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Leonid Dubrovinsky - One of the best experts on this subject based on the ideXlab platform.
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evolution of diamond forming systems of the mantle transition zone ringwoodite peritectic reaction mg fe 2sio4 experiment at 20 gpa
Geochemistry International, 2019Co-Authors: A V Spivak, E S Zakharchenko, Dariia Simonova, Yu. A. Litvin, Leonid DubrovinskyAbstract:The peritectic reaction of ringwoodite (Mg,Fe) 2 SiO 4 and silicate-carbonate melt with formation of magnesiowustite (Fe,Mg)O, Stishovite SiO 2 and Mg, Na, Ca, K-carbonates is revealed by experimental study at 20 GPa of melting relations of the multicomponent MgO−FeO−SiO 2 −Na 2 CO 3 −CaCO 3 −K 2 CO 3 system of the Earth’s mantle transition zone. A reaction of CaCO 3 and SiO 2 with the formation of Ca-perovskite CaSiO 3 is also detected. It is shown that the peritectic reaction of ringwoodite and melt with the formation of Stishovite physic-chemically controls the fractional ultrabasic-basic evolution of both magmatic and diamond-forming systems of the deep horizons of the transition zone up to its boundary with the Earth’s lower mantle.
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Evolution of Diamond-Forming Systems of the Mantle Transition Zone: Ringwoodite Peritectic Reaction (Mg,Fe)_2SiO_4 (Experiment at 20 GPa)
Geochemistry International, 2019Co-Authors: A V Spivak, E S Zakharchenko, Yu. A. Litvin, D. A. Simonova, Leonid DubrovinskyAbstract:—The peritectic reaction of ringwoodite (Mg,Fe)_2SiO_4 and silicate–carbonate melt with formation of magnesiowustite (Fe,Mg)O, Stishovite SiO_2, and Mg, Na, Ca, K-carbonates is revealed by experimental study at 20 GPa of phase relations in the multicomponent diamond-forming MgO–FeO–SiO_2–Na_2CO_3–CaCO_3–K_2CO_3 system of the Earth mantle transition zone. An interaction of CaCO_3 and SiO_2 with a formation of Ca-perovskite CaSiO_3 is also detected. It is shown that the peritectic reaction of ringwoodite and melt with the formation of Stishovite controls physicochemically the fractional ultrabasic-basic evolution of both magmatic and diamond-forming systems of deep horizons of the transition zone up to its boundary with the Earth lower mantle.
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the Stishovite paradox in the evolution of lower mantle magmas and diamond forming melts experiment at 24 and 26 gpa
Doklady Earth Sciences, 2017Co-Authors: Yu. A. Litvin, A V Spivak, D. A. Simonova, Leonid DubrovinskyAbstract:Experimental studies of phase relations in the oxide–silicate system MgO–FeO–SiO2 at 24 GPa show that the peritectic reaction of bridgmanite controls the formation of Stishovite as a primary in situ mineral of the lower mantle and as an effect of the Stishovite paradox. The Stishovite paradox is registered in the diamond-forming system MgO–FeO–SiO2–(Mg–Fe–Ca–Na carbonate)–carbon in experiments at 26 GPa as well. The physicochemical mechanisms of the ultrabasic–basic evolution of deep magmas and diamondforming media, as well as their role in the origin of the lower mantle minerals and genesis of ultradeep diamonds, are studied.
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magmatic evolution of the material of the earth s lower mantle Stishovite paradox and origin of superdeep diamonds experiments at 24 26 gpa
Geochemistry International, 2016Co-Authors: Yu. A. Litvin, A V Spivak, Leonid DubrovinskyAbstract:The ultrabasic–basic magmatic evolution of the lower mantle material includes important physicochemical phenomena, such as the Stishovite paradox and the genesis of superdeep diamonds. Stishovite SiO2 and periclase–wustite solid solutions, (MgO · FeO)ss, associate paradoxically in primary inclusions of superdeep lower mantle diamonds. Under the conditions of the Earth’s crust and upper mantle, such oxide assemblages are chemically impossible (forbidden), because the oxides MgO and FeO and SiO2 react to produce intermediate silicate compounds, enstatite and ferrosilite. Experimental and physicochemical investigations of melting phase relations in the MgO–FeO–SiO2–CaSiO3 system at 24 GPa revealed a peritectic mechanism of the Stishovite paradox, (Mg, Fe)SiO3 (bridgmanite) + L = SiO2 + (Mg, Fe)O during the ultrabasic–basic magmatic evolution of the primitive oxide–silicate lower mantle material. Experiments at 26 GPa with oxide–silicate–carbonate–carbon melts, parental for diamonds and primary inclusions in them, demonstrated the equilibrium formation of superdeep diamonds in association with ultrabasic, (Mg, Fe)SiO3 (bridgmanite) + (MgO · FeO)ss (ferropericlase), and basic minerals, (FeO · MgO)ss (magnesiowustite) + SiO2 (Stishovite). This leads to the conclusion that a peritectic mechanism, similar to that responsible for the Stishovite paradox in the pristine lower mantle material, operates also in the parental media of superdeep diamonds. Thus, this mechanism promotes both the ultrabasic–basic evolution of primitive oxide–silicate magmas in the lower mantle and oxide–silicate–carbonate melts parental for superdeep diamonds and their paradoxical primary inclusions.
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Stishovite and post Stishovite polymorphs of silica in the shergotty meteorite their nature petrographic settings versus theoretical predictions and relevance to earth s mantle
Journal of Physics and Chemistry of Solids, 2004Co-Authors: A. El Goresy, Leonid Dubrovinsky, T G Sharp, Ming ChenAbstract:Abstract The Shergotty meteorite contains three dense silica polymorphs in distinct petrographic settings: (1) two post-Stishovite SiO 2 polymorphs in individual multiphase grains coexisting with glass with nearly labradorite composition, and (2) large individual Stishovite grains in shock-melt pockets which also contain the new CAS phase (Calcium-aluminosilicate; CaAl 4 Si 2 O 11 ; [Phys Earth Planet Interiors 97 (1996) 97; Geophys Abstract 5(2003)] and hollandite structured plagioclase composition. Prismatic and wedge-shaped grains of the original accessory tridymite (or cristobalite) in the Shergotty meteorite were densified during a major impact event on the Shergottite–Nakhlite–Chaissingite (SNC) parent body and inverted either to (1) multiphase assemblages of several post-Stishovite polymorphs depicting prominent tweed pattern or to (2) Large homogeneous Stishovite grains in melt pockets. In the first setting we identified an orthorhombic and a monoclinic post-Stishovite silica polymorph, respectively. TEM investigations of a grain containing the orthorhombic polymorph revealed an α-PbO 2 like phase that could be assigned to either Pnc2 (with the cell parameters: a =4.55±0.01 A, b =4.16±0.03 A, c =5.11±0,04 A), or Pbcn space group and dense SiO 2 glass. The X-ray diffraction pattern from a second grain revealed a polymorph with a monoclinic lattice with the space group P 21/ c , that is related to the baddeleyite (ZrO 2 ) structure with the cell parameters: a =4.375(1) A, b =4.584(1) A, c =4.708(1) A, β =99.97(3), ρ =4.30(2) g/cm 3 . TEM-SAED pattern of this grain revealed the presence of the α-PbO 2 -like SiO 2 polymorph, Stishovite, secondary cristobalite, and dense silica glass. The coexistence of several high-density polymorphs and dense silica glass in the same grain suggests that several post-Stishovite phases were formed during the shock event in Shergotty. Some of these polymorphs were highly unstable and vitrified, presumably in the decompression stage. Based on diamond anvil experiments on cristobalite a peak shock pressure in excess of 40 GPa could be deduced. The petrographic setting and texture of the single Stishovite grains in the melt pockets is different. The mono-phase individual grains occur exclusively as large (>10 μm) rounded objects inside melt pockets together with hollandite structured plagioclase composition and the new CAS phase [1] . Stishovite in melt pockets is barren of any sign of a tweed pattern and contains no silica glass. This suggests that the mechanisms of phase transitions were different in the two lithologies. Stishovite in the melt pockets probably did not form by a retrograde transformation from a post-Stishovite polymorph.
Eiji Ohtani - One of the best experts on this subject based on the ideXlab platform.
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discovery of Stishovite in apollo 15299 sample
American Mineralogist, 2015Co-Authors: Shohei Kaneko, Eiji Ohtani, Naohisa Hirao, Masaaki Miyahara, Tomoko Arai, Kazuhisa SatoAbstract:High-pressure polymorphs recovered in terrestrial craters are evidence of meteoroid impact events on the Earth’s surface. Despite countless impact craters on the Moon, high-pressure polymorphs have not been reported to date in returned Apollo samples. On the other hand, recent studies report that the high-pressure polymorphs of silica, coesite, and Stishovite occur in shocked lunar meteorites. We investigated regolith breccia 15299, which was returned by the Apollo 15 mission, using the combined techniques of focused ion beam (FIB), synchrotron X-ray diffraction (XRD), and transmission electron microscopy (TEM). The regolith breccia 15299 studied here consists of a mafic impact melt breccia with millimeter-sized, coarse-grained, low-Ti basalt clasts. The mafic melt breccia consists of fragments of minerals (olivine, pyroxene, plagioclase, silica, and ilmenite) and glass. Several quartz, tridymite, and cristobalite grains of 10–100 μm across occur in the mafic impact melt breccia. Vesicular melt veins of less than ~200 μm wide cut across the mafic melt breccia matrix and mineral fragments. Some silica grains are entrained in the melt veins. One of the silica grains entrained in the melt veins consist of Stishovite [ a = 4.190(1), c = 2.674(1) A, V = 46.95 A3, space group P 42/ mnm ] along with tridymite and silica glass. This is the first report of high-pressure polymorphs from returned lunar samples. TEM images show that the Stishovite is needle-like in habit, and up to ~400 nm in size. Considering the lithologies and shock features of 15299, it is inferred that the Stishovite possibly formed by the Imbrium impact or subsequent local impact event(s) in the Procellarum KREEP Terrane (PKT) of the nearside of the Moon.
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Discovery of coesite and Stishovite in eucrite
Proceedings of the National Academy of Sciences of the United States of America, 2014Co-Authors: Masaaki Miyahara, Eiji Ohtani, Akira Yamaguchi, Shin Ozawa, Takeshi Sakai, Naohisa HiraoAbstract:Howardite–eucrite–diogenite meteorites (HEDs) probably originated from the asteroid 4 Vesta. We investigated one eucrite, Bereba, to clarify a dynamic event that occurred on 4 Vesta using a shock-induced high-pressure polymorph. We discovered high-pressure polymorphs of silica, coesite, and Stishovite originating from quartz and/or cristobalite in and around the shock-melt veins of Bereba. Lamellar Stishovite formed in silica grains through a solid-state phase transition. A network-like rupture was formed and melting took place along the rupture in the silica grains. Nanosized granular coesite grains crystallized from the silica melt. Based on shock-induced high-pressure polymorphs, the estimated shock-pressure condition ranged from ∼8 to ∼13 GPa. Considering radiometric ages and shock features, the dynamic event that led to the formation of coesite and Stishovite occurred ca. 4.1 Ga ago, which corresponds to the late heavy bombardment period (ca. 3.8–4.1 Ga), deduced from the lunar cataclysm. There are two giant impact basins around the south pole of 4 Vesta. Although the origin of HEDs is thought to be related to dynamic events that formed the basins ca. 1.0 Ga ago, our findings are at variance with that idea.
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the solidus of carbonated eclogite in the system cao al2o3 mgo sio2 na2o co2 to 32 gpa and carbonatite liquid in the deep mantle
Earth and Planetary Science Letters, 2010Co-Authors: Konstantin D Litasov, Eiji OhtaniAbstract:Abstract Melting phase relations have been determined in a model carbonated eclogite (5 wt.% CO 2 ) at 10.5–32.0 GPa and 1300–1850 °C. The assemblage of silicate minerals coexisting with partial melts changes with pressure from garnet–omphacite–kyanite–Stishovite at 10 GPa via garnet–corundum–Stishovite at 16–20 GPa to Mg–perovskite–Ca–perovskite–CF phase–Stishovite at 27–32 GPa. Magnesite is the only carbonate stable in this system through the studied pressure range. The solidus temperature was defined by the appearance of partial melt. The solidus of carbonated eclogite is bracketed at 1380–1460 °C at 10.5 GPa, 1460–1560 °C at 16.5 GPa, 1530–1630 °C at 20 GPa, and 1600–1790 °C at 27 and 32 GPa. The slope of solidus curve is less steep at 10–32 GPa than at lower pressures. The solidus curve of Fe-free carbonated eclogite roughly coincides with an average mantle geotherm. Partial melts formed by melting of carbonated eclogite at 10.5–32.0 GPa have magnesiocarbonatite compositions with Ca/Mg ratios higher than in similar melts in peridotite assemblages, and contain high Na 2 O-contents. It has been demonstrated that carbonatite-like melt can be generated by partial melting of carbonated eclogites at pressure up to at least 32 GPa, i.e. to lower mantle depths.
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the solidus of carbonated eclogite in the system cao al2o3 mgo sio2 na2o co2 to 32gpa and carbonatite liquid in the deep mantle
Earth and Planetary Science Letters, 2010Co-Authors: Konstantin D Litasov, Eiji OhtaniAbstract:Abstract Melting phase relations have been determined in a model carbonated eclogite (5 wt.% CO 2 ) at 10.5–32.0 GPa and 1300–1850 °C. The assemblage of silicate minerals coexisting with partial melts changes with pressure from garnet–omphacite–kyanite–Stishovite at 10 GPa via garnet–corundum–Stishovite at 16–20 GPa to Mg–perovskite–Ca–perovskite–CF phase–Stishovite at 27–32 GPa. Magnesite is the only carbonate stable in this system through the studied pressure range. The solidus temperature was defined by the appearance of partial melt. The solidus of carbonated eclogite is bracketed at 1380–1460 °C at 10.5 GPa, 1460–1560 °C at 16.5 GPa, 1530–1630 °C at 20 GPa, and 1600–1790 °C at 27 and 32 GPa. The slope of solidus curve is less steep at 10–32 GPa than at lower pressures. The solidus curve of Fe-free carbonated eclogite roughly coincides with an average mantle geotherm. Partial melts formed by melting of carbonated eclogite at 10.5–32.0 GPa have magnesiocarbonatite compositions with Ca/Mg ratios higher than in similar melts in peridotite assemblages, and contain high Na 2 O-contents. It has been demonstrated that carbonatite-like melt can be generated by partial melting of carbonated eclogites at pressure up to at least 32 GPa, i.e. to lower mantle depths.
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high hydrogen solubility in al rich Stishovite and water transport in the lower mantle
Earth and Planetary Science Letters, 2007Co-Authors: Konstantin D Litasov, Eiji Ohtani, Hiroyuki Kagi, Anton Shatskiy, D L Lakshtanov, Jay D Bass, Eiji ItoAbstract:Abstract Stishovite is an important phase in subducting oceanic crust. The post-garnet assemblage from a precursor eclogite lithology contains up to 25% Stishovite at pressures above 25 GPa. This Stishovite may contain up to 5 wt.% Al 2 O 3 . We measured the hydrogen contents of Stishovite samples synthesized at 20–25 GPa and 1200–1800 °C from several starting materials containing up to 10 wt.% Al 2 O 3 . FTIR spectra of Stishovite show major bands at 3111–3134 cm − 1 , with the frequencies increasing as H 2 O and Al 2 O 3 contents increase, and several minor bands at 2659–2667, 3240, 3261, 3312–3334, and 3351 cm − 1 . The H 2 O content of Al-free Stishovite is in the range of 16–30 wt. ppm. The maximum H 2 O content of Al-bearing Stishovite (4.4 wt.% Al 2 O 3 ) synthesized at 20 GPa and 1400 °C is 3010 ± 300 wt. ppm. Most hydrogen in Stishovite is associated with Al 3+ substitutional defects on the octahedral (Si 4+ ) site. The hydrogen can occupy 40% of vacancies created by incorporation of Al 3+ at 20 GPa. This observation along with some anomalies in the FTIR spectra may indicate an alternative mechanism of Al 3+ incorporation into Stishovite via the formation of oxygen vacancies or interstitial Al i defects. We report the highest H 2 O concentrations in Al-Stishovite to date, and argue that it is the most important carrier of water into the lower mantle post-garnet eclogitic assemblages.
Konstantin D Litasov - One of the best experts on this subject based on the ideXlab platform.
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the solidus of carbonated eclogite in the system cao al2o3 mgo sio2 na2o co2 to 32 gpa and carbonatite liquid in the deep mantle
Earth and Planetary Science Letters, 2010Co-Authors: Konstantin D Litasov, Eiji OhtaniAbstract:Abstract Melting phase relations have been determined in a model carbonated eclogite (5 wt.% CO 2 ) at 10.5–32.0 GPa and 1300–1850 °C. The assemblage of silicate minerals coexisting with partial melts changes with pressure from garnet–omphacite–kyanite–Stishovite at 10 GPa via garnet–corundum–Stishovite at 16–20 GPa to Mg–perovskite–Ca–perovskite–CF phase–Stishovite at 27–32 GPa. Magnesite is the only carbonate stable in this system through the studied pressure range. The solidus temperature was defined by the appearance of partial melt. The solidus of carbonated eclogite is bracketed at 1380–1460 °C at 10.5 GPa, 1460–1560 °C at 16.5 GPa, 1530–1630 °C at 20 GPa, and 1600–1790 °C at 27 and 32 GPa. The slope of solidus curve is less steep at 10–32 GPa than at lower pressures. The solidus curve of Fe-free carbonated eclogite roughly coincides with an average mantle geotherm. Partial melts formed by melting of carbonated eclogite at 10.5–32.0 GPa have magnesiocarbonatite compositions with Ca/Mg ratios higher than in similar melts in peridotite assemblages, and contain high Na 2 O-contents. It has been demonstrated that carbonatite-like melt can be generated by partial melting of carbonated eclogites at pressure up to at least 32 GPa, i.e. to lower mantle depths.
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the solidus of carbonated eclogite in the system cao al2o3 mgo sio2 na2o co2 to 32gpa and carbonatite liquid in the deep mantle
Earth and Planetary Science Letters, 2010Co-Authors: Konstantin D Litasov, Eiji OhtaniAbstract:Abstract Melting phase relations have been determined in a model carbonated eclogite (5 wt.% CO 2 ) at 10.5–32.0 GPa and 1300–1850 °C. The assemblage of silicate minerals coexisting with partial melts changes with pressure from garnet–omphacite–kyanite–Stishovite at 10 GPa via garnet–corundum–Stishovite at 16–20 GPa to Mg–perovskite–Ca–perovskite–CF phase–Stishovite at 27–32 GPa. Magnesite is the only carbonate stable in this system through the studied pressure range. The solidus temperature was defined by the appearance of partial melt. The solidus of carbonated eclogite is bracketed at 1380–1460 °C at 10.5 GPa, 1460–1560 °C at 16.5 GPa, 1530–1630 °C at 20 GPa, and 1600–1790 °C at 27 and 32 GPa. The slope of solidus curve is less steep at 10–32 GPa than at lower pressures. The solidus curve of Fe-free carbonated eclogite roughly coincides with an average mantle geotherm. Partial melts formed by melting of carbonated eclogite at 10.5–32.0 GPa have magnesiocarbonatite compositions with Ca/Mg ratios higher than in similar melts in peridotite assemblages, and contain high Na 2 O-contents. It has been demonstrated that carbonatite-like melt can be generated by partial melting of carbonated eclogites at pressure up to at least 32 GPa, i.e. to lower mantle depths.
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Stishovite single crystal growth and application to silicon self diffusion measurements
American Mineralogist, 2010Co-Authors: Anton Shatskiy, Konstantin D Litasov, Eiji Ito, Daisuke Yamazaki, Titus Cooray, Yuriy M Borzdov, Takuya Matsuzaki, Anais Ferot, Tomoo KatsuraAbstract:Large single crystals of Stishovite were successfully synthesized at 11 GPa from a silica solution in water. The potential of both slow cooling and thermal gradient methods were examined. The thermal gradient method provided crystals of 0.8 × 0.8 × 1.3 mm in size grown at 1350 °C and a thermal gradient of 50 °C/mm using Stishovite as a silica source. The use of quartz as a source resulted in the appearance of numerous Stishovite crystals in the solution interior resulting in diminished space for the growth of large crystals. This can be explained by a significant difference in the solubility of metastable quartz and Stishovite in water, estimated to be 85.3 and 5.6 wt% SiO 2 at 1000 °C and 11 GPa, respectively. Crystals up to 0.8 × 1.3 × 1.5 mm were grown by the slow cooling method in the system SiO 2 + 14.7 wt% H 2 O as temperature was decreased from 1600 to 1000 °C with a cooling rate of 2 °C/min. The size of single crystals obtained was large enough to carry out silicon self-diffusion experiments, which were performed at a pressure of 14 GPa and temperatures from 1400 to 1800 °C. The lattice diffusion coefficients along the [110] and [001] directions can be expressed as D [110] (m 2 /s) = 4.10 × 10 −12 exp [−322 (kJ/mol)/R T ] and D [001] (m 2 /s) = 5.62 × 10 −12 exp [−334 (kJ/mol)/R T ], respectively, where R is the gas constant and T is the absolute temperature.
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high hydrogen solubility in al rich Stishovite and water transport in the lower mantle
Earth and Planetary Science Letters, 2007Co-Authors: Konstantin D Litasov, Eiji Ohtani, Hiroyuki Kagi, Anton Shatskiy, D L Lakshtanov, Jay D Bass, Eiji ItoAbstract:Abstract Stishovite is an important phase in subducting oceanic crust. The post-garnet assemblage from a precursor eclogite lithology contains up to 25% Stishovite at pressures above 25 GPa. This Stishovite may contain up to 5 wt.% Al 2 O 3 . We measured the hydrogen contents of Stishovite samples synthesized at 20–25 GPa and 1200–1800 °C from several starting materials containing up to 10 wt.% Al 2 O 3 . FTIR spectra of Stishovite show major bands at 3111–3134 cm − 1 , with the frequencies increasing as H 2 O and Al 2 O 3 contents increase, and several minor bands at 2659–2667, 3240, 3261, 3312–3334, and 3351 cm − 1 . The H 2 O content of Al-free Stishovite is in the range of 16–30 wt. ppm. The maximum H 2 O content of Al-bearing Stishovite (4.4 wt.% Al 2 O 3 ) synthesized at 20 GPa and 1400 °C is 3010 ± 300 wt. ppm. Most hydrogen in Stishovite is associated with Al 3+ substitutional defects on the octahedral (Si 4+ ) site. The hydrogen can occupy 40% of vacancies created by incorporation of Al 3+ at 20 GPa. This observation along with some anomalies in the FTIR spectra may indicate an alternative mechanism of Al 3+ incorporation into Stishovite via the formation of oxygen vacancies or interstitial Al i defects. We report the highest H 2 O concentrations in Al-Stishovite to date, and argue that it is the most important carrier of water into the lower mantle post-garnet eclogitic assemblages.
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the post Stishovite phase transition in hydrous alumina bearing sio2 in the lower mantle of the earth
Proceedings of the National Academy of Sciences of the United States of America, 2007Co-Authors: D L Lakshtanov, Konstantin D Litasov, Vitali B. Prakapenka, Stanislav V Sinogeikin, Holger Hellwig, Jingyun Wang, Carmen Sanchesvalle, Jeanphilippe Perrillat, Bin ChenAbstract:Silica is the most abundant oxide component in the Earth mantle by weight, and Stishovite, the rutile-structured (P42/mnm) high-pressure phase with silica in six coordination by oxygen, is one of the main constituents of the basaltic layer of subducting slabs. It may also be present as a free phase in the lower mantle and at the core–mantle boundary. Pure Stishovite undergoes a displacive phase transition to the CaCl2 structure (Pnnm) at ≈55 GPa. Theory suggests that this transition is associated with softening of the shear modulus that could provide a significant seismic signature, but none has ever been observed in the Earth. However, Stishovite in natural rocks is expected to contain up to 5 wt % Al2O3 and possibly water. Here we report the acoustic velocities, densities, and Raman frequencies of aluminum- and hydrogen-bearing Stishovite with a composition close to that expected in the Earth mantle at pressures up to 43.8(3) GPa [where (3) indicates an uncertainty of 0.3 GPa]. The post-Stishovite phase transition occurs at 24.3(5) GPa (at 298 K), far lower than for pure silica at 50–60 GPa. Our results suggest that the rutile–CaCl2 transition in natural Stishovite (with 5 wt % Al2O3) should occur at ≈30 GPa or ≈1,000-km depth at mantle temperatures. The major changes in elastic properties across this transition could make it visible in seismic profiles and may be responsible for seismic reflectors observed at 1,000- to 1,400-km depth.
Raymond Jeanloz - One of the best experts on this subject based on the ideXlab platform.
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equation of state of Stishovite and interpretation of sio2 shock compression data
Journal of Geophysical Research, 2003Co-Authors: Wendy R Panero, L R Benedetti, Raymond JeanlozAbstract:[1] We present an isothermal equation of state for the high-pressure Stishovite phase of SiO2 based on synchrotron X-ray diffraction at 300 K to 60 GPa. The inferred equation of state is the same for Stishovite synthesized from pure SiO2 glass as from a natural, basalt glass that forms Stishovite in association with other mineral phases, yielding a zero-pressure bulk modulus K0T = 312.9(±3.4) GPa and pressure derivative K′0 = 4.8(±0.2). Our measurements of Stishovite at static, high pressures predict a shock-compressed (Hugoniot) state that is 2.4 (±1.6)% denser than observed for SiO2, suggesting that an amorphous phase, rather than Stishovite, is produced under impact loading.
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transport of water into the lower mantle role of Stishovite
Journal of Geophysical Research, 2003Co-Authors: Wendy R Panero, L R Benedetti, Raymond JeanlozAbstract:When subjected to lower-mantle pressures and temperatures, natural 'anhydrous' basalt containing 0.2 wt.% H{sub 2}O forms a phase assemblage in which SiO{sub 2} Stishovite is a significant carrier of hydrogen (up to 500 ppm H{sub 2}O by weight, as hydroxide), whereas the coexisting (Mg, Fe, Al, Ca)SiO{sub 3} perovskite appears to be not (upper bound of 50 ppm (wt) H{sub 2}O). Contrary to the devolatilization characteristically observed at lower pressures, we find that the abundance of H{sub 2}O in residual Stishovite increases from {approx}100 to {approx}400 ppm by weight upon partially melting the high-pressure mineral assemblage at 28-60 GPa. We infer that the trace concentration of Al within residual Stishovite increases upon partial melting, thereby increasing the coupled abundance of H in this crystalline phase. The 'anhydrous' component of subducted oceanic crust can thus recycle a significant amount of water into the lower mantle over the age of the Earth, with subducted Stishovite potentially returning {approx} 10{sup 2} times the amount of water present in today's atmosphere.
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high pressure infrared sepctra of quartz coesite Stishovite and silica glass
Journal of Geophysical Research, 1993Co-Authors: Quentin Williams, Russell J Hemley, M B Kruger, Raymond JeanlozAbstract:High-pressure infrared absorption spectra of alpha-quatz, coesite, Stishovite, and SiO2 glass are consistent with the primary compression mechanism of the initially tetrahedrally bonded phases being the bending of the Si-O-Si angle at pressures less than 10-20 GPa. At higher pressures, up to 40 GPa, we observe a decline in the intensity of the infrared SiO4 asymmetric-stretching vibrations of all three phases, with an increase in the relative amplitude between 700 and 900/cm. This change in intensities is attributed to an increase in the average coordination number of silicon through extreme distortion of tetrahedra. At pressures above approximately 20 GPa, the low-pressure crystalline polymorphs gradually become amorphous, and the infrared spectra provide evidence for an increase in silicon coordination in these high-density amorphous phases. The pressure-amorphized samples prepared from quartz and coesite differ structurally both from each other and from silica glass that has been compressed, and the high pressure spectra indicate that these materials are considerably more disordered than Stishovite under comparable pressure conditions. Average mode Grueneisen parameters calculated for quartz, Stishovite and fused silica from both infrared and Raman spectra are compatible with the corresponding thermodynamic value of the Grueneisen parameter, however, that of coesite is significantly discrepant.
Kei Hirose - One of the best experts on this subject based on the ideXlab platform.
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post Stishovite transition in hydrous aluminous sio2
Physics of the Earth and Planetary Interiors, 2016Co-Authors: Kei Hirose, Koichiro Umemoto, Katsuyuki Kawamura, Renata M WentzcovitchAbstract:Abstract Lakshtanov et al. (2007) showed that incorporation of aluminum and some water into SiO2 significantly reduces the post-Stishovite transition pressure in SiO2. This discovery suggested that the ferroelastic post-Stishovite transition in subducted MORB crust could be the source of reflectors/scatterers with low shear velocities observed in the mid to upper lower mantle. A few years later, a similar effect was observed in anhydrous Al-bearing silica. In this paper, we show by first principles static calculations and by molecular dynamics using inter-atomic potentials that hydrogen bonds and hydrogen mobility play a crucial role in lowering the post-Stishovite transition pressure. A cooperative redistribution of hydrogen atoms is the main mechanism responsible for the transition pressure reduction in hydrous aluminous Stishovite. The effect is enhanced by increasing hydrogen concentration. This perspective suggests a potential relationship between the depth of seismic scatterers and the water content in Stishovite.
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acoustic velocity measurements for Stishovite across the post Stishovite phase transition under deviatoric stress implications for the seismic features of subducting slabs in the mid mantle
American Mineralogist, 2013Co-Authors: Yuki Asahara, Kei Hirose, Yasuo Ohishi, Naohisa Hirao, Haruka Ozawa, Motohiko MurakamiAbstract:Understanding effects of non-hydrostatic pressure on phase transitions in minerals relevant to the Earth’s mantle is important to translate the observable seismic signals to not directly observable mineralogical models for the deep Earth. SiO 2 can occur as a free phase in subducting slabs, which contain sedimentary layers and/or mid-ocean-ridge basalts. In this study, we report on the effect of deviatoric strain on the pressure-induced phase transition in SiO 2 and its consequences on the seismic signal. The acoustic velocity in polycrystalline Stishovite across the post-Stishovite phase transition was measured by Brillouin scattering in the pure SiO 2 system at room temperature under deviatoric stress. High-pressure synchrotron X-ray diffraction data were also collected at SPring-8. A linear fit to the symmetry-breaking strain values and the pressure of the transverse velocity minimum indicate a transition pressure between 25 and 35 GPa, which is about 20 GPa lower than that under hydrostatic conditions. The transverse velocity dropped by about 3% at around 25 GPa in this study. This is much smaller than the prediction from ab initio calculations that a transverse velocity reduces by ~60% at around 50 GPa under hydrostatic conditions. The results of the present study indicate that the deviatoric stress lowers the transition pressure and reduces the acoustic velocity change associated with the post Stishovite phase transition. Sedimentary and mid ocean ridge basalt (MORB) layers in a subducting slab are likely sites for finding Stishovite and its high-pressure polymorphs in the deep earth. Seismic observations of deep earthquakes occurring in subducting slabs indicate the existence of considerable stress in down-going slabs. This study suggests that nonhydrostatic deviatoric stress is one of the possible reasons for the absence of general seismic features that can be directly related to the post-Stishovite phase transition in subducting slabs at 1500 km depth. The phase transition of Stishovite under deviatoric stress, which occurs at shallower depths, can affect the local seismic scattering structures and the rheological behavior of a subducting slab in the mid-lower mantle region.
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equation of state of al bearing Stishovite to 40 gpa at 300 k
American Mineralogist, 2002Co-Authors: Shigeaki Ono, Kei Hirose, Takuma Suto, Yasuhiro Kuwayama, Tetsuya Komabayashi, Takumi KikegawaAbstract:The compression behavior of Al-bearing Stishovite was investigated by powder X-ray diffraction up to 40 GPa with the BL13A beamline at the Photon Factory (KEK, Japan). A reliable equation of state for Stishovite was obtained using a diamond anvil cell coupled with a yttrium-aluminum-garnet (YAG) laser-heating. A sample containing 2.1 wt% Al 2 O 3 was heated using a YAG laser at each pressure increment to relax deviatoric stress. X-ray diffraction measurements were carried out at 300 K using the angle-dispersive technique. A least squares refinement of the data yielded equation of state parameters where the bulk modulus K o = 282 (±2) GPa when the first pressure derivative of the bulk modulus K o′ was fixed at 4. The effect of Al is to decrease slightly the bulk modulus of Stishovite and increase the density of the subducted oceanic crust. The enhanced compressibility of Al-bearing Stishovite certainly has geophysical and geochemical implications for the fate of the subducted slab, as this mineral is the main constituent of subducted mid-oceanic ridge basalt (MORB) in the Earth’s mantle.
