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Notes: Abstract This work applies the well-known supernova-trigger hypothesis for solar system formation to explain in detail many properties of the Allende meteorite. The Allende carbonaceous chondrite meteorite is an assemblage of millimetre- to centimetre-sized Ca-Al-rich inclusions (CAI's), fine-grained alkali-rich spinel aggregates, amoeboid olivine aggregates, olivine chondrules and sulfide chondrules set in an extremely fine-grained black matrix. Detailed isotopic, chemical and textural properties show that these components formed in the above order as independent cosmic grains. Some CAI's containmicron-sized metal nuggets in which the normally incompatible refractory (Mo, Re, W) and platinum group (Pt, Os, Ir, Ru) metals are alloyed together in approximately ‘cosmic’ proportions, suggesting that these nuggets also condensed as cosmic grains. From the consistent pattern of enclosure of earlier components on the above list within later ones, it appears that in the environment where these materials formed, condensation moved inexorably in the direction of increasing olivine and decreasing refractory element and16O content (from ∼4% excess16O to ∼‘normal’ terrestrial oxygen isotopic composition). Condensation sequences are all short and incomplete, from which it is concluded that condensing materials were soon separated from the condensing environment and isolated until all were brought together in a final ‘snowstorm’ of fine-grained, olivine crystals constituting the meteorite matrix. These major properties can be accounted for in a model in which a supernova remnant (SNR) in the ‘snowplow’ phase, whose oxygen was initially pure16O, pushes into a dark interstellar cloud. In the model, condensation of CAI's begins in the SNR shell when it has been diluted with ∼2500 times its mass of matter from the cloud, which also in part explains the rarity of observed isotopic anomalies in CAI's. The retardation of the SNR by the cloud propels condensed grains ahead toward the cloud under their own momentum. Continuing dilution by the cloud and continuing removal of the most refractory elements in grains can explain the evolving patterns of fractionation and depletion of refractory elements, including REE's, in successive condensates. Features such as rims on CAI's and concentric zonation of fine-grained aggregates can also be satisfied in the model. A presolar origin and a short (∼ 10 000 years) formation time for inclusions in carbonaceous chondrites are major implications of the model.

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.