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Notes: Abstract Diffusion rates for the three tetravalent cations U, Th and Hf have been measured in synthetic zircon. Diffusant sources included oxide powders and ground pre-synthesized silicates. Rutherford backscattering spectrometry (RBS) was used to measure depth profiles. Over the temperature range 1400–1650 °C, the following Arrhenius relations were obtained (diffusion coefficients in m2sec−1): log D Th = (1.936 ± 0.9820) + (− 792 ± 34 kJ mol−1 /2.303 RT) log D U = (0.212 ± 2.440) + (− 726 ± 83 kJ mol−1 /2.303 RT) log D Hf = (3.206 ± 1.592) + (− 812 ± 54 kJ mol−1 /2.303 RT) The data show a systematic increase in diffusivity with decreasing ionic radius (i.e., faster diffusion rates for Hf than for U or Th), a trend also observed in our earlier study of rare earth diffusion in zircon. Diffusive fractionation may be a factor in the Lu-Hf system given the much slower diffusion rates of tetravalent cations when compared with the trivalent rare earths. The very slow diffusion rates measured for these tetravalent cations suggest that they are essentially immobile under most geologic conditions, permitting the preservation of fine-scale chemical zoning and isotopic signatures of inherited cores.

Notes: Abstract In the absence of an externally applied stress, the segregation of small amounts of granitic or tonalitic melts from their residual mafic crystals is possible only if the melt forms an interconnected network phase. Accordingly, this research focuses on melt connectivity at low melt fraction (〈4 wt% or 5 vol.%). Connectivity of granitic and tonalitic melts in amphibole-rich rock was assessed by performing two types of piston-cylinder experiments at 1 GPa and 800 °C. The first involved annealing samples that consisted of either alternating layers or homogeneous mixtures of calcic amphibole and metaluminous obsidian powder. The second type of experiment involved creating diffusion couples. Here, an upper cylinder of amphibole-saturated granitic or tonalitic melt was placed against a lower cylinder consisting of an amphibole-rich rock containing zero or a small melt (granitic or tonalitic) fraction. The upper part of the diffusion couple was doped with β emitter (151Sm or 14C) and functioned as an infinite melt reservoir. The lower part of the diffusion couple was considered to be the host rock. The experiments approached textural equilibrium which allowed us to characterize the wetting behaviour of the calcic amphibole by the hydrous silicic melt (granitic or tonalitic). These particular experiments also provided information concerning diffusive transport, because the β emitter could diffuse through the connected melt (liquid) in the amphibole-rich rock. The dihedral angle measurements show that melt connectivity was achieved. This conclusion is based on the fact that the dihedral angles, θ, consistently yielded median apparent values of 53°〈θ〈58° for an amphibole-rich rock/granitic melt system, and 46°〈θ〈48° for an amphibole-rich rock/tonalitic melt system. However, the frequency distribution of θ angles is found to be relatively broad. The results of the diffusion-couple experiments, assessed using the β radiographic technique, complement the dihedral (wetting) angle measurements by showing that melt connectivity is achieved at a melt fraction less than 4wt% (5 vol.%).

Notes: Abstract The diffusivity (D) of dissolved SiO2 in quartz-saturated H2O was determined at 1 GPa and ∼530–870 °C using a custom-designed Ag diffusion cell consisting of two chambers – both containing quartz + H2O – connected by a narrow capillary. During a diffusion experiment, quartz saturation was maintained at different levels in the two chambers by placing the diffusion cell in the thermal gradient of a standard piston-cylinder assembly. The diffusivity was computed from the total mass of SiO2 transported from the “hot” to the “cold” chamber during the course of an experiment. Over the temperature range investigated, the results conform to an Arrhenius-type dependence of D SiO2 (m2/s) upon T(K)−1: The significance of the constants in this equation (in particular, the ∼52 kJ/mole apparent activation energy) is uncertain, because the SiO2 content of the fluid varies markedly with temperature, due to the strong temperature dependence of quartz solubility. Nevertheless, the above expression is probably a good representation of the temperature dependence of D SiO2 in the crust, where aqueous fluids are likely to approach quartz saturation at all depths. One experimental result at 0.6 GPa suggests little dependence of D SiO2 upon pressure at crustal conditions. At the low end of the temperature range investigated, the measured diffusivities are identical to values calculated from the Stokes-Einstein equation using high P-T viscosity estimates for H2O. Disagreement between measured and calculated diffusivities at higher temperatures (a factor of ∼4 at 850 °C) may be due to one or more of the following factors: (1) inadequacy of the Stokes-Einstein relationship as a description of transport in supercritical H2O; (2) inaccuracy of viscosity estimates of H2O; or (3) concentration effects on diffusion over the temperature range investigated. Given the presence of interconnected porosity in deep-seated rocks, the diffusive transport distances for aqueous silica implied by the above equation are impressive even on a geologic scale, exceeding 0.5 km in 106 years at temperatures of 500 °C or higher. The combined effect of the high D SiO2 with the high and strongly temperature-dependent solubility of quartz at crustal conditions raises the possibility of significant diffusive fluxes through a stationary fluid in a normal geothermal gradient.