pure CaSiO3. The present study shows that CM-perovskite nucleates during the initial stage of Ca(Mg, Fe, Al)Si2O6 pyroxene transformation. Therefore, cold subducting slabs and impacted meteorites are the possible places in which CM-perovskite could exist. The Ca-rich glassy phase in a shocked chondrite (Tomioka and Kimura 2003) might have formed by vitrification of a metastable CM-perovskite-like phase.

Partitioning of potassium between iron and silicate at the core‐mantle boundary

[1] Chemical reactions and the partitioning of potassium between metallic iron and potassium-silicate were studied at the core-mantle boundary (CMB) condition, 2900 km deep in the Earth, in a laser-heated diamond-anvil cell (LHDAC). Analytical transmission electron microscope (ATEM) analysis of the recovered samples from 134 GPa and 3500 K, prepared with Focused Ion Beam (FIB) techniques, revealed significant dissolution of potassium (0.8 wt.%) into molten iron, indicating that the partition coefficient of potassium between iron and silicate is 0.15. Our results show that the Earths core can contain 35 ppm total potassium, i.e., 4.1 × 10 -3 ppm of 40 K, which could serve as a heat source corresponding to 0.23 TW in the Earths core.