Multiple sources or late injection of short-lived r-nuclides in the early solar system?

Comparisons between the predicted abundances of short-lived r-nuclides (107Pd, 129I, 182Hf, and 244Pu) in the interstellar medium (ISM) and the observed abundances in the early solar system (ESS) conclusively showed that these nuclides cannot simply be derived from galactic chemical evolution (GCE) if synthesized in a unique stellar environment. It was thus suggested that two di erent types of stars were responsible for the production of light and heavy r-nuclides. Here, new constraints on the 244Pu=238U production ratio are used in an open nonlinear GCE model. It is shown that the two r-process scenario cannot explain the low abundance of 244Pu in the ESS and that this requires either than actinides be produced at an additional site (A-events) or more likely, that 129I and 244Pu be inherited from GCE and 107Pd and 182Hf be injected in the ESS by the explosion of a nearby supernova.Comment: 4 pages, 1 figure, Nucl. Phys. A, in press (proceedings of NIC8

Thulium anomalies and rare earth element patterns in meteorites and Earth: Nebular fractionation and the nugget effect

This study reports the bulk rare earth element (REEs, La-Lu) compositions of 41 chondrites, including 32 falls and 9 finds from carbonaceous (CI, CM, CO and CV), enstatite (EH and EL) and ordinary (H, L and LL) groups, as well as 2 enstatite achondrites (aubrite). The CI-chondrite-normalized REE patterns and Eu anomalies in ordinary and enstatite chondrites show more scatter in more metamorphosed than in unequilibrated chondrites. This is due to parent-body redistribution of the REEs in various carrier phases during metamorphism. The dispersion in REE patterns of equilibrated ordinary chondrites is explained by the nugget effect associated with concentration of REEs in minor phosphate grains. Terrestrial rocks and samples from ordinary and enstatite chondrites display negative Tm anomalies of ~-4.5 % relative to ca chondrites. In contrast, CM, CO and CV (except Allende) show no significant Tm anomalies. Allende CV chondrite shows large excess Tm (~+10 %). These anomalies are similar to those found in group II refractory inclusions in meteorites but of much smaller magnitude. The presence of Tm anomalies in meteorites and terrestrial rocks suggests that either (i) the material in the inner part of the solar system was formed from a gas reservoir that had been depleted in refractory dust and carried positive Tm anomalies or (ii) CI chondrites are enriched in refractory dust and are not representative of solar composition for refractory elements. The observed Tm anomalies in ordinary and enstatite chondrites and terrestrial rocks, relative to carbonaceous chondrites, indicate that material akin to carbonaceous chondrites must have represented a small fraction of the constituents of the Earth.Comment: Geochimica et Cosmochimica Acta, in press, 58 pages, 6 tables, 14 figure

Radiogenic p-isotopes from type Ia supernova, nuclear physics uncertainties, and galactic chemical evolution compared with values in primitive meteorites

The nucleosynthesis of proton-rich isotopes is calculated for multi-dimensional Chandrasekhar-mass models of Type Ia supernovae (SNe Ia) with different metallicities. The predicted abundances of the short-lived radioactive isotopes 92Nb, 97, 98Tc, and 146Sm are given in this framework. The abundance seeds are obtained by calculating s-process nucleosynthesis in the material accreted onto a carbon-oxygen white dwarf from a binary companion. A fine grid of s-seeds at different metallicities and 13C-pocket efficiencies is considered. A galactic chemical evolution model is used to predict the contribution of SN Ia to the solar system p-nuclei composition measured in meteorites. Nuclear physics uncertainties are critical to determine the role of SNe Ia in the production of 92Nb and 146Sm. We find that, if standard Chandrasekhar-mass SNe Ia are at least 50% of all SN Ia, they are strong candidates for reproducing the radiogenic p-process signature observed in meteorites.Peer reviewedFinal Accepted Versio

Strontium Stable Isotope Composition of Allende Fine-Grained Inclusions

Isotopic anomalies are departures from the laws of mass-dependent fractionation that cannot be explained by radioactive decay, cosmogenic effects, or exotic isotopic fractionation processes such as nuclear field shift or magnetic effects [1 and references therein]. These anomalies often have a nucleosynthetic origin and provide clues on the stellar origin and solar system processing of presolar dust. Anomalies are most often found in refractory elements of relatively low mass, so Sr is a prime target for study. The four stable isotopes of strontium are useful for discerning the various nucleosynthetic origins of early solar system building blocks and the timing of accretion processes. Strontium-84 is the least abundant (0.56%) of these isotopes, but is particularly significant in being a p-process only nuclide that is produced in core-collapse or type Ia supernovae [2,3]. The more abundant isotopes ^(86)Sr (9.86%), ^(87)Sr (7.00%) and ^(88)Sr (82.58%) are produced in s- and r-processes in asymptotic giant branch stars and other stellar types [4]. Additionally, ^(87)Sr is produced by ^(87)Rb decay in proportions that dominate over possible nucleosynthetic variations but provide timings of early solar system processes, most notably volatile element depletion [5-7]. Furthermore, variations in strontium isotopic ratios caused by high-temperature massdependent fractionation [8] are also important [9-12], as they provide insights into nebular and accretionary processes

Nucleosynthetic osmium isotope anomalies in acid leachates of the Murchison meteorite

We present osmium isotopic results obtained by sequential leaching of the Murchison meteorite, which reveal the existence of very large internal anomalies of nucleosynthetic origin. The Os isotopic anomalies are correlated, and can be explained by the variable contributions of components derived from the s, r and p-processes of nucleosynthesis. Much of the s-process rich osmium is released by relatively mild leaching, suggesting the existence of an easily leachable s-process rich presolar phase, or alternatively, of a chemically resistant r-process rich phase. The s-process composition of Os released by mild leaching diverges slightly from that released by aggressive digestion techniques, perhaps suggesting that the presolar phases attacked by these differing procedures condensed in different stellar environments. The correlation between 190Os and 188Os can be used to constrain the s-process 190Os/188Os ratio to be 1.275 pm 0.043. Such a ratio can be reproduced in a nuclear reaction network for a MACS value for 190Os of ~200 pm 22 mbarn at 30 keV. We also present evidence for extensive internal variation of 184Os abundances in the Murchison meteorite. This suggests that p process rich presolar grains (e.g., supernova condensates) may be present in meteorites in sufficient quantities to influence the Os isotopic compositions of the leachates.Comment: 40 pages, 9 figures, 2 tables. Accepted for publication in Earth and Planetary Science Letter

Evidence from Ab Initio and Transport Modeling for Diffusion-Driven Zirconium Isotopic Fractionation in Igneous Rocks

We use density functional theory to calculate the equilibrium isotopic fractionation factors of zirconium (Zr) in a variety of minerals including zircon, baddeleyite, Ca-catapleiite, ilmenite, geikielite, magnetite, apatite, K-feldspar, quartz, olivine, clinopyroxene, orthopyroxene, amphibole, and garnet. We also report equilibrium isotopic fractionation factors for Hf in zircons, Ca-catapleiite, and ilmenite. These calculations show that coordination environment is an important control on Zr and Hf isotopic fractionation, with minerals with Zr and Hf in low coordinations predicted to be enriched in the heavy isotopes of Zr and Hf, relative to those with Zr and Hf in high coordinations. At equilibrium, zircon, which hosts Zr and Hf in 8-fold coordination, is predicted to have low 94Zr/90Zr and 179Hf/177Hf ratios compared to silicate melt, which hosts Zr and Hf in 6-fold coordination. However, our modeling results indicate that little equilibrium isotopic fractionation for Zr is expected during magmatic differentiation and zircon crystallization. We show through isotopic transport modeling that the Zr isotopic variations that were documented in igneous rocks are likely due to diffusion-driven kinetic isotopic fractionation. The two settings where this could take place are (i) diffusion-limited crystallization of zircon (DLC model) and (ii) diffusion-triggered crystallization of zircon (DTC model) in the boundary layer created by the growth of Zr-poor minerals. Fractional crystallization of zircons enriched in light Zr isotopes by diffusion can drive residual magmas toward heavy Zr isotopic compositions. Our diffusive transport model gives the framework to interpret Zr isotope data and gain new insights into the cooling history of igneous rocks and the setting of zircon crystallization

I-Xe studies of aqueous alteration in the Allende CAI Curious Marie

The Allende fine-grained inclusion Curious Marie is a unique CAI. It is depleted in uranium but contains large ^(235)U excess [1], providing new evidence that ^(247)Cm was alive in the Early Solar System, as has been previously suggested [2], and leading to an updated (^(247)Cm/^(235)U)initial ratio of (1.1±0.3)×10^(-4)