Measurements of O¹⁸/O¹⁶ ratios in phenocrysts from basalts of Hawaii, Tahiti, Tristan da Cunha, New Zealand, and Ross Island show that isotopie fractionations between mineral pairs are uniform from rock to rock. Typical values of the fractionations, expressed as 1,000 In a, are: plagioclase-magnetite, 2.1; plagio-clase-augite, 0.6; plagioclase-olivine, 0.9. On the basis of the laboratory-calibrated plagioclase magnetite pair, the isotopic fractionations represent a temperature of 1,020°-1,100° C, in agreement with field observations. For the corresponding mineral pairs in mafic intrusive rocks, the fractionations are about twice as great, indicating temperatures of isotopie equilibration below 800° C, probably as a result of retrograde exchange during slow cooling.
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Anderson et al. (1971) studied this question.
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