The Dissolution Rate of Quartz in Basanite Melt: An Investigation of the Effects of Experiment Geometry on Mineral Dissolution Rates

Cliff S. J Shaw (cliff.shaw@uni-bayreuth.de)

Bayerisches Geoinstitut, Unversität Bayreuth, 95440 Bayreuth, Germany

Data on the dissolution rate of minerals in silicate melts are important for our understanding magmatic processes. However, interpretation of the available data is complicated since the experiments were done using different sample geometries. This study aims to quantify dissolution rates as a function of sample geometry by dissolving quartz cylinders and spheres in basanite melt at 0.5 GPa and 1350°C. Dissolution rates depend on the shape and position of the crystal in the capsule. The dissolution rate of cylinders at the top of the capsule is proportional to the square root of time, thus, dissolution is controlled by diffusion. Cylinders on the capsule bottom dissolve faster, at a rate proportional to time, thus diffusion is no longer the rate limiting step. Cylinders in the middle of the capsule and spheres at the top give rates intermediate between the two extremes. Compositional data show that in experiments with cylinders at the top of the capsule dissolution is by diffusion alone with D_SiO2 in the range 7 - 10 µm²/sec^-1. Experiments with quartz cylinders at the bottom of the capsule show evidence for compositional convection i.e. plumes of silica-rich melt detached from the crystal melt interface. Dissolution rates are faster with this geometry since the thickness of the diffusive boundary layer is periodically decreased. Compositional profiles in where a cylinder was placed in the middle of the capsule confirm that dissolution proceeds by both diffusive and diffusive - convective processes. Experiments using spheres give extremely complex patterns that cannot be explained by diffusion or compositional convection alone but require an element of forced convection associated with sinking of the crystal. These results indicate that choice of sample geometry is very important in dissolution experiments and that many previous dissolution experiments give rates that reflect control by convection.

Dissolution of Orthopyroxene, Spinel and Clinopyroxene in Si-Undersaturated Alkaline Magma: Experimental Constraints on the Textural Evolution of Mantle Xenoliths

Cliff S. J. Shaw (cliff.shaw@uni-bayreuth.de)

Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Germany

Mantle xenoliths evolve over a range of pressures and temperatures. Before entrainment they equilibrate under static conditions with short-lived perturbations e.g., passage of magma or mantle metasomatism. Once xenoliths are entrained in a magma, they are subjected to large changes in pressure, temperature, etc.

Disequilibrium textures are common where xenoliths are penetrated by melt. Orthopyroxene may be surrounded by reaction rims of olivine + clinopyroxene + Si-rich glass. Spinel is often surrounded by an Al-depleted and Cr-enriched sieve-textured rim and clinopyroxene often has a sieve-textured rim depleted in Na and Al and enriched in Ca and Ti. The thickness of these reaction zones increases in the order spinel - clinopyroxene - orthopyroxene.

To determine if these textures indicate in situ reactions in the mantle or reaction during transport, a series of experiments were performed from 1 atmosphere to 2 GPa and the resulting textures and compositions compared with those observed in nature. Textures similar in to those in natural samples have been produced by reaction between basanite and orthopyroxene from 1 atmosphere to 1 GPa. Above 1 GPa the reaction rims are compositionally and texturally distinct. Reaction rims been formed around Cr-rich spinels at 0.5 and 1 GPa however, unlike the rims around orthopyroxene these appear to require large amounts of superheating. Experiments 10° above the liquidus of the basanite do not form sieve texture whereas those at 60° above the liquidus do form sieve texture. Reaction rims similar to those around clinopyroxene in natural samples have been formed by clinopyroxene - basanite reaction at 0.5 and 1 GPa and 10 - 30° of superheating. All of these textures develop over a short time span - hours to days - indicating that they more likely reflect dynamic processes associated with xenolith entrainment rather than any process that operated in situ within the mantle.

Carbon Diffusion in Olivines

Svetlana Shilobreeva (shilobre@postman.ru)¹, Leonid Kuzmin (kuzminl@mail.ru)² & Alexandr Kazantsev (kazants@sci.lebedev.ru)³

¹ Vernadsky Institute of Geochemistry and Analytical Chemistry of Russian Academy of Sciences, Kosygin st.19, Moscow,117975, Russia

² Laboratory of Atomic Nucleus, Institute of Nuclear Research of Russian Academy of Sciences, prospect 60-Letiya Oktyabrya, 7a, Moscow, 117312, Russia

³ State Research Institute of Rare Metals, Bolshoy Tolmachevsky per.5, Moscow,109017, Russia

Carbon is one of the most important chemical elements on the Earth. It can play an indicative role in cosmo- and geochemistry. Knowledge of carbon content and its distribution in minerals is required for understanding and modelling of processes during evolution of volatiles in the Earth. Several experimental and theoretical attempts were made to quantify carbon mobility and to clarify the mechanisms of carbon diffusion in silicates (Oberheuser et al.,1983; Tsong et al., 1985; Watson, 1986; Tingle and Green, 1985; Kuzmin et al., 1998). Carbon diffusion from the bulk to the surface of silicate crystal is still under discussion. The values of diffusivity, D, in olivines is also being disputed. As shown recently, the diffusion, alongside with the thermal process, can also include a radiation enhanced mechanism caused by natural radioactivity (Kuzmin et al., 1998). A technique for in situ determining radiation-enhanced diffusivities in oxide and silicate crystals was developed using a deuteron beam both for defect generation and direct measurement of concentration profiles from energy spectra of protons of the ¹²C(d,p)¹³C nuclear reaction. Polyenergetic ion implantation was proposed to be used to determine thermal diffusivities of carbon in forsterite crystals by secondary ion mass spectrometry. Data on thermal D_therm and radiation-enhanced D_rad carbon diffusivities are first found for forsterite and olivine crystals. The data show the strong dependence of diffusivities on carbon migration mechanism (interstitial and vacancy) in crystals: D_therm=1.10^-15 cm²/s (1100 K, 1 bar) and D_rad=1.6.10^-12 cm²/s (300-370 K, deuteron dose at energy 1.4 MeV is 2.8.10¹⁶ atom/cm²). The found data confirm the hypothesis proposed by the authors on the vacancy mechanism of carbon atom introduction and migration in olivine crystals of mantle origin.

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Watson EB, Geophys. Research. Letters, 13, 529-532, (1986).

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Kuzmin LE, Shilobreeva SN, Kazantsev AM & Minaev VM, Short Reports on physics, Lebedev Phys. Inst, 9, 12-17, (1988).