Crystallization and Oxidation Processes in Heat-treated Basalt Glass

Dorothee J. M. Burkhard (burkhard@mailer.uni-marburg.de)

University of Marburg, Institute for Mineralogy, D-35032 Marburg, Germany

While much has been done to understand crystallization and redox effects in basalt, little is known about such processes during heat treatment of basalt glass, although fundamental in order to evaluate possible post-eruptive interactions between lava flows or to improve the design of fire-resistant insulation materials. We therefore heat-treated tholeiitic basalt glass (< 1 wt.% Fe₂O₃ of about 11. wt.% FeO_tot in Air and observe crystallization in two distinct events, related to a surface-controlled growth of mainly augite dendrites with Fe-Ti oxides between 775 and 900°C, and an additional volumetric crystal growth including that of anorthite-rich plagioclase above 920°C. In Argon atmosphere, only one crystallization event occurs with a peak around 890°C, inducing surface and a minor volumetric growth of mainly pyroxene, with a significantly smaller overall growth rate. Fe-Ti oxides are not found. The heat-treatment causes oxidation, the degree of which depends on temperature, time, exposed surface area to bulk, and external atmosphere, i.e., Air or Argon. As demonstrated by ¹⁸O, there is no exchange with atmospheric oxygen, i.e., the oxygen acceptance coupled to the Fe²⁺ => Fe³⁺ electron loss must be made available from within the glass. One possibility is that in the course of the relaxation of the glass structure during heating, oxidizing, residual gases are released. As a result of the heat-treatment, extensive diffusion of mainly Ca to the sample surface is observed, if treated in Air, while a treatment in Argon results in an only minor increase in Mg at the sample surface. The activation energy E_a for oxidation is much smaller than E_a for Ca-diffusion. Hence, it is unlikely that oxidation depends on such diffusion processes as suggested by Cooper et al. (1996). However, diffusion depends on the oxygen gradient between sample and external atmosphere. It is therefore possible that an enhanced diffusion enables the enhanced crystallization observed in Air.

Cooper RF, Fanselow JB & Poker DB, Geoch. Cosmochimica Acta, 60, 3253-3265, (1996).

Melting Experiments on the Pyrope - Almandine - Grossular Join at Pressures 6.5 GPa: The Application to the Genesis of Diamond-bearing Eclogites

Valentina G. Butvina (archi@geol.msu.ru)¹, Andrei V. Bobrov¹ & Yurii A. Litvin²

¹ Chair of Petrology, Geological Department, Moscow State University, Vorob'evy Gory, Moscow 119899, Russia

² Institute of Experimental Mineralogy, Russian Academy of Sciences, Chernogolovka, Moscow District 142432

The origin of zone Fe/Mg distribution and, especially, a growing magnesiality towards grain edges known as "inverse zoning" in natural garnets from diamond- bearing eclogites (Marakushev and Bobrov, 1998) is still an open question important in terms of the mantle dynamics and genesis of the deep mantle rocks. Compositions of the mantle garnets can be most closely represented by the pyrope Mg₃Al₂Si₃O₁₂ - almandine Fe₃Al₂Si₃O₁₂ - grossular Ca₃Al₂Si₃O₁₂ system. Melting relations of the pyrope - almandine join, pyrope₅₀almandine₅₀ - grossular and almandine₅₀grossular₅₀ - pyrope internal sections of the system were studied in the experiments at 6.5 GPa and 1500 - 1900°C (the conditions correspond to the PT field of diamond stability). An anvil-with-hole high-pressure apparatus and W/WO₂ buffer to stabilize the Fe²⁺ form of iron (Litvin, 1981) were used. Temperature of congruent melting of the pure almandine Fe₃Al₂Si₃O₁₂ phase was found to be 1600°C at 6.5 GPa. On experimental grounds, melting relations for the pyrope Mg₃Al₂Si₃O₁₂ - almandine Fe₃Al₂Si₃O₁₂ join at 6.5 GPa are characterized by the Roseboom's type I diagram representing complete series of solid solutions without extreme points. It became apparent that the inverse zoning of the natural eclogitic garnets can not be formed by crystallization of eclogitic magmas at fixed pressure. Melting relations on the pyrope - almandine join allow garnet crystallization in the course of pressure lowering as the most suitable mechanism of formation of the Fe/Mg inverse zoning in the mantle garnets. This mechanism is compatible with the convection currents rising in the mantle. For the pyrope Mg₃Al₂Si₃O₁₂ - almandine Fe₃Al₂Si₃O₁₂ - grossular Ca₃Al₂Si₃O₁₂ system, experimental studies at 6.5 GPa on the both pyrope₅₀almandine₅₀ - grossular and almandine₅₀grossular₅₀ - pyrope joins shows pseudo binary phase relations and appearance of not only corundum Al₂O₃ phase as in a case of the pyrope - grossular join at 2.8 GPa (Malinovsky et al, 1983) but kyanite Al₂SiO₅ and coesite SiO₂ as well. Formation of corundum, kyanite and coesite in the experimental system pyrope - almandine - grossular at 6.5 GPa is in agreement with the mineral assembly for natural diamond - bearing eclogites and indicative as the new arguments for the origin thereof.

Marakushev AA & Bobrov AV, Doklady Earth Sciences, 358, 526-530, (1998).

Litvin YuA, Geokhimiya, 8, 1234-1242, (1981).

Malinovsky YuI, Doroshev AM & Kalinin AA, Doklady USSR Ac. Sci, 268, 189-194, (1983).

The Kinetics of Ostwald Ripening for Quartz and Albite in Hydrous Silicic Melts. Implications for the Segregation of Granitic Melts

Hugues Cabane (cabane@opgc.univ-bpclermont.fr), Didier Laporte & Ariel Provost

Laboratoire Magmas et Volcans, 5 rue Kessler, 63038 Clermont-Ferrand Cedex, France

Silicate melts in partially molten rocks form an interconnected network of grain-edge channels and films and may separate from the solid residuum at very low degrees of melting. An important parameter that controls the rate of melt segregation is the mean size of the grains because permeability is proportional to the square of grain size. According to the principle of interfacial energy minimisation, the mean grain size of a partially molten system will tend to grow with time by a dissolution-precipitation mechanism known as Ostwald ripening. We conducted an experimental study of the kinetics of Ostwald ripening for quartz and albite in hydrous silicic melts. The experiments were performed in a piston-cylinder apparatus at 1 GPa or in a externally-heated pressure vessel at 0.2 GPa. The samples were studied by scanning electron microscopy and the main textural parameters (grain size distribution, mean grain size, grain shape, etc.) were characterised by image analysis.

Major changes in grain shape and grain size occurred during the experiments. For quartz, the grain shapes evolved from angular fragments with embayments to equilibrium shape with planar facets coexisting with smoothly curved, convex surfaces. After a transient regime of a few hours, the mean grain size d increased as a function of the cubic root of time t, d³-d₀ ³=kt (where d₀ is the initial mean size and k is the growth rate) in agreement with theoretical studies of Ostwald ripening. k increases from ~2 µm³/s for 6.5 wt% water in the liquid to ~20 µm³/s for a water-saturated liquid. Using our data for quartz in equilibrium with a water-undersaturated granitic melt, we found that Ostwald ripening may increase the permeability of a partially molten rock by one order of magnitude in 6000 years.