The spacings of coherent cryptoperthite lamellae in phenocrysts of two rhyolitic lava flows from Big Bend, Texas have been determined by transmission electron microscopy. The lamellar spacings (660–2140Å) along a section through the lower and upper flows vary systematically, and this pattern of variation is in agreement with that predicted by heat flow calculations and reheating of the lower flow by the upper flow. The values of the observed lamellar spacings can be approximately explained by using the published experimental data for the coarsening of cryptoperthite lamellae and the estimated thermal history from heat flow calculations. The calculated spacings are very sensitive to the initial temperature of exsolution, and hence to the factors which affect this temperature.

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