Thirty-eight determinations of the Hf/Zr ratio fall on a normal distribution curve with mean value 0.018 and standard deviation 0.0055. Altered varieties of zircon show standard deviations increasing with degree of alteration. The Hf/Zr ratio is characteristic of petrographic provinces. The data may be explained by assuming that alteration took place by diffusion in supercritical water.

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