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Oxygen isotope temperatures of “equilibrated” ordinary chondrites

1972; Elsevier BV; Volume: 36; Issue: 2 Linguagem: Inglês

10.1016/0016-7037(72)90004-x

1872-9533

Naoki Onuma, Robert N. Clayton, T. K. Mayeda,

Nuclear Physics and Applications

Measurements have been made of O18O16 ratios of coexisting minerals (plagioclase, pyroxene, olivine) from nine ordinary chondrites. The O18 content of a given mineral increases systematically from H through L to LL group. For example, mean δO18 for olivine is 3.7%. for H group, 4.l%. for L group, and 4.3%. for LL group. Isotope fractionations for mineral pairs fall on a concordancy line, suggesting that these minerals were crystallized in oxygen isotope equilibrium. The isotopic temperature is estimated to be 950 ± 100°C for seven of the type 5 and type 6 chondrites, while Shaw (L6) has a higher temperature (1300°C), and Bjurböle (L4) has a lower temperature (690°C). These temperatures probably represent the maxima attained during dry thermal metamorphism.