Toshiro Nagase

A new hydrous silicate, a water reservoir, in the upper part of the lower mantle

We synthesized a new hydrous silicate in the pressure range from 20 GPa to 24 GPa at 800–1300°C. This phase, named tentatively as phase G, has a hexagonal unit cell, a=4.790 (3) A and c=4.344 (3) A, and V=86.3 (2) A³ and the atomic ratio Mg/Si=0.66±0.03. SIMS analysis revealed that it contains 14.5±2.0wt% water. Phase G has a chemical formula of Mg1.14Si1.73H2.81O6 and a density of 3.37g/cm³. Phase G coexists with periclase and Mg-perovskite under the lower mantle conditions, and thus it can be a reservoir of water in cold slabs penetrating into 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 ∼800 km 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 1,000 K, we estimate that metastable garnet should survive of the order of 10 Myr even at 1,600 K. 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.

Structure and crystal chemistry of Phase G, A new hydrous magnesium silicate synthesized at 22 GPa and 1050°C

A single crystal of phase G, Mg1.24Si1.76H2.48O6 synthesized at conditions of 1050°C and 22 GPa, using a multi-anvil apparatus was studied at the Photon Factory BL-10A beamline at the National Laboratory for High Energy Physics, Tukuba, Japan. With a (111) Si double-crystal monochromator and a single crystal measuring 47 × 35 × 12 µm, intensities of 95 independent reflections were collected with sin θ/λ 1.5 σ Io at a wave length of 0.6990 A. The unit cell parameters obtained through the refinement of 23 reflections are: Trigonal, a=4.790 (3) A, c=4.344 (3) A, V=86.3 (2) A³. The result of structure analysis in space group P 1m (No. 162) indicates that the structure of phase G has close structural similarity to that of stishovite,SiO2. The calculated density of phase G is 3.43 g/cm³, which is larger than any other known dense hydrous magnesium silicate, suggesting that phase G might be stable even under lower mantle conditions.

Evidence for fractional crystallization of wadsleyite and ringwoodite from olivine melts in chondrules entrained in shock-melt veins.

Peace River is one of the few shocked members of the L-chondrites clan that contains both high-pressure polymorphs of olivine, ringwoodite and wadsleyite, in diverse textures and settings in fragments entrained in shock-melt veins. Among these settings are complete olivine porphyritic chondrules. We encountered few squeezed and flattened olivine porphyritic chondrules entrained in shock-melt veins of this meteorite with novel textures and composition. The former chemically unzoned (Fa24–26) olivine porphyritic crystals are heavily flattened and display a concentric intergrowth with Mg-rich wadsleyite of a very narrow compositional range (Fa6–Fa10) in the core. Wadsleyite core is surrounded by a Mg-poor and chemically stark zoned ringwoodite (Fa28–Fa38) belt. The wadsleyite–ringwoodite interface denotes a compositional gap of up to 32 mol % fayalite. A transmission electron microscopy study of focused ion beam slices in both regions indicates that the wadsleyite core and ringwoodite belt consist of granoblastic-like intergrowth of polygonal crystallites of both ringwoodite and wadsleyite, with wadsleyite crystallites dominating in the core and ringwoodite crystallites dominating in the belt. Texture and compositions of both high-pressure polymorphs are strongly suggestive of formation by a fractional crystallization of the olivine melt of a narrow composition (Fa24–26), starting with Mg-rich wadsleyite followed by the Mg-poor ringwoodite from a shock-induced melt of olivine composition (Fa24–26). Our findings could erase the possibility of the resulting unrealistic time scales of the high-pressure regime reported recently from other shocked L-6 chondrites.

Natural dissociation of olivine to (Mg,Fe)SiO3 perovskite and magnesiowüstite in a shocked Martian meteorite

We report evidence for the natural dissociation of olivine in a shergottite at high-pressure and high-temperature conditions induced by a dynamic event on Mars. Olivine (Fa34-41) adjacent to or entrained in the shock melt vein and melt pockets of Martian meteorite olivine-phyric shergottite Dar al Gani 735 dissociated into (Mg,Fe)SiO3 perovskite (Pv)+magnesiowüstite (Mw), whereby perovskite partially vitrified during decompression. Transmission electron microscopy observations reveal that microtexture of olivine dissociation products evolves from lamellar to equigranular with increasing temperature at the same pressure condition. This is in accord with the observations of synthetic samples recovered from high-pressure and high-temperature experiments. Equigranular (Mg,Fe)SiO3 Pv and Mw have 50–100 nm in diameter, and lamellar (Mg,Fe)SiO3 Pv and Mw have approximately 20 and approximately 10 nm in thickness, respectively. Partitioning coefficient, KPv/Mw = [FeO/MgO]/[FeO/MgO]Mw, between (Mg,Fe)SiO3 Pv and Mw in equigranular and lamellar textures are approximately 0.15 and approximately 0.78, respectively. The dissociation of olivine implies that the pressure and temperature conditions recorded in the shock melt vein and melt pockets during the dynamic event were approximately 25 GPa but 700 °C at least.

New silica clathrate minerals that are isostructural with natural gas hydrates

Silica clathrate compounds (clathrasils) and clathrate hydrates are structurally analogous because both materials have framework structures with cage-like voids occupied by guest species. The following three structural types of clathrate hydrates are recognized in nature: cubic structure I (sI); cubic structure II (sII); and hexagonal structure H (sH). In contrast, only one naturally occurring silica clathrate mineral, melanophlogite (sI-type framework), has been found to date. Here, we report the discovery of two new silica clathrate minerals that are isostructural with sII and sH hydrates and contain hydrocarbon gases. Geological and mineralogical observations show that these silica clathrate minerals are traces of low-temperature hydrothermal systems at convergent plate margins, which are the sources of thermogenic natural gas hydrates. Given the widespread occurrence of submarine hydrocarbon seeps, silica clathrate minerals are likely to be found in a wide range of marine sediments.

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.

Discovery of seifertite in a shocked lunar meteorite

Many craters and thick regoliths of the moon imply that it has experienced heavy meteorite bombardments. Although the existence of a high-pressure polymorph is a stark evidence for a dynamic event, few high-pressure polymorphs are found in a lunar sample. α-PbO₂-type silica (seifertite) is an ultrahigh-pressure polymorph of silica, and is found only in a heavily shocked Martian meteorite. Here we show evidence for seifertite in a shocked lunar meteorite, Northwest Africa 4734. Cristobalite transforms to seifertite by high-pressure and -temperature condition induced by a dynamic event. Considering radio-isotopic ages determined previously, the dynamic event formed seifertite on the moon, accompanying the complete resetting of radio-isotopic ages, is ~2.7 Ga ago. Our finding allows us to infer that such intense planetary collisions occurred on the moon until at least ~2.7 Ga ago.

Triclinic liddicoatite and elbaite in growth sectors of tourmaline from Madagascar

Abstract Crystals of liddicoatite-elbaite tourmaline from a pegmatite in Jochy, Madagascar are composed of o{021̄1}, r{101̄1}, c{0001}, a{112̄̄0}, and m{101̄0} sectors, which correspond to the prominent crystal faces, respectively. Therefore, the sectors were produced during growth, not by strain after growth. The o, m, and r sectors of one specimen are biaxial between crossed polars [2V(-) = 30°, 20°, and 15°, respectively] and triclinic, as indicated by X-ray diffraction. The a sector is optically biaxial and orthorhombic. The c sector is optically uniaxial and essentially trigonal as indicated by single-crystal X-ray diffraction. The o, r, and c sectors are of liddicoatite component, whereas the a sector of the one specimen corresponds to fluor-elbaite. Another crystal specimen comprises a and m sectors, which are polysynthetically twinned, resulting in striations parallel to the c axis on the prism faces, and of liddicoatite. All five sectors have vacancies in the X-site (Ca, Na, ⃞ ).

Crystal structure and compressibility of superhydrous phase B, Mg20Si6H8O36

Crystal structure of superhydrous phase B, Mg20Si6H8O36, synthesized at 17 GPa and 1000 °C using the uniaxial, split‐sphere, multi‐anvil apparatus, was analyzed by the single‐crystal X‐ray diffraction method. Compressibility of the unit cell was measured up to 6.5 GPa with single crystal X‐ray method using a diamond anvil high pressure cell and synchrotron radiation at the BL‐10 A beam line at the National Laboratory for High Energy Physics. The observed bulk modulus K0T=145 GPa is 12% larger than that of α‐Mg2SiO4 and smaller than that of β‐Mg2SiO4.