We have analyzed molybdenum, ruthenium, and barium isotopes simultaneously in 21 individual high-density presolar graphite grains from the Murchison CM2 meteorite using the Chicago Instrument for Laser Ionization (CHILI). While all grains clearly suffered from contamination with molybdenum of solar composition, probably from the parent asteroid of the host meteorite, six grains showed s-process enrichments in at least one of the elements analyzed, pointing toward an AGB star origin of these grains. For one of the grains, we found almost pure s-process ruthenium, challenging existing AGB star models that assume full homogenization of freshly produced s-process material in the convective AGB star envelope prior to grain condensation. Furthermore, our results indicate that these grains condensed before about half of the 99Tc decayed to 99Ru and that technetium did not condense into the grains.

We have analyzed molybdenum, ruthenium, and barium isotopes simultaneously in 55 individual presolar silicon carbide (SiC) grains from the Murchison CM2 meteorite using the Chicago Instrument for Laser Ionization (or CHILI). Most grains show clear s -process signatures, which are strongly correlated for molybdenum and ruthenium. For all three elements, we provide estimates for s -process contributions from low-mass AGB stars with unprecedented precision. Variations in s -process production observed for some nuclides reflect a strong dependence on physical properties, neutron density, temperature, and timing, affecting various s -process branch points. Significant contamination can be excluded for a majority of grains. Instead, distributions along mixing lines in three-isotope diagrams reflect mixing between initial parent star material and matter synthesized in the star. The results suggest that the ratios between p - and r -process isotopes of molybdenum, ruthenium, and barium in presolar SiC from many parent stars are the same as the ones inferred for the solar system. This indicates that the products of these processes were well mixed by the time the molecular cloud collapsed to form the stars that eventually grew the SiC grains, and that this mixture did not change between formation of the precursor stars and formation of the Sun.

Correlated errors of experimental data are a common but often neglected problem in physical sciences. Various tools are provided here for thorough propagation of uncertainties in cases of correlated errors. Discussed are techniques especially applicable to three-isotope plots common in geo-and cosmochemistry, where common denominators in isotope ratios, as well as instrumental effects, such as mass-dependent isotope fractionation, could lead to significant correlation of errors. Furthermore, various techniques for calculating linear regressions are compared to each other, showing that the method suggested by Mahon (1996) [1] gives the best results after correcting for some typographical errors therein and eliminating a minor mistake. We also provide a method for calculating linear regressions through a fixed point, avoiding previously made mistakes.

We report the Sr and Ba isotopic compositions of 18 presolar SiC grains of types Y (11) and Z (7), rare types commonly argued to have formed in lower-than-solar metallicity asymptotic giant branch (AGB) stars. We find that the Y and Z grains show higher Sr-88/Sr-87 and more variable Ba-138/Ba-136 ratios than mainstream (MS) grains. According to FRANEC Torino AGB models, the Si, Sr, and Ba isotopic compositions of our Y and Z grains can be consistently explained if the grains came from low-mass AGB stars with 0.15 Z(circle dot) <= Z < 1.00 Z(circle dot), in which the C-13 neutron exposure for the slow neutron-capture process is greatly reduced with respect to that required by MS grains for a 1.0 Z(circle dot) AGB star. This scenario is in line with the previous finding based on Ti isotopes, but it fails to explain the indistinguishable Mo isotopic compositions of MS, Y, and Z grains.

The provenance of GEMS (glass with embedded metal and sulfides) in cometary type interplanetary dust particles is investigated using analytical scanning transmission electron microscopy and secondary ion mass spectrometry. We review the current state of knowledge and closely examine the densities, elemental compositions and distributions, iron oxidation states, and isotopic compositions of a subset of GEMS in chondritic porous interplanetary dust. We find that GEMS are underdense with estimated densities that are 35-65% of compositionally equivalent crystalline aggregates. GEMS low densities result in a lower contribution to the bulk compositions of IDPs than has been assumed based on their volume fraction. We also find that element/Si ratios, assumed to be primary (indigenous), are instead perturbed by contamination and secondary alteration, including pulse heating during atmospheric entry. Fe in pyrrhotite inclusions was oxidized and Mg, S, Ca, and Fe were depleted relative to lithophile Al and Si, resulting in reduction in element/Si ratios. Because they trap outgassing elements, Fe-rich oxide rims that formed on the surface of GEMS are serendipitous "witness plates" to the changes in composition that accompany atmospheric entry. As a result of alteration, GEMS elemental compositions cannot reliably inform about their provenance. Except for highly anomalous oxygen isotope ratios measured in some large GEMS grains that indicate a contribution from circumstellar dust, oxygen isotope compositions are generally poor indicators of provenance. Prior work indicates that most GEMS fall close to the terrestrial oxygen isotope composition, which, however, does not exclude a presolar interstellar origin. Nitrogen isotopic compositions are more diagnostic. Elevated 15N/14N ratios indicate that GEMS accreted in conjunction with formation of organic matter by ion-molecule reactions in a cold (<50 K) presolar environment like the extreme outer nebula or interstellar medium. Considering all observations, we conclude that GEMS are most likely processed interstellar silicates. (c) 2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

N. Dauphas, A. M. Davis, J. M. Korsmeyer, M. J. Krawczynski, T. Stephan, Department of the Geophysical Sciences, The University of Chicago, Chicago, IL, USA, Chicago Center for Cosmochemistry, Enrico Fermi Institute, The University of Chicago, Chicago, IL, USA, Department of Chemistry, The University of Chicago, Chicago, IL, USA, Department of Earth and Planetary Sciences, Washington University in St. Louis, St. Louis, MO, USA. E-mail: regula@uchicago.edu.

CALCIUM–ALUMINUM-RICH INCLUSION FROM THE EFREMOVKA CV3 CHONDRITE. J. M. Korsmeyer1,2, T. Stephan2,3, A. M. Davis2,3,4, H. E. Bloom2,3, G. J. MacPherson5, and M. A. Ivanova6, 1Department of Chemistry, The University of Chicago, Chicago, IL, USA, 2Chicago Center for Cosmochemistry, 3Department of the Geophysical Sciences, The University of Chicago, Chicago, IL, USA, 4Enrico Fermi Institute, The University of Chicago, Chicago, IL, USA, 5Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC, USA, 6Vernadsky Institute, Moscow, Russia. E-mail: jkorsmeyer@uchicago.edu

GRAPHITE. H. E. Bloom, T. Stephan, A. M. Davis, P. R. Heck, P. Hoppe, J. M. Korsmeyer, and S. Amari, Department of the Geophysical Sciences, The University of Chicago, Chicago, IL, USA, Chicago Center for Cosmochemistry, Enrico Fermi Institute, The University of Chicago, Chicago, IL, USA, Robert A. Pritzker Center for Meteoritics and Polar Studies, Negaunee Integrative Research Center, Field Museum of Natural History, Chicago, IL, USA, Max Planck Institute for Chemistry, Mainz, Germany, Department of Chemistry, The University of Chicago, Chicago, IL, USA, McDonnell Center for the Space Sciences and Physics Department, Washington University in St. Louis, St. Louis, MO, USA. E-mail: bloomh@uchicago.edu.

GRAPHITE—s-, r-, AND p-PROCESSES AND THE ROLE OF CONTAMINATION. T. Stephan, H. E. Bloom, P. Hoppe, A. M. Davis, J. M. Korsmeyer, A. Regula, P. R. Heck, and S. Amari, Department of the Geophysical Sciences, The University of Chicago, Chicago, IL, USA, Chicago Center for Cosmochemistry, Max Planck Institute for Chemistry, Mainz, Germany, Enrico Fermi Institute, The University of Chicago, Chicago, IL, USA, Department of Chemistry, The University of Chicago, Chicago, IL, USA, Robert A. Pritzker Center for Meteoritics and Polar Studies, Negaunee Integrative Research Center, Field Museum of Natural History, Chicago, IL, USA, McDonnell Center for the Space Sciences and Physics Department, Washington University in St. Louis, St. Louis, MO, USA. E-mail: tstephan@uchicago.edu.

We report the Sr and Ba isotopic compositions of 18 presolar SiC grains of types Y (11) and Z (7), rare types commonly argued to have formed in lower-than-solar metallicity asymptotic giant branch (AGB) stars. We find that the Y and Z grains show higher 88Sr/87Sr and more variable 138Ba/136Ba ratios than mainstream (MS) grains. According to FRANEC Torino AGB models, the Si, Sr, and Ba isotopic compositions of our Y and Z grains can be consistently explained if the grains came from low-mass AGB stars with 0.15 Z⊙ ≤ Z < 1.00 Z⊙, in which the 13C neutron exposure for the slow neutron-capture process is greatly reduced with respect to that required by MS grains for a 1.0 Z⊙ AGB star. This scenario is in line with the previous finding based on Ti isotopes, but it fails to explain the indistinguishable Mo isotopic compositions of MS, Y, and Z grains.

Cluster analysis of presolar silicon carbide grains based on literature data for 12 C/ 13 C, 14 N/ 15 N, δ 30 Si/ 28 Si, and δ 29 Si/ 28 Si including or not inferred initial 26 Al/ 27 Al data, reveals nine clusters agreeing with previously defined grain types but also highlighting new divisions. Mainstream grains reside in three clusters probably representing different parent star metallicities. One of these clusters has a compact core, with a narrow range of composition, pointing to an enhanced production of SiC grains in asymptotic giant branch (AGB) stars with a narrow range of masses and metallicities. The addition of 26 Al/ 27 Al data highlights a cluster of mainstream grains, enriched in 15 N and 26 Al, which cannot be explained by current AGB models. We defined two AB grain clusters, one with 15 N and 26 Al excesses, and the other with 14 N and smaller 26 Al excesses, in agreement with recent studies. Their definition does not use the solar N isotopic ratio as a divider, and the contour of the 26 Al-rich AB cluster identified in this study is in better agreement with core-collapse supernova models. We also found a cluster with a mixture of putative nova and AB grains, which may have formed in supernova or nova environments. X grains make up two clusters, having either strongly correlated Si isotopic ratios or deviating from the 2/3 slope line in the Si 3-isotope plot. Finally, most Y and Z grains are jointly clustered, suggesting that the previous use of 12 C/ 13 C = 100 as a divider for Y grains was arbitrary. Our results show that cluster analysis is a powerful tool to interpret the data in light of stellar evolution and nucleosynthesis modeling and highlight the need of more multi-element isotopic data for better classification.

Amorphous silicates containing abundant nano-inclusions have been reported in the Paris CM chondrite (Leroux et al., 2015). They have chemical and morphological similarities to glass with embedded metal and sulfides (GEMS) found in interplanetary dust particles (IDPs) and micrometeorites believed to originate from comets. We used scanning transmission electron microscopy (STEM) with energy-dispersive X-ray spectroscopy (EDS) and nanodiffraction to study the chemistry and mineralogy of these inclusions in order to understand the origin of the GEMS-like material in Paris and its possible relationships to other materials found in primitive chondritic materials including IDP GEMS. EDS and diffraction analyses indicate compositional and mineralogical differences between the nanophase inclusions in cometary GEMS and Paris GEMS-like material. Metal inclusions are notably absent within Paris amorphous silicate. Ni-rich sulfides, including pentlandite, are common in even the least altered matrix material of Paris, while they are absent in GEMS-bearing IDPs and Ultracarbonaceous Antarctic Micrometeorites (UCAMMs). From examination of the inclusions, we cannot yet confirm or refute the possibility that GEMS-like material in Paris is related to cometary GEMS. The distinct compositions and mineralogy of the Paris material may be due to aqueous alteration of cometary GEMS precursors, but they may also denote an independent origin for meteoritic GEMS-like assemblages.

Molybdenum isotopes measured in most individual presolar silicon carbide grains are dominated by s -process contributions from the helium intershells of asymptotic giant branch (AGB) stars. The much smaller isotopic variations in molybdenum in meteorites and their components are largely controlled by s -process enrichments or depletions relative to terrestrial composition but lie along two parallel s -process mixing lines separated by what has been suggested to be an r -process contribution. The two mixing lines are populated by carbonaceous-chondrite- and noncarbonaceous-chondrite-related meteorites (CC and NC groups, respectively). We have compared molybdenum isotopic data for presolar grains with those from meteorites and renormalized the meteorite data in a way that is consistent with s -, r -, and p -process contributions observed in presolar SiC grains. The results indicate that (1) there seems to be a fixed ratio between p - and r -process contributions in all data, (2) the dichotomy in molybdenum isotopes between the CC and NC groups can be explained by variations in the isotope makeup of the s -process contribution to the meteoritic samples, (3) this variability is similar to the variations in s -process molybdenum from different AGB stars deduced from presolar grain analyses, and (4) the larger range of isotopic compositions found in refractory inclusions is also consistent with s -process isotope variability.