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The variation of viscosity with temperature has been determined over a wide range of temperature for a Muong Nong tektite material. The viscosity at the liquidus temperature of 1320øC is about 2 x 10 • P. Crystallization heat treatments in the range of temperature between 900 ø and 1300øC does not produce significant crystallization in the natural sample except when the sample is heated in contact with a synthetic tektite composition. In this case, cristobalite crystals grow from the contact surface into the natural sample. For comparison, two synthetic microtektite compositions with lower SiO2 contents than the Muong Nong material were also examined. These compositions could not be formed free of crystals in sizes as large as the Muong Nong sample. Heat flow calculations were performed for 2.5 cm and 10 cm spheres of tektite material cooling by radiation to their surroundings. The combined results of the crystallization study and the heat flow calculations indicate that the Muong Nong material is an excellent glass former and can be formed as a glass in bodies even larger than the natural samples. The results also indicate that the microtektite compositions require faster cooling rates to be formed as glasses than the Muong Nong material and that nucleation behavior is very likely important in determining the size of bodies found as tektites. 104 cm -3. In contrast, the bubbles distributed through the rest 5485

A mathematical model is developed to describe the concentration profiles as a function of time of a solute dissolved in two coexisting finite phases under continuous cooling conditions. A temperature‐dependent partitioning of the solute between the two phases is included. The model relates to the use of experimentally determined solute profiles to infer the rates at which various rocks cooled. The model employs finite difference equations and the Thomas tridiagonal method to solve the appropriate differential equations describing the solute partitioning. To illustrate the application of the model, the partitioning of zirconia between coexisting ilmenite and ulvöspinel is considered. It is shown that cooling rates can be calculated not only from solute concentration profiles within the grains but also from the ratio of the solute concentrations at a given distance on both sides of the interface or from the ratio of the average solute concentrations in the two phases. The cooling rates of a suite of Apollo 15 Elbow Crater gabbros are calculated from measured ratios of concentrations at 10 μm from the interfaces between the phases. A comparison is made of calculated concentration ratios 10 μm from the interface and cooling rates for equilibrium versus uniform initial concentrations. It is shown that the Elbow Crater gabbros could not have had a uniform concentration profiles at the previously suggested initial temperature of 1350 K. Also considered are the effects of the initial temperature, the grain sizes, and the diffusion coefficients of Zr in the phases on the calculated cooling rates. Finally, suggestions are made for other systems where the present model can be applied to estimate the thermal histories of rocks.