Oxygen isotope studies of minerals in stony meteorites

Abstract

Oxygen isotope analyses demonstrate the following sequence (as in terrestrial igneous rocks) of increasing O^(18) content for coexisting minerals of stony meteorites: olivine, pyroxene, plagioclase, free silica. Except for the carbonaceous chondrites, the O^(18)/O^(16) ratio of a given mineral is quite uniform in each meteorite class. Differences exist between classes of meteorites, however, as well as between certain meteorites and terrestrial igneous rocks. For example, the O^(18)/O^(16) ratios of meteoritic pyroxenes vary from δ = −0.5 per mil to δ = + 8.6 per mil (relative to SMOW), whereas pyroxenes in 8 terrestrial igneous rocks have δ-values which range only from + 5.5 to +6.6. The oxygen isotope data suggest a separation of the stony meteorites into three groups, as follows: 1. I. Basaltic achondrites, hypersthene achondrites, and mesosiderites—with pyroxene δ-values of 3.7 to 4.4. 2. II. Hypersthene-olivine chondrites, bronzite-olivine chondrites, enstatite chondrites, enstatite achondrites, and nakhlites—with pyroxene δ-values of 5.3 to 6.3. 3. III. Types I, II and III carbonaceous chondrites, and ureilites—with highly variable olivine and pyroxene δ-values. It is suggested that the meteorites within each of these three groups are genetically related to one another, but that the three groups may themselves be only distantly related or unrelated. The chondrites have O^(18)/O^(16) ratios similar to their terrestrial analogs, the ultramafic rocks. However, whereas terrestrial basaltic rocks are 1 to 2 per mil richer in O^(18) than ultramafic rocks, the basaltic meteorites are 0.5 to 1.5 per mil lower than chondrites; thus there are serious restrictions placed on any postulated derivation of the basaltic meteorites from chondrites. In addition, the carbonaceous chondrites cannot be simply normal chondrites which have suffered a low-temperature alteration. The olivine in carbonaceous meteorites is lower in O^(18)/O^(16) and isotopically much more variable than chondrite olivine, implying that it is derived from a distinctly different source material.

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