6 search hits
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Water solubility in diopside
(2008)
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Polina Gavrilenko
- (1) Water solubility in pure diopside was measured. Water-saturated diopside crystals were synthesized using piston-cylinder and multi-anvil presses at 20-30 and 100 kbar and 800-1100oC from an oxide and hydroxide starting mixture containing 10 % excess silica. The water concentration in diopside was determined from polarized infrared measurements on doubly polished single crystals. Water contents were calculated by integrating the absorption bands and using published extinction coefficients for water in diopside. All measured infrared spectra of pure diopside fall into two groups. The differences in the spectra point towards substitution mechanisms involving different vacancies, which in turn could be the result of different oxide activities in the starting material. Therefore, a separate series of experiments was carried out with starting materials with an excess or deficiency of MgO or SiO2. These experiments yielded diopside with different absorption spectra. Starting materials with low silica activity yielded Type I bands, which are therefore likely to be related to Si vacancies. Type II bands form at high silica activity and may therefore be related to Mg or Ca vacancies. Water solubility in pure diopside varies from 121 up to 568 ppm H2O. Water solubility at 30 kbar increases from 700 to 1000oC and drops again above 1000oC. At 900oC, water solubility increases to a maximum at 25 kbar and then decreases rapidly to higher pressures. Due to the low solubility of aluminum in clinopyroxene at high pressure, the data on pure diopside are probably a good guide for the water solubility in clinopyroxenes under the conditions of the deeper upper mantle. Since water solubility in diopside under those conditions is order of magnitude below the water solubility in olivine, clinopyroxene is not expected to be a major storage site for water in the deeper upper mantle, even if its modal abundance is significant. (2) Water-saturated Al-containing diopside was synthesized in an end-loaded piston-cylinder apparatus at 1.5-2.5 GPa and 900-1100oC. The compositions of the starting materials for Al-bearing diopside are along the join diopside (CaMgSi2O6) – Ca-Tschermak’s component (CaAl2SiO6) with different ratios of these two end members. The water solubility strongly increases with the presence of Al up to 2500 ppm H2O. The water solubility in aluminous diopside increasing with decreasing temperature. Estimated partition coefficients of water between clinopyroxene and orthopyroxene are close to unity, with Dcpx/opx possibly increasing with temperature. Together with previously published data on water in orthopyroxene, the results of this study clearly show that in the uppermost mantle, most of the water is dissolved in the pyroxenes. The relative importance of clinopyroxene and orthopyroxene is primarily a function of their modal abundance. This observation is consistent with the model of Mierdel et al (2007), which suggests that the Earth’s asthenosphere is due to a minimum in water solubility in nominally anhydrous minerals. (3) In order to determine the effect of water on the equation of state of diopsides, high-pressure single crystal X-ray diffraction experiments with a diamond anvil cell were performed. The compressibility of diopside decreases with increasing water and Al content in the structure. The bulk modulus Ko and its first pressure derivative K’ for the four diopside crystals are 106(1) GPa and 6.1(5) for pure anhydrous diopside (0 ppm H2O); 107(1) GPa and 6.5(4) for pure diopside with 63 ppm of H2O; 108(1) GPa and 6.3(4) for pure diopside with 600 ppm H2O; and 113(1) GPa and 5.7(5) for Al-bearing hydrous (containing 0.374 Al a.p.f.u.) diopside with 2510 ppm H2O. The results on compressibility of diopside contrast with previous work, which showed that compressibility of most other main mantle phases increases with water content. In addition, from the refinement of the crystal structures of both hydrous and dry diopside and comparison with the structure of Ca-Tschermak’s pyroxene it was possible to see the influence of protonation of oxygen atoms. Because of the contrasting effect of water on the equation of state of olivine and of pyroxenes in the upper mantle, detecting water from observations of seismic velocities alone is probably nearly impossible.
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Experimental Constraints on Silicate Perovskite Forming Reactions and Elastic Properties: Geophysical Implications for Chemical Heterogeneity in the Deep Mantle
(2008)
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Saikia Ashima
- Three experimental investigations have been performed in order to understand how the composition of the mantle may influence mineral stability and elastic properties and how these may influence seismic properties of the deep mantle. The phase relations of calcium perovskite have been studied in high pressure and temperature experiments to examine the effect of its formation on seismic discontinuity features at 520 km depth in the mantle. The effect of varying composition on the compressibility of magnesium silicate perovskite has been examined in order to understand the geophysical consequences of chemical heterogeneity in the lower mantle. Calorimetric measurements of the pyrope-majorite garnet solid solution have been made to provide essential thermodynamic data for modeling the formation reactions of both magnesium and calcium silicate perovskite.
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High-pressure and high-temperature structural and electronic properties of (Mg,Fe)O and FeO
(2007)
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Innokentiy Kantor
- Magnesium-rich MgO-FeO solid solution, known as the mineral ferropericlase, constitutes a significant part of the Earth as the second most abundant mineral in the lower mantle after (Mg,Fe)SiO3 perovskite. A combined experimental and theoretical study was carried out in order to determine structural and electronic properties of ferropericlase over a broad pressure and temperature range. The phase diagram of FeO (wüstite), the end member of the (Mg,Fe)O solid solution, was found to be more complex than previously thought. It was discovered that the magnetic ordering (Néel) transition does not coincide with the structural cubic-to-trigonal symmetry breaking transition in non-stoichiometric FeO. The magnetic ordering transition was determined for the first time by a combined Mössbauer spectroscopy and neutron diffraction study. A full agreement between these two methods was observed, indicating that in the case of FeO the Mössbauer spectra reflect long-range magnetic ordering. A quasi-single crystal X-ray diffraction study of FeO compared with previous results shows that the transition pressure depends not only on stress conditions, but also on wüstite composition, and probably the order of the transition (second- or weak first-order) is also stress dependent. Above ~70 GPa after laser annealing the X-ray diffraction pattern of FeO could not be explained as a trigonal structure, but as a monoclinic structure with space group P21/m. (Mg,Fe)O solid solution was studied over a wide pressure and temperature range and over a compositional range from 5 to 20 mole % of FeO component. The detailed analysis of (Mg,Fe)O Mössbauer spectra shows clear evidence for the distribution of the hyperfine parameter quadrupole splitting (D), which provides a key to determining its local structure. It is shown that by analyzing the D distribution, a short-range order parameter could be estimated for the low-Fe (Mg,Fe)O solid solution. Samples quenched from high temperature at ambient pressure during synthesis show local cation distribution close to randomness, as was reported previously. Upon compression, however, a rapid increase of short-range order with the tendency for Fe clusterization was observed. This non-random atomic distribution was shown to be stable at high pressures and also at high temperatures. Such a tendency for Fe ions to separate could lead to the miscibility gap in the (Mg,Fe)O solid solution series at high pressures and temperatures, as was observed. At pressures higher than 50 GPa a spin-pairing transition of Fe2+ was observed. Clear and pronounced changes in the Mössbauer spectra are fully consistent with a high- to low-spin transition: the centre shift decreases, indicating an increase of electron density at the nuclei. Quadrupole splitting also vanishes to zero, indicating significant spherical symmetrisation of the valence electrons and electrical field gradient disappearance. The absolute magnitude of these changes is in full agreement with ab initio calculations made in this study. The onset of the spin transition is similar for all the samples studied, but the width is strongly composition dependent. The higher the iron content, the broader the transition width, which reaches about 50 GPa for the (Mg0.8Fe0.2)O sample. Such a broad transition range is not typical for phase transitions with significant volume collapse. Analysis of literature data together with the results of this study lead to an interpretation of spin crossover as a thermal equilibrium process without phase transition. The compositional and temperature dependence of spin crossover in ferropericlase can be described fairly well within such a model, taking into account the local structure of the solid solution. The results of this model were also confirmed by ab initio simulations. The model proposed in this work predicts that spin crossover in ferropericlase will occur over a large depth range of the lower mantle. No discontinuities in density or elastic properties are expected to be produced in the lower mantle due to spin crossover in ferropericlase, contrary to previous suggestions.
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Mantle-Melting at High Pressure: Experimental Constraints on Magma Ocean Differentiation
(2005)
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Christian Liebske
- In this study geochemical processes and a geophysical parameter were investigated that are relevant to the crystallisation of a deep magma ocean, that likely existed during Earth’s accretion. The melting relations of potential magma ocean compositions, such as peridotitic and chondritic bulk compositions, were investigated using multianvil apparatus at pressures of 25-26 GPa and temperatures up to 2400°C. Compositional effects on the melting relations were investigated by varying bulk Mg/Si and Mg/(Mg+Fe) ratios (the latter is denoted as Mg-number, Mg#). At 26 GPa, peridotite liquids show a crystallisation sequence of ferropericlase (Fp) followed down temperature by Mg-silicate perovskite (MgPv) + Fp, which is in contrast to the sequence of MgPv followed by MgPv + Fp in chondritic composition. The melting relations along the different compositions depend primarily on the bulk Mg/Si ratio and not on the Mg#. Melting relations and eutectic compositions were studied in the simple binary MgO-SiO2 system between 10 and 26 GPa. Combining the new results with previously published data shows that the eutectic composition between Mg2SiO4 and MgSiO3, up to 20 GPa, moves towards MgO with increasing pressure. Between 20 and 23 GPa the direction in which the eutectic is moving with pressure reverses. At higher pressures, this trend is again reversed and the eutectic composition moves towards MgO. The multiple changes in the direction in which the eutectic is moving as a function of pressure explains qualitatively the differences in liquidus phase relations in the more complex peridotite and chondrite compositions. The effect of bulk chemical composition on the partitioning of major, minor and trace elements between MgPv and coexisting silicate melts was investigated using micro-beam techniques. MgPv/melt partition coefficients for Mg (DMg) and Si (DSi) are related to the melt Mg/Si ratio, such that DSi becomes smaller than DMg at chondritic Mg/Si melt ratios. This shows that the Earth’s upper mantle Mg/Si ratio is unlikely to be derived from chondrites by MgPv fractionation. Partition coefficients of tri- and tetravalent elements increase with increasing Al concentration of MgPv. A crystal chemical model indicates that Al3+ substitutes predominantly onto the Si-site in MgPv, but most other elements substitute onto the Mg-site. This is consistent with a charge-compensating substitution mechanism. A crystal fractionation model, based on refractory lithophile element ratios, is developed to constrain the amount of MgPv and Ca-silicate perovskite (CaPv) that could have fractionated in a magma ocean and could still be present as a chemical heterogeneity in the lower mantle today. It is shown that a fractionated crystal pile composed of 96% MgPv and 4% CaPv could comprise up to 13 wt% of the entire mantle. Fe3+/Fetotal ratios have been determined for MgPv, crystallised at temperatures below and above the peridotite solidus, using Mössbauer and electron-energy-loss spectroscopy. The amount of Fe3+ in MgPv is positively correlated to the Al content of this phase. In recovered samples, homogeneously distributed Fe-rich metal on the sub-micron scale was observed on grain boundaries, although the MgPv has Fe3+/Fetotal ratios between 0.2 and 0.5. This suggests that the amount of Fe3+ in MgPv is independent of oxygen fugacity and that the presence of Fe-rich metal in the samples is the result of disproportionation of FeO to Fe3+ and Fe-metal. This has potentially implications for the mantle oxidation state and the mantle geochemistry during magma ocean solidification. The viscosity of peridotite liquid, as an analogue for a magma ocean composition, was investigated at high pressure using in-situ falling sphere viscometry. Experiments were performed between 2.5 and 13 GPa at temperatures between 2043 to 2523 K. Measured viscosities range from 0.018 (±0.003) to 0.13 (±0.01) Pa s. Up to 9 GPa the data indicate an isothermal increase in viscosity with increasing pressure but viscosity decreases between 9 and 13 GPa at constant temperature. The observed change in the pressure dependence of the viscosity is likely associated with structural changes in the liquid upon compression. The new high pressure data are combined with 1 bar viscosities for peridotite liquid (Dingwell et al. 2004), and a non-Arrhenian Vogel-Fulcher-Tamman equation, to which an empirical pressure-dependent term has been added, is presented to parameterize all experimental data. This approach reproduces measured viscosities to within 0.08 log10-units on average. The model can be used to calculate magma ocean viscosities to depths of 400 km. At likely magma ocean temperatures, viscosities down to transition zone pressures are extremely low and comparable to water at room temperature. The results of the different aspects of this study were used to investigate magma ocean crystallisation and its effect on the geochemistry and the evolution of the Earth’s mantle.
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Austausch von Silizium durch die dreiwertigen Kationen von Eisen und Aluminium in CaSiO3-Perowskiten: Mechanismen und Auswirkungen auf die Mineralogie der Übergangszone
(2004)
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Ulrich Bläß
- Um den Einfluss dreiwertiger Kationen sowohl auf die strukturellen Eigenschaften von CaSiO3-Perowskit als auch die daraus resultierenden Auswirkungen auf den Erdmantel abschätzen zu können, sind Hochdruck- Hochtemperaturexperimente in den beiden Systemen CaSiO3 – CaFeO2,5 und CaSiO3 – CaAlO2,5 bei Drücken zwischen 3 und 25 GPa und Temperaturen zwischen 800° und 1900°C durchgeführt worden. Die Ergebnisse im eisenhaltigen System zeigen bei den P-T-Bedingungen der Übergangszone eine nur geringe Eisen-Löslichkeit über den Defektmechanismus (4 mol% bei 16 GPa). Diese vermutlich ungeordnete Phase ist durch eine breite Mischungslücke von einem neuartigen Defektperowskit mit Ca(Fe0,4Si0,6)O2,8 Stöchiometrie getrennt, in welchem die Ordnung von Sauerstoffdefekten innerhalb diskreter Lagen zu einer 10-fachen Überstruktur entlang der pseudokubischen [111]-Richtung führt. Bei einer Sauerstofffugazität von ca. Re-ReO2 ist dieser Defektperowskit zwischen ca. 11 bis 22 GPa und 1000° bis >1800°C stabil, zerfällt jedoch bei einer niedrigeren Sauerstofffugazität von Mo-MoO. Experimente oberhalb der Stabilität des geordneten Defektperowskits geben Hinweise auf eine erheblich ansteigende Eisen-Löslichkeit in reinem CaSiO3-Perowskit. Auf eisenreicheren Zusammensetzungen oder bei tieferen Drücken treten abgesehen von dem Endglied Ca2Fe2O5 mit Brownmillerit-Struktur keine weiteren Defektperowskite auf. Dessen Hochdruckstabilitätsgrenze ist experimentell bestimmt worden und liegt bei ca. 4,5 GPa. Die Gleichgewichts-Phasenbeziehungen aller Zusammensetzungen sind für ausgewählte Drücke beschrieben. Neben den beschriebenen Perowskit-Phasen treten als weitere Phasen im wesentlichen Dicalciumsilikat, eine calciumarme Ferritphase und teilweise Andradit auf. Vergleichbare Experimente im aluminiumhaltigen System haben die Existenz zweier Defektperowskite mit Ca(Al0,4Si0,6)O2,8 und Ca(Al0,5Si0,5)O2,8 Stöchiometrie aufgedeckt. Dabei weist erstere Phase eine analoge 10-fache Überstruktur wie der eisenhaltige Defektperowskit auf. Ihr Stabilitätsfeld ist mit ca. 9 GPa bis 18 GPa zu etwas niedrigeren Drücken verschoben bei einer gleichzeitig erhöhten Liquidus-Temperatur. Aufgrund ähnlicher Gitterkonstanten ist eine ausgedehnte Mischkristallbildung mit dem eisenhaltigen Defektperowskit zu vermuten. Der andere Defektperowskit mit Ca(Al0,5Si0,5)O2,8 Stöchiometrie besitzt dagegen entlang der pseudokubischen [111]-Richtung eine 8-fache Überstruktur mit vermutlich gleicher Ordnung von Sauerstoffleerstellen innerhalb der Defektlagen. Sein Stabilitätsfeld ist zu erheblich niedrigeren Drücken verschoben und liegt zwischen ca. 4 und 12 GPa bei ebenfalls relativ hohen Liquidus-Temperaturen. Syntheseexperimente bei P-T-x-Bedingungen unter denen keiner der geordneten Defektperowskite stabil ist, führen zu einer metastabilen Bildung von extrem feinkörnigen Defektperowskiten, welche teilweise eine partielle Ordnung der Defektlagen und eine domänenreiche Mikrostruktur aufweisen. Aufgrund dieser Resultate können in dem aluminiumhaltigen System keine Gleichgewichts-Phasenbeziehungen aufgezeigt werden. Da jedoch der Silizium-Aluminium-Austausch mit dem Einbau von Sauerstoffdefekten gekoppelt ist, ist eine mit dem eisenhaltigen System vergleichbare Löslichkeit von Aluminium in reinem CaSiO3-Perowskit zu erwarten. Alle drei Defektperowskite sind ausführlich mittels optischer Mikroskopie, Elektronenstrahlmikrosonde, Röntgen- und Elektronenbeugung, Mössbauer- und Elektronen-Energieverlust-Spektroskopie sowie bezüglich ihrer Mikrostruktur am Transmissionselektronenmikroskop untersucht worden. Die Mikrostruktur aller drei geordneter Defektperowskite ist relativ arm an Versetzungen, weist jedoch eine reiche Verzwillingung nach zwei verschiedenen Gesetzmäßigkeiten auf. Zum einen handelt es sich um ferroelastische Zwillinge, welche zusammen mit einer geringfügigen Gitterverzerrung von einer rhomboedrischen Symmetrie die Existenz einer rhomboedrischen Hochtemperaturphase andeuten. Zum anderen treten Wachstumszwillinge auf, bei welchen die Überstruktur der einzelnen Domänen entlang je einer der vier möglichen pseudokubisch dreizähligen Achsen ausgebildet ist. Mögliche Strukturmodelle dieser Defektperowskite bestehen entlang der pseudokubischen [111]-Achse abwechselnd aus Defektlagen mit Silizium in vierfacher oder eventuell auch fünffacher Koordination und Blöcken mit Perowskitstruktur, welche die dreiwertigen Kationen enthalten. Das potentielle Auftreten der Defektperowskite im Erdmantel ist diskutiert. Aufgrund der Phasenbeziehungen ist eine Bildung innerhalb peridotitischer oder MORB Zusammensetzungen nicht zu erwarten. Jedoch zeigen die Experimente dieser Arbeit, dass die Konzentration an dreiwertiger Kationen in CaSiO3-Perowskiten der Übergangszone auf wenige Prozent beschränkt ist, aber möglicherweise in den oberen Bereichen des Unteren Erdmantels in Abhängigkeit von den Verteilungskoeffizienten gegenüber Magnesiumsilikat-Perowskit erheblich ansteigt.
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Water in the Earth’s Interior: Thermodynamics and kinetics of hydrogen incorporation in olivine and wadsleyite
(2004)
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Sylvie Demouchy
- (1) Hydrogen diffusion in olivine The kinetics of hydration of dry single crystals of San Carlos olivine was determined by performing experiments under water-saturated conditions. The experiments were performed at 1.5 GPa, 1000°C for 5 hours in a piston cylinder apparatus, or at 0.2 GPa, 900°C, for 1 and 20 hours in TZM cold-seal vessels. Polarized Fourier-transform infrared spectrometry (FTIR) was employed to quantify the hydroxyl distributions in the samples after the experiments. The new data obtained show a strong anisotropy of diffusion, with the diffusion coefficient D[100]>D[010]> D[001] at 900°C for short duration experiments. This initial mechanism of diffusion possibly involved a redox-exchange between proton and polaron. After longer duration experiment, the anisotropy of diffusion is different with D[001]>D[010]» D[100]. For this second stage of diffusion a model of hydrogen-metal vacancy associated defects is proposed, where the vacancies are the slower diffusing species with the diffusion laws: [100], [010] = 10-(5.6±3.2) exp [-(175 ± 76)/RT] [001] = 10-(1.4±0.5) exp [-(258 ± 31)/RT] (2) Hydrogen diffusion in forsterite The kinetics of hydration linked to magnesium-vacancy diffusion within dry synthetic forsterite single crystals was determined by performing similar experiments and analyses as in the previous section. The experiments were performed at 1.5 GPa, 1000°C for 3 hours in piston cylinder apparatus, or at 0.2 GPa, 900-1110°C, for 3-20 hours in TZM cold-seal vessels. The chemical diffusion coefficients are marginally slower than in iron-bearing olivine for the same diffusion process, but the anisotropy of diffusion is the same, with the [001] axis the fastest direction of diffusion and [100] the slowest. Fits of the diffusion data to an Arrhenius law yield similar activation energies for each of the crystallographic axes; a global fit to all the diffusion data gave an activation energy around 211 ± 18 kJmol-1. Thus hydration likely occurs by coupled diffusion of protons and octahedrally coordinated metal vacancies. The diffusion rates are fast enough to modify water contents within xenoliths ascending from the mantle but they are probably too slow to permit a total equilibration in a new dry or wet environment. (3)Dehydration profiles in natural mantle-derived olivine within basalt First evidence for water diffusion in a natural mantle-derived olivine are presented from peridotite samples. The samples are olivine crystals within lherzolite xenoliths from the Quaternary alkali basalts of the Pali-Aike volcanic field in Patagonia. Water content and distribution was studied using unpolarized and polarized FTIR and analyses shows that olivine, Cr-diopside and orthopyroxene contain a significant amount of water, with up to 13 wt ppm H2O for olivine and up to 250 wt ppm H2O in the pyroxenes. In contrast, analysis of optically clear-parts of small garnet crystals indicates that they are dry. Oriented Infrared profiles show that olivine grains larger than 0.5 mm have hydroxyl-depleted rims. These water concentration profiles suggest that partial dehydration occurred during the ascent of the xenolith-bearing magma to the Earth’s surface, confirming that dehydration is occurring in the nature. From a combination of analyses of natural xenoliths with experimental diffusion works, ascent duration of the host magma is estimated to several hours, suggesting a fast rise up to the surface. (4)Temperature and pressure dependence of water solubility in iron-free wadsleyite Previous experimental studies indicate that the maximum solubility of water in wadsleyite may vary as a function of pressure and temperature. Therefore wadsleyite samples were synthesized using a multi-anvil press. One series of experiments were performed at a fixed pressure of 15 GPa and at various temperatures and in a second series the temperature was fixed at 1200°C and pressure was varied from 13 to 18 GPa. The starting material corresponds to a composition of Mg2SiO4 + 5wt% H2O. The water content was quantified by ion probe (SIMS). Results show that at 15 GPa, the water concentration decreases significantly with increasing temperature from 2.5 wt% H2O at 900oC down to 0.93 wt% H2O at 1400oC; the corresponding wadsleyite Mg/Si ratios increase from 1.79 to 1.93 over this temperature range. Up to 17 GPa, no significant effect of pressure on the water content was observed. Moreover, together with previous results on ringwoodite, these data imply a strong decrease of the water partition coefficient between wadsleyite and ringwoodite with temperature. (5) Computer simulation on hydrous point defect in iron-free wadsleyite The general utility lattice program (GULP), a semi-empirical method, was used to simulate the formation of point defects (Mott-Littleton method) in wadsleyite and especially hydrogen incorporation and their corresponding infrared frequencies.
