Oxygen isotopic analyses of coexisting minerals in igneous rocks indicate the crystallization history and mineral paragenesis of these rocks. Detailed variations in the O18/O16 fractionations among minerals can be readily explained on the basis of equilibrium crystallization of magma with incomplete reaction. In every instance in which conclusive information exists, the isotopic results are in excellent agreement with the crystallization sequence determined by conventional geologic criteria. Most rocks analyzed fall into a consistent pattern with respect to petrologic type.

As is to be expected on the basis of their lower-temperature origin, O18/O16 fractionations among coexisting minerals in metamorphic rocks are in general somewhat larger than in igneous rocks. These data also suggest, however, that oxygen isotopic equilibrium is not always established during metamorphism. The higher the grade the greater is the apparent tendency for a metamorphic rock to approach the igneous-rock range of oxygen isotopic composition. This is most probably a result of exchange with a reservoir of relatively constant oxygen isotopic composition during metamorphism.

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