Abstract
The 18O/ 16O ratios of coexisting silicate minerals in igneous and metamorphic rocks are approximately proportional to the following chemical index:Si+ 0.58Al equivalents/total cation equivalents Measured phenocryst-groundmass isotopic fractionations in volcanic rocks are negligible ( ±0.2per mil) when corrected for differences in their chemical indices. Apparently, solid and liquid silicates of equivalent chemistry do not fractionate the isotopes of oxygen with respect to one another. Isotopic zoning within phenocrysts and whole-rock isotopic variations accompanying magmatic differentiation are thus expected to be minimal and chiefly due to variations in chemical index. Isotopic fractionations among minerals in plutonic rocks are considerably larger than those found among similar minerals in volcanic rocks. Furthermore, continental plutonic rocks are commonly enriched in 18O relative to oceanic volcanic rocks of equivalent chemistry.
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