O 18/O 16 ratios of some chondritic meteorites and terrestrial ultramafic rocks

Abstract

Oxygen isotopic analyses were made on the silicate portions of 27 chondrites, 2 pallasites and 10 terrestrial ultramafic rocks using a fluorine extraction procedure. The presence of sulphides, free iron, iron oxides (formed by weathering of the meteorites), and olivine contributes difficulties to the extraction of the oxygen. Consequently, before fluorinations were done the samples were subjected to several pretreatments. Heating in vacuum and in an atmosphere of hydrogen removed the oxygen added to the meteorites during weathering. Dry chlorination at 350°C removed the iron and sulphide phases. Fusion of the samples with known amounts of quartz of known O 18 O 16 ratio converted the olivine to a pyroxene glass and permitted attainment of 100 per cent yields in the extraction of the oxygen. The total isotopic variation in the analyzed chondrites is only 1.2 per mil. The pallasite olivines are about 2 per mil lower in O 18 O 16 than the average chondrite. It is possible to detect some grouping within the 1.2 per mil range in the δ-values of the chondrites. For example, the average δ-value for the olivine-bronzite chondrites (H-type) is about 0.5 per mil lower than for the olivine-hypersthene (L-type) chondrites. Separated chondrules from 4 chondrites give essentially the same δ-values as the matrix. The δ-values for terrestrial dunites and olivines are within the range of δ-values reported by previous workers and are isotopically similar to the olivine-hyperstheno chondrites.