ALBERT

Description: Studies of individual presolar SiC grains have shown that most are enriched in Si-29, Si-30, and C-13, and depleted in N-15, compared to solar-system abundances and that many have large excesses of Mg-26, most plausibly from in situ decay of Al-26. Stone et al., observed that Si from a family of platy SiC grains define a linear array on a 3-isotope plot that does not pass through normal solar-system Si. In contrast, Si-isotope data from over 100 3-4 micron SiC grains from Murchison from an elongate ellipse enclosing the Stone et al. linear array but also including 'normal' solar-system Si. To investigate whether this difference in Si isotopes indicates different populations of SiC in the two meteorites and to improve the characterization of Orgueil SiC, we used the PANURGE ion microprobe to measure Si, C, N, and Mg isotopes and Al and Na concentrations in a suite of 2-5 micron SiC grains from a new sample of Orgueil.

Description: The discovery in chondritic meteorites of diamond, SiC, and poorly crystallized graphite that formed around other stars demonstrated conclusively that presolar dust survived the formation of the solar system to be incorporated into meteorites. The presolar nature of these grains is shown by the highly unusual isotopic compositions of their constituent elements. To date, all recognized types of presolar grains have been carbon rich and apparently formed around carbon stars, those with C/O greater than 1. The discovery of the first oxygen-rich grain with isotopic characteristics consistent with a presolar origin is reported. Oxygen-rich grains presumably form only around stars with C/O less than 1.

Description: We report on the isotopic composition of Pb and the concentration of Pb, U, and Th in a sample of LEW88516 (LEW). LEW was a 13-g stone recovered from Antarctica and was classified as a new member of the Shergottite group. This work was undertaken with the hope that LEW might yield new information to elucidate further the origin and evolution of Shergottites and other SNC meteorites. We have previously studied U-Th-Pb in other Shergottites, namely Shergotty, Zagami, EETA79001, and ALHA77005, as well as Nakhla. The results indicate that the initial leads of these Shergottites were well defined, were distinct from each other, and have high Pb-204/Pb-206 (from 0.0652 to 0.0739). These leads evolved in different reservoirs over most of solar system history in a low U-238/Pb-204(micro) is approximately equal to 5 environment. The U-Th-Pb isotopic systems are quite regular, which unambiguously indicates an event of U-Th-Pb fractionation at approximately 200 m.y. The details of the data arrays are complex. The young age is in general agreement with some of the ages obtained by other methods, but precise concordance between the different methods is not established. The new results on LEW are remarkably similar to those of ALHA77005 and support the other observations based on the mineralogy, petrology, and bulk composition. The clear distinction between the Shergottites and Nakhla is confirmed. We consider that the Shergottites and possibly all the SNC's were derived from an impact on the regolith of a differentiated terrestrial type planetary body (Mars?) with a high content of volatiles as compared to the earth.

Description: Chemical equilibria in stellar atmospheres have been investigated by many authors. Lattimer, Schramm, and Grossman presented calculations in both O rich and C rich environments and predicted possible presolar condensates. A recent paper by Cherchneff and Barker considered a C rich composition with PAH's included in the calculations. However, the condensation sequences of C bearing species have not been investigated in detail. In a carbon rich gas surrounding an AGB star, it is often assumed that graphite (or diamond) condenses out before TiC and SiC. However, Lattimer et al. found some conditions under which TiC condenses before graphite. We have performed molecular equilibrium calculations to establish the stability fields of C(s), TiC(s), and SiC(s) and other high temperature phases under conditions of different pressures and C/O. The preserved presolar interstellar dust grains so far discovered in meteorites are graphite, diamond, SiC, TiC, and possibly Al2O3.

Description: We present new analyses of Th-232/U-238 in CI and CM meteorites. The relative abundance of these nuclides is important in estimates of the age of r-process elements. The cosmochronology based upon the Th-232/U-238 ratio (kappa) depends on the precise determinations of these two different elements in meteorites and on the production ratios. Both parameters are subject to substantial errors. Recent recalculations of this chronology have used selected values from compilations but do not adequately address the errors in terms of a reliable data base. Morgan and Lovering provided extensive neutron activation analyses for ordinary chondrites which yield an average kappa of 3.6 +/- 0.4. Their work on carbonaceous chondrites showed a wide range in kappa from 2 to 6. More recent investigations by isotopic dilution have established the following: (1) highly variable kappa from 2.7 to 11 in Allende Ca-Al-rich inclusions and a value of 3.6 in the Orgueil CI1 chondrite; (2) a range from 2.71 to 6.63 for 7 L-type chondrites and a range from 2.7 to 4.4 for 6 L, H, and LL chondrites. A further investigation of this subject matter is presented.

Description: We are investigating the Re-Os isotope systematics of two groups of magmatic iron meteorites (2A, 4A) in an attempt to establish precise 'total rock' isochrons by the Re-Os system. The Re-187/Os-187 isotope system is recognized as a method by which the ages of iron meteorites can be directly determined and that can provide information on the timing of FeNi segregation and core formation in planetesimals. The Re-Os isotope system permits the direct absolute dating of the metal phase in iron meteorites. Indirect dating of iron meteorites has been achieved in the past through the Rb-Sr, K-Ar, and most recently, Sm-Nd for silicate inclusions, where present. Relative dating has been obtained directly by extensive studies of the short-lived system Pd-107/Ag-107 for the metal and sulfide phases and indirectly using I-129/Xe-129 in silicate and sulfide inclusions.

Description: The timing of events leading to the formation of silicate-rich and metal-rich regions in planetesimals remains an important problem in the study of planetary formation and differentiation in the early solar system. The IAB irons are especially important as they are considered to represent a magmatic differentiation series. Iron meteorites present a particular challenge for chronological studies, due to the relative paucity of phases serving as hosts for radioactive parent-daughter nuclides. Recent work using the Re-Os system, following on the pioneering work by Herr et al. and Luck and Allegre, appears promising, but investigators up to now have concentrated on whole rock isochrons. Silicate clasts enclosed within iron meteorites can provide information about the chronology and thermal history of irons. Extensive work on Rb-Sr, K-Ar, and I-Xe has been reported on silicate inclusions in iron meteorites. We report the initial results from our Sm-Nd study of an inclusion with the Caddo IAB iron, the first Sm-Nd isotopic study of a silicate clast embedded within an iron meteorite. Our results include measurements of the standard long-lived Sm-147/Nd-143 (tau = 152 AE) system, as well as the shorter-lived SM-146/Nd-142 (tau = 0.149 AE) system, which has been shown to be very useful in deciphering the history of the early solar system. The Caddo silicate clast was described by Palme et al., who kindly provided us with a major part of the inclusion. The inclusion is coarse-grained consisting predominantly of olivine, clinopyroxene, and plagioclase, with lesser amounts of orthopyroxene, Fe-Ni metal, sulfide, and phosphate. The relatively large grain size (up to 3 mm) and 120 degree grain boundaries suggest extensive metamorphism at high temperatures. Based on study of a thin section, there is evidence for metal invading along grain boundaries in some regions of the inclusion, suggesting that the Fe-Ni metal was molten when the silicate clast was incorporated. Metamorphic recrystallization may have occurred during this event.

Description: We have found a approximately 3 micrometer Al2O3 grain (B39) in the Bishunpur LL3.1 chondrite that is enriched in 0-17 by a factor of approximately 6.8 (0-16/0-17 = 385 +/- 9) and depleted in 0-18 by almost 60% (0-16/0-18 = 853 +/- 30) relative to solar system oxygen and has an initial Al-26/Al-27 = 1.7 +/- 0.2 x 10 (exp 3), approximately 34 times greater than the initial solar system value. The isotopic compositions of B39 and two other Al2O3 grains previously reported from the Orgueil and Murchison meteorites show that these grains formed directly from the ejecta of low-mass AGB stars with C/O less than 1. A simple theoretical analysis is presented showing that the oxygen systematics of the grains are a natural consequence of main-sequence evolution followed by first dredge-up Al-26 is the result of third dredge-up. Circumstellar Al2O3 grains provide very precise isotopic data for stellar ejecta that complement spectroscopic observations of oxygen-rich stars. Isotopic differences indicate that the Al2O3 grains come from separate stars of different mass and intital oxygen composition that originated in molecular clouds different from the one in which the solar system formed.

Description: We carried out a theoretical evaluation of the contribution of Asymptotic Giant Branch (AGB) stars to some short-lived (10(exp 6) less than or equal to Tau-bar less than or equal to 2 x 10(exp 7) yr) isotopes in the Interstellar Medium (ISM) and in the early solar system using stellar model calculations for thermally pulsing evolutionary phases of low-mass stars. The yields of s-process nuclei in the convective He-shell for different neutron exposures tau(sub 0) were obtained, and AGB stars were shown to produce several radioactive nuclei (especially Pd-107, Pb-205, Fe-60, Zr-93, Tc-99, Cs-135, and Hf-182) in diferent amounts. Assuming either contamination of the solar nebula from a single AGB star or models for continuous injection and mixing from many stars into the ISM, we calculate the ratios of radioactive to stable nuclei at the epoch of the Sun's formation. The dilution factor between the AGB ejecta and the early solar system matter is obtained by matching the observed Pd-107/Pd-108 and depends on the value of tau(sub 0). It is found that small masses M(sub He) of He-shell material (10(exp -4)-10(exp -7) solar mass) enriched in s-process nuclei are sufficient to contaminate 1 solar mass of the ISM to produce the Pd-107 found in the early solar system. Predictions are made for all of the other radioactive isotopes. The optimal model to explain several observed radioactive species at different states of the proto-solar nebula involves a single AGB star with a low neutron exposure (tau(sub 0) = 0.03 mbarn(sup -1)) which contaminated the cloud with a dilution factor of M(sub He)/solar mass approximately 1.5 x 10(exp -4). This will also contribute newly synthesized stable s-process nuclei in the amount of approximately 10(exp -4) of their abundances already present in the proto-solar cloud. Variations in the degree of homogenization (approximately 30%) of the injected material may account for some of the small general isotopic anomalies found in meteorites. It is also found that Fe-60 is produced in small but significant quantities that may be sufficient to explain the observations if the time elapsed delta from the contamination of the ISM to the formation of protoplanetary bodies is not higher than delta = 5 x 10(exp 6) yr. If delta is longer, up to 10 x 10(exp 6) yr, this would require the single AGB star to experience enhanced neutron densities (n(sub n) approximately 3 x 10(exp 9)n/cu cm) in the s-processing zone in order to compensate for the branching at Fe-59. The alternative model of long-term continuous ejection of matter from many AGB stars does not appear to match the observations. We also estimate the Al-26 production from the H-shell and find that the Al-26 abundance in the early solar system may be readily explained in a self-consistent manner. Moreover, Al-26 from AGB stars may contribute substantially to the galactic Al-26 gamma-source, while no significant gamma-flux from Co-60 (deriving from Fe-60 decay) is to be expected.