Melt extraction, permeability, and textural development during melting at high pressures experimentation to 1000 g in a centrifuging piston cylinder (max. 1 GPa, 1400 C)
- Doctoral Thesis
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In this thesis a series of piston cylinder experiments has been conducted on partially molten systems, to understand the textural and mechanical relationship between the solid and liquid phases. Three different experimental apparati have been used for these the experiments. A standard end-loaded piston cylinder to perform annealing experiments; an end-loaded piston cylinder equipped for in-situ measurement of electrical conductivity of samples has been exploited to determine the conductivity of silicate-metallic samples and a newly developed centrifuging piston cylinder, which applies high temperature, pressure and acceleration to partially molten samples. These apparati and the assemblages to run experiments with them are described in detail. Before the description and analysis of the four systems investigated in detail, all nonsuccessful experiments are briefly reviewed. Most of the latter experiments have been performed in an olivine plus Fe-S system and a natural gabbro system, with negative results. A series of annealing experiments and conductivity measurements has been performed to investigate the relationship between solid olivine and molten Fe-S. A percolation threshold larger than 15 vol% has been found for this system as bulk conductivity similar to that of pure olivine has been measured for mixtures containing less than 20 vol% of Fe-S. The measurements of dihedral angles at different oxygen contents of the metallic alloy has revealed that the dihedral angle itself is always larger than 60° for oxygen contents up to 16.6 atom%. A grain growth law for olivine surrounded by molten ironsulfide has been computed. From these data the final conclusion that in an accreting planetary body , in which the silicate fraction remain crystalline, only the metal melt in excess of 15 vol% could segregate to form a core, the rest being trapped in the silicate mantle. In the system “peridotite plus Fe-S” gravitational separation of Fe-S melt from solid or partially molten peridotites has been studied, together with determination of a percolation threshold for the system. Segregation rates of small Fe-S droplets in a peridotite matrix were determined to 2.4·10-4 to 1.1·10-3 mm/h in presence of silicate melt, dropping to just 8.4-8.6·10-5 mm/h in solid peridotite. This corresponds to permeabilities of 10-16-10-17 m2 for the silicate melt present system, and to a permeability of 10-19 m2 for silicate melt absent system. These data yield a calculated core formation time for the Earth of ca. 300 My, for Mars of about 240 My and for Vesta of about 220 My. These values are at least one order of magnitude larger than Hf/W ages for core formation in these planetary bodies. Conductivity experiments show results similar on the “olivine plus Fe-S” and the “peridotite plus Fe-S” systems. Bulk conductivities were measured and are similar for mixtures containing up to 15 vol% of Fe-S. From these results the following conclusions arise: Fe-S melt percolation in solid or partially molten peridotites is too slow, to account for core formation in earth size bodies and the percolation threshold for peridotite plus Fe-S melt is larger than 15 vol%. Melt extraction experiments have been performed, with the use of the centrifuging piston cylinder, in a system constituted of olivine aggregates with small amounts of basaltic melt added (system olivine plus MORB). A permeability between 1.57·10-10 and 4.48·10- 8 10 m2 has been found for aggregates containing 5 to 13 vol% of melt, which correspond to extraction rates of 3.2 to 16.7 m/yr. These values do not agree to those computed with the models of McKenzie (1984) or supposed by Faul (1997), however they are close to those of Faul’s (1997) model. A comparison with the segregation rates calculated on the base the isotopic ratios of Th, U and Ra in natural MORB melt yields good agreement with the permeabilities of this study. A last series of experiments has also been done with the centrifuging piston cylinder dealing with a system constituted by basaltic melt containing 30 vol% of olivine (system MORB plus olivine). The settling velocity for a suspension of olivine grains has been measured to be 1/10 of the settling velocity of a single isolated grain (settling with a Stokes’ velocitiy), confirming experimentally the observations of Schwindinger (1999). A formation time of ca. 0.32 m/yr has been computed for olivine orthocumulates, which is in agreement with evidences from a natural olivine orthocumulate present in the succession of the Murotomisaki gabbroic Complex (Japan). After mechanical settling is completed, gravity induced dissolution-repricipitation processes lead to the formation of adcumulates where the previous porous space is filled with the cumulus phase. A possible formation time for olivine adcumulates of ca. 1 cm/yr, after maximum close packing of grains is reached, has been estimated on the base of experimental data. This last value is valid only for adcumulate layers which are a few centimeters thick Show more
ContributorsSupervisor: Bagdassarov, Nikolai
Supervisor: Schmidt, Max W.
SubjectSTRUCTURE + TEXTURE + FABRICS (PETROGRAPHY); SEQUENCE OF SEPARATION FROM THE MAGMA (PETROGRAPHY); GESTEINSDIFFERENTIATION (PETROGRAPHIE); MORB, MID OCEAN RIDGE BASALTS (PETROGRAPHIE); EXPERIMENTAL GEOLOGY (EARTH SCIENCES); MORB, MID OCEAN RIDGE BASALTS (PETROGRAPHY); STRUKTUR + TEXTUR + GEFÜGE (PETROGRAPHIE); PERIDOTIT + DUNIT + OLIVINGESTEINE (PETROGRAPHIE); ROCKS DIFFERENTIATION (PETROGRAPHY); AUSSCHEIDUNGSFOLGE DES MAGMAS (PETROGRAPHIE); ANALOGIE- UND SIMULATIONSAPPARATE (ERDWISSENSCHAFTEN); ANALOGY AND SIMULATION APPARATUS (EARTH SCIENCES); PERIDOTITE + DUNITE + OLIVINITE (PETROGRAPHY); EXPERIMENTALGEOLOGIE (ERDWISSENSCHAFTEN)
Organisational unit03592 - Schmidt, Max
Diss., Eidgenössische Technische Hochschule ETH Zürich, Nr. 17727, 2008.
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