Naoki Onuma

DISTRIBUTION OF THE PRE-SOLAR COMPONENT IN ALLENDE AND OTHER CARBONACEOUS CHONDRITES

Excess 16 O, relative to terrestrial abundances, has been found in all samples of C2, C3 and C4 carbonaceous chondrites which have been analyzed, amounting to nine meteorites thus far. Whole-rocks and mineral separates from all the C3 and C4 meteorites fall on a single mixing line consistent with admixture of 1–5% of excess 16 O. The anhydrous silicates of C2 meteorites fall on the same line, but the hydrous silicate matrix of C2s define a mass-fractionation trend parallel to the terrestrial trend, but displaced towards higher 16 O. All meteorites analyzed are isotopically heterogeneous on a sub-millimeter scale. Detailed analyses of separated phases of several Allende Ca Al-rich inclusions reveal a consistent pattern of large 16 O enrichments in spinel, pyroxene and sometimes olivine, and small 16 O enrichments in melilite, feldspathoids and grossular. The heterogeneous distribution of the 16 O excesses, together with their enhancement in minerals believed to be early solar nebular condensates, implies the existence of pre-solar “carriers” of the isotopic anomaly, probably grains or molecules with oxygen which was nearly pure 16 O. These carriers have not been unequivocally indentified, but pre-solar grains of corundum or spinel, and pre-solar molecules of SiO are possibilities. The isotopic anomalies may also have been disturbed by diffusion-controlled processes of exchange between early condensates and their surroundings, either in the nebula, or later in the parent body. No direct correlation has yet been observed between the oxygen isotope anomalies and recently observed isotope anomalies in neon, magnesium and xenon.

OXYGEN ISOTOPE COSMOTHERMOMETER.

Variations in oxygen isotopic abundances of meteoritic minerals, chondrules, whole meteorites, and planets are discussed in terms of a model involving isotopic exchange between primordial dust and a cooling solar nebular gas. From the temperature-dependence of the isotopic fractionation factors temperatures have been assigned to the processes of initial condensation, chondrule formation and planetary accretion. Separated phases from carbonaceous chondrites fall into three isotopic groups representing widely differing conditions of formation: (a) low-iron olivine and pyroxene, and calcium-aluminum silicates condensed at temperatures >1000°K; (b) high-iron olivine and pyroxene melted to form chondrules after prior cooling and exchange to temperatures of 530–620°K; (c) hydrous silicates condensed at temperatures below 400°K. n nThe ordinary chondrites, the Earth and the Moon have remarkably similar isotopic compositions, which are not readily accounted for in terms of mixtures of known materials of the primitive meteorites. Mean accretion temperatures of 450–470°K are estimated for members of this group. n nThe model permits an estimate of the O18O16 ratio of the solar nebular gas, which gives δO18 = −1 ± 2%. relative to SMOW.

Oxygen isotope temperatures of “equilibrated” ordinary chondrites

Abstract Measurements have been made of O 18 O 16 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 Bjurbole (L4) has a lower temperature (690°C). These temperatures probably represent the maxima attained during dry thermal metamorphism.

OXYGEN ISOTOPE FRACTIONATION BETWEEN MINERALS AND AN ESTIMATE OF THE TEMPERATURE OF FORMATION.

Oxygen isotopic compositions of separated minerals from three type A and four type B rocks are very uniform. The δ18O values are: plagioclase, 6.20; clinopyroxene, 5.75; ilmenite, 4.45 (parts per thousand relative to Standard Mean Ocean Water). The isotopic distribution corresponds to equilibrium at 1120�C. The isotopic composition of lunar pyroxenes falls within the range for pyroxenes of terrestrial mafic and ultramafic rocks, ordinary chondrites, enstatite chondrites, and enstatite achondrites, but above the range for basaltic achondrites, hypersthene achondrites, and mesosiderites. Glass isolated from the lunar soil has a δ18O value of 6.2, significantly richer in 18O than the crystalline rock fragments in the soil.

Oxygen isotopic composition of minerals in the Kenna ureilite

The δ18O and δ17O values of olivine from Kenna are 7.6 and 3.0%, respectively, relative to SMOW. These values are typical of ureilites which form a unique group on a δ17O -δ18O graph. The ureilites are related to, but not directly derived from, the anhydrous phases of C2 and C3 meteorites. The 18O/16O fractionation between pyroxene and olivine is 0.60, indicating a temperature of last equilibration of 1000 ± 100°C.