A component of primitive nuclear composition in carbonaceous meteorites.

The oxygen of anhydrous, high-temperature minerals in carbonaceous meteorites is strongly depleted in the heavy stable isotopes (17)O and (18)O. The effect is the result of nuclear rather than chemical processes and probably results from the admixture of a component of almost pure (16)O. This component may predate the solar system and may represent interstellar dust with a separate history of nucleosynthesis.

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 delta(18)O 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 degrees 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 delta(18)O value of 6.2, significantly richer in (18)O than the crystalline rock fragments in the soil.

Water in SNC meteorites: evidence for a martian hydrosphere.

The Shergotty-Nakhla-Chassigny (SNC) meteorites, purportedly of martian origin, contain 0.04 to 0.4 percent water by weight. Oxygen isotopic analysis can be used to determine whether this water is extraterrestrial or terrestrial. Such analysis reveals that a portion of the water is extraterrestrial and furthermore was not in oxygen isotopic equilibrium with the host rock. Lack of equilibrium between water and host rock implies that the lithosphere and hydrosphere of the SNC parent body formed two distinct oxygen isotopic reservoirs. If Mars was the parent body, the maintenance of two distinct reservoirs may result from the absence of plate tectonics on the planet.

A silicate inclusion in Puente del Zacate, a IIIA iron meteorite.

The IIIA and IIIB iron meteorites are considered to have formed in the cores of asteroids. A silicate inclusion within the IIIA meteorite Puente del Zacate consisting of olivine (Fa4), low-calcium pyroxene (Fs6Wo1), chromium diopside (Fs3Wo47), plagioclase (An14Or4), graphite, troilite, chromite, daubreelite, and iron metal resembles inclusions in IAB iron meteorites. The oxygen isotopic composition of the Puente del Zacate inclusion is like chromite and phosphate inclusions in other IIIA and IIIB irons. The Puente del Zacate inclusion may have been derived from the lower mantle of the IIIAB parent asteroid.

The fall, recovery, orbit, and composition of the Tagish Lake meteorite: a new type of carbonaceous chondrite.

The preatmospheric mass of the Tagish Lake meteoroid was about 200,000 kilograms. Its calculated orbit indicates affinity to the Apollo asteroids with a semimajor axis in the middle of the asteroid belt, consistent with a linkage to low-albedo C, D, and P type asteroids. The mineralogy, oxygen isotope, and bulk chemical composition of recovered samples of the Tagish Lake meteorite are intermediate between CM and CI meteorites. These data suggest that the Tagish Lake meteorite may be one of the most primitive solar system materials yet studied.

Aqueous alteration of the Bali CV3 chondrite: evidence from mineralogy, mineral chemistry, and oxygen isotopic compositions.

A petrographic, geochemical, and oxygen isotopic study of the Bali CV3 carbonaceous chondrite revealed that the meteorite has undergone extensive deformation and aqueous alteration on its parent body. Deformation textures are common and include flattened chondrules, a well-developed foliation, and the presence of distinctive (100) planar defects in olivine. The occurrence of alteration products associated with the planar defects indicates that the deformation features formed prior to the episode of aqueous alteration. The secondary minerals produced during the alteration event include well-crystallized Mg-rich saponite, framboidal magnetite, and Ca-phosphates. The alteration products are not homogeneously distributed throughout the meteorite, but occur in regions adjacent to relatively unaltered material, such as veins of altered material following the foliation. The alteration assemblage formed under oxidizing conditions at relatively low temperatures (<100 degrees C). Altered regions in Bali have higher Na, Ca, and P contents than unaltered regions which suggests that the fluid phase carried significant dissolved solids. Oxygen isotopic compositions for unaltered regions in Bali fall within the field for other CV3 whole-rocks, however, the oxygen isotopic compositions of the heavily altered material lie in the region for the CM and CR chondrites. The heavy-isotope enrichment of the altered regions in Bali suggest alteration conditions similar to those for the petrographic type-2 carbonaceous chondrites.