^1,2Gregory A. Brennecka, ²Lars E. Borg, ³Stephen J. Romaniello, ^3,4Amanda K. Souders, ^1,2Quinn R. Shollenberger, ²Naomi E. Marks, ³Meenakshi Wadhwa
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.10.003]
¹Institut für Planetologie, Westfälische Wilhelms-Universität, Münster, Germany
²Nuclear & Chemical Sciences Division, Lawrence Livermore National Laboratory, 7000 East Avenue L-231, Livermore, CA 94550 USA
³School of Earth and Space Exploration, Arizona State University, PO Box 871404, Tempe, AZ 85287-1404 USA
⁴Department of Geosciences, Texas Tech University, Lubbock, TX 79409, USA
Copyright Elsevier

Although there is limited direct evidence for supernova input into the nascent Solar System, many models suggest it formed by the gravitational collapse of a molecular cloud that was triggered by a nearby supernova. Existing lines of evidence, mostly in the form of short-lived radionuclides present in the early Solar System, are potentially consistent with this hypothesis, but still allow for alternative explanations. Since the natural production of ¹²⁶Sn is thought to occur only in supernovae and this isotope has a short half-life (¹²⁶Sn→¹²⁶Te, t_1/2=235 ky), the discovery of extant ¹²⁶Sn would provide unequivocal proof of supernova input to the early Solar System. Previous attempts to quantify the initial abundance of ¹²⁶Sn by examining Sn-Te systematics in early solids have been hampered by difficulties in precisely measuring Te isotope ratios in these materials. Thus, here we describe a novel technique that uses hydride generation to dramatically increase the ionization efficiency of Te—an approximately 30-fold increase over previous work. This introduction system, when coupled to a MC-ICPMS, enables high-precision Te isotopic analyses on samples with 126Sn. This sample set shows no evidence of live ¹²⁶Sn, implying at most minor input of supernova material during the time at which the CAIs formed. However, based on the petrology of this sample set combined with the higher than expected concentrations of Sn and Te, as well as the lack of nucleosynthetic anomalies in other isotopes of Te suggest that the bulk of the Sn and Te recovered from these particular refractory inclusions is not of primary origin and thus does not represent a primary signature of Sn-Te systematics of the protosolar nebula during condensation of CAIs or their precursors. Although no evidence of supernova input was found based on Sn-Te systematics in this sample set, hydride generation represents a powerful tool that can now be used to further explore Te isotope systematics in less altered materials.

^1,2Changkun Park, ¹Kazuhide Nagashima, ¹Alexander N. Krot, ¹Gary R. Huss, ³Andrew M. Davis, ⁴Martin Bizzarro
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.10.002]
¹Hawai‘i Institute of Geophysics and Planetology, School of Ocean, Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
²Division of Earth-System Sciences, Korea Polar Research Institute, Incheon 21990, South Korea
³Department of the Geophysical Sciences, Enrico Fermi Institute, and Chicago Center for Cosmochemistry, University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637-1433, USA
⁴Centre for Star and Planet Formation, Geological Museum, University of Copenhagen, Øster Voldgade 5-7, DK-1350, Denmark
Copyright Elsevier

Calcium-aluminum-rich inclusions with isotopic mass fractionation effects and unidentified nuclear isotopic anomalies (FUN CAIs) have been studied for more than 40 years, but their origins remain enigmatic. Here we report in situ high precision measurements of aluminum-magnesium isotope systematics of FUN CAIs by secondary ion mass spectrometry (SIMS). Individual minerals were analyzed in six FUN CAIs from the oxidized CV3 carbonaceous chondrites Axtell (compact Type A CAI Axtell 2271) and Allende (Type B CAIs C1 and EK1-4-1, and forsterite-bearing Type B CAIs BG82DH8, CG-14, and TE). Most of these CAIs show evidence for excess 26Mg due to the decay of 26Al. The inferred initial 26Al/27Al ratios [(26Al/27Al)0] and the initial magnesium isotopic compositions (δ26Mg0) calculated using an exponential law with an exponent β of 0.5128 are (3.1±1.6)×10−6 and 0.60±0.10‰ (Axtell 2271), (3.7±1.5)×10−6 and −0.20±0.05‰ (BG82DH8), (2.2±1.1)×10−6 and −0.18±0.05‰ (C1), (2.3±2.4)×10−5 and −2.23±0.37‰ (EK1-4-1), (1.5±1.1)×10−5 and −0.42±0.18‰ (CG-14), and (5.3±0.9)×10−5 and −0.05±0.08‰ (TE) with 2σ uncertainties. We infer that FUN CAIs recorded 26Al and magnesium isotopic heterogeneity in the CAI-forming region(s). Comparison of 26Al-26Mg systematics, stable isotope (oxygen, magnesium, calcium, and titanium) and trace element studies of FUN and non-FUN igneous CAIs indicates that there is a continuum among these CAI types. Based on these observations and evaporation experiments on CAI-like melts, we propose a generic scenario for the origin of igneous (FUN and non-FUN) CAIs: (i) condensation of isotopically normal solids in an 16O-rich gas of approximately solar composition; (ii) formation of CAI precursors by aggregation of these solids together with variable abundances of isotopically anomalous grains—possible carriers of unidentified nuclear (UN) effects; and (iii) melt evaporation of these precursors accompanied by crystallization under different temperatures and gas pressures, leading to the observed variations in mass-dependent isotopic fractionation (F) effects.