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Calcium-aluminium-rich inclusions (CAIs) and chondrules are the oldest dated components of chondrites. They record the events and processes during the formation and early evolution of the solar system in their morphology, mineral phases and isotopic compositions. 26Al-26Mg isotopic systematics of two CAIs from Leoville (CV3.1-3.4), two chondrules from Queen Alexandra range 99177 (CR3.00), one mega chondrule from Semarkona (LL3.00), and a plagioclase rich chondrule from Chainpur (LL3.4) were carried out to understand the formation processes of these rare kinds of objects and to obtain constraints on early solar system events and processes. Petrographic and isotopic properties of Leoville CAI 1 (Type B2) suggest its formation c. 0.1 Ma after typical non-igneous CAIs characterized by the canonical ratio of 26Al/27Al = 5.25 × 10-5, from a partial melt heated to a maximum temperature of ~1420°C and cooled slowly at =0.5°C/h. Leoville CAI 3 (Type A) plausibly formed early within 1 Ma of the canonical CAI value and subsequently experienced parent body aqueous alteration. The analysed chondrules did not yield significant 26Mg excess due to their small Al/Mg ratio or resetting by secondary processes. [ABSTRACT FROM AUTHOR]

The sputtering rate of presolar silicon carbide grains due to galactic cosmic rays has been computed for their experimentally deduced lifetimes (∼1 Gyr) in the interstellar medium. An ion target simulator, SDTrimSP, was used to model the sputtering of interstellar grains with varying sizes and thicknesses of the ice mantle formed around the grain during their journey through the interstellar medium. Temperature, composition, and density for four different types of molecular cloud environments (quiescent, low-mass young stellar objects (YSOs), intermediate-mass YSOs, and high-mass YSO weak processing) considered indicate the sputtering rate on the mantle ice composition depends on water composition to a certain extent. The model simulations indicate galactic cosmic ray(s) with an energy range from 10 MeV to 1 GeV are just capable of sputtering/destructing ∼13%–15% of the grain itself. This value, stretched over 1 Gyr is not as significant as the other destruction processes and therefore can be classified as a minor destruction process. The effect of galactic cosmic rays on the ice mantle and core is also noted with particular emphasis on amorphization/recoils generated inside the SiC core and their distribution within the grain. [ABSTRACT FROM AUTHOR]

60 Fe– 60 Ni relative chronometry ( t 1 / 2 = 2.62 Ma ) studies within silicate phases of two chondrules (Ch1b, Ch5) from the Queen Alexandra Range (QUE) 97008 (L3.05) meteorite yield 60 Fe/ 56 Fe abundances of ( 6.3 ± 4.0 ) × 10 − 7 and ( 6.6 ± 2.5 ) × 10 − 7 for QUE 97008 Ch1b and Ch 5, respectively at the time of their formation. The 26 Al– 26 Mg isotopic studies carried out in these chondrules in a previous independent study allow to infer Solar system initial 60 Fe/ 56 Fe abundances of ( 8.3 ± 7.3 ) × 10 − 7 and ( 11.2 ± 7.1 ) × 10 − 7 consistent with the value of ∼ 7 × 10 − 7 inferred previously from in situ studies carried out using secondary ion mass spectrometry. The high abundance of 60 Fe inferred from the present study carried out using NanoSIMS is not in agreement with some bulk and mineral separate studies carried out using MC-ICPMS. The paper discusses possible potential reasons for the observed difference in abundances. It is found that none of them can fully reconcile the results from the two analytics. However, the present study extends the limited data set of in situ, multi-isotope systematics within chondrules to another slightly more altered meteorite of another type (low iron ordinary chondrite L type) and suggests existence of discernable fossil records of 60 Fe in meteorites other than Semarkona (LL3.00). [ABSTRACT FROM AUTHOR]

We report the structural and chemical investigation of nine presolar silicate grains from the CH3/CBb3 chondrite Isheyevo and CR2 chondrite Northwest Africa (NWA) 801. Five of these grains belong to group 1, likely condensed in low- to intermediate-mass asymptotic giant branch (AGB) stars, super-AGB stars, or core-collapse supernovae, while the remaining four grains belong to group 4 and have a supernova origin. The advanced transmission electron microscopy and associated electron spectroscopy analyses show a diverse range of chemical and structural compositions for presolar silicates. Two GEMS (glass with embedded metal and sulfide)-like silicates, each from different groups, condensed under nonequilibrium conditions in stellar outflows. Two nonstoichiometric silicates from group 1 have dissimilar formation and alteration histories. An amorphous silicate from group 1 with olivine-like [(Mg,Fe)2SiO4] composition likely formed as a crystalline olivine that subsequently amorphized in the interstellar medium. An oldhamite (CaS) grain within a stoichiometric enstatite (MgSiO3) from group 1 probably formed by heterogeneous condensation in circumstellar outflows. Of the two crystalline grains from group 4, one is an antigorite [(Mg,Fe)3Si2O5(OH)4], while the other is a nontronite [Na,Fe2(Si,Al)4O10(OH)2.nH2O], both formed as a crystalline forsterite and later altered to have hydrated silicate composition. A group-4 silicate has a chemical composition similar to a low Ca-pyroxene [(Ca,Mg)(Si,Al)2O6]. Our data imply that presolar grains from different groups can have a similar range of grain-formation conditions. [ABSTRACT FROM AUTHOR]

We report correlated Si, and Ti isotopic compositions and elemental concentrations of 238 presolar SiC grains from the Murchison CM2 meteorite. Combined with measurements of the C and N isotopic compositions of these 238 grains, 220 were determined to be of type mainstream, 10 type AB, 4 type Y and 4 type Z. SiC grains of diameter ≳2.5 µm, to ensure enough material to attempt Ti measurements, were randomly chosen without any other prejudice. The Ti isotopic compositions of the majority of the grains are characterized by enrichments in 46 Ti, 47 Ti, 49 Ti, and 50 Ti relative to 48 Ti, and show linear isotopic correlations indicative of galactic chemical evolution and neutron capture of the grains parent stars. The variability in the observed Ti signal as a function of depth in most of the grains indicates the presence of distinct subgrains, likely TiC that have been previously observed in TEM studies. Vandium-51 concentrations correlate with those of Ti, indicating V substitutes for Ti in the TiC matrix in many of the grains. No isotopic anomalies in 52 Cr/ 53 Cr ratios were observed, and Cr concentrations did not correlate with those of either Ti or V. [ABSTRACT FROM AUTHOR]

We report C, N, O, Si, Al–Mg, K, Ca, and Ti isotopic analyses of presolar graphite grains from the Orgueil CI chondrite. NanoSIMS isotopic measurements were made on 345 grains from seven density fractions, with grain sizes >1 μm: low-density grains from OR1b, OR1c, and OR1d; and high-density grains from OR1f, OR1g, OR1h, and OR1i. In all fractions, except OR1b and OR1h, we found presolar graphite as demonstrated by the large range of 12 C/ 13 C ratios (4–2480) measured in individual grains. Some isotopic properties are dependent on density: low-density grains contain 18 O, 15 N, and 28 Si excesses, while the majority of high-density grains contain normal N and O, and are generally enriched in 29 Si and 30 Si. The 15 N, 18 O, and 28 Si excesses and very high derived isotopic ratios for the extinct radionuclides 26 Al, 41 Ca, and 44 Ti in low-density grains indicate an origin from supernovae. In order to explain the isotopic ratios measured in these grains, we present mixing scenarios between different layers of supernovae and discuss the limitations of various theoretical models. Silicon-30 and 12 C excesses in high-density grains and lower values for short-lived radionuclides ( 26 Al and 41 Ca) indicate an origin in asymptotic giant branch stars with low metallicities. Some supernova grains, with 44 Ca excesses, are also present amongst the high-density grains. Grains with low 12 C/ 13 C ratios (without evidence for 44 Ti) and large excesses in 42,43 Ca and 46,47,49,50 Ti probably originate from post-asymptotic giant branch stars, that have suffered a very late thermal pulse, and can achieve low 12 C/ 13 C ratios and large neutron capture signatures in Ca and Ti isotopes. We conclude that most low-density graphite grains originate from supernovae while high-density graphite grains have multiple stellar sources: low-metallicity and born-again asymptotic giant branch stars, Type II supernovae, and possibly, J-type stars. [ABSTRACT FROM AUTHOR]

Abstract: The magnesium isotopic compositions of 26 hibonite-bearing inclusions from the CM chondrite Murchison, as well as isotopic measurements on a subset of these samples for oxygen, titanium, and lithium–beryllium–boron are reported along with oxygen isotopic data for an additional 13 hibonites that were previously investigated for other isotope systems (magnesium, potassium, calcium, and titanium) and rare earth element concentrations. Magnesium isotopic compositions divide CM hibonites into two distinct populations which correlate perfectly with their mineralogy and morphology, as previously discovered by Ireland [Ireland T. R. (1988) Correlated morphological, chemical, and isotopic characteristics of hibonites from the Murchison carbonaceous chondrite. Geochim. Cosmochim. Acta 52, 2827–2839]: Spinel-HIBonite spherules (SHIBs) bear evidence of in situ 26Al decay, whereas PLAty-Crystals (PLACs) and Blue AGgregates (BAGs) either lack resolvable 26Mg-excesses or exhibit 26Mg deficits by up to ∼4‰. High precision, multiple collector SIMS analyses show that 6 of 7 SHIBs investigated fall on a single correlation line implying 26Al/27Al = (4.5 ± 0.2) × 10−5 at the time of isotopic closure, consistent with the “canonical”26Al abundance characteristic of internal isochrons in many calcium–aluminum-rich inclusions (CAIs). One SHIB sample exhibits Δ26Mg∗ consistent with a “supracanonical”26Al/27Al ratio of (6.4 ± 0.5) × 10−5. The PLAC hibonites contain highly anomalous titanium isotopic compositions, with δ50Ti values ranging from −80‰ to almost +200‰, whereas SHIBs generally lack large Ti isotopic anomalies. Eight out of 11 26Al-free PLAC hibonite grains record 10B/11B excesses that correlate with Be/B; the inferred initial 10Be/9Be ratio of (5.1 ± 1.4) × 10−4 is lower than the best-constrained 10Be/9Be of (8.8 ± 0.6) × 10−4 in a CV CAI. The data demonstrate that 10Be cannot be used as a relative chronometer for these objects and that most of the 10Be observed in CAIs must be produced by irradiation of precursor solids in the early solar system. The lack of 26Al in PLAC hibonites indicates that significant amounts of 26Al were not formed in the same spallogenic processes that made 10Be in PLAC precursors. This is most easily understood as indicating very early formation of the PLAC hibonites, prior to the incorporation and mixing of 26Al into the solar nebula, although an alternative scenario, which invokes irradiation under different solar flare conditions, cannot be ruled out. Lithium isotopes are normal within uncertainties, probably reflecting contamination and/or postcrystallization exchange. The oxygen isotopic compositions of SHIBs and PLACs are all highly 16O-enriched, but are not derived from a homogeneous reservoir: Δ17O values span a range of ∼−28‰ to −15‰. The ranges of 16O-enrichment in SHIBs and PLACs overlap and are less “anomalous” than the most 16O-enriched compositions found in meteorites [Kobayashi S., Imai H. and Yurimoto H. (2003) New extreme 16O-rich chondrule in the early solar system. Geochem. J. 37, 663–669]. Both PLACs and SHIBs formed in 16O-enriched reservoirs characterized by small-scale heterogeneities in the gas phase. If such heterogeneities were generated by an admixture of relatively 16O-poor gas created by self-shielding during CO photolysis and transported to the hot inner regions of the accretion disk, then this process must have been initiated very early on, prior to the arrival of fresh radioactivity into the inner solar system. Oxygen isotope heterogeneities persisted throughout the formation interval of PLACs, CAI precursors, and SHIBs which could be as long as 3 × 105 years based on 26Al records. One SHIB and one BAG exhibit mass fractionated oxygen isotopic compositions similar to those seen in FUN inclusions and in several platy hibonite crystals [Lee T., Mayeda T. K. and Clayton R. N. (1980) Oxygen isotopic anomalies in Allende inclusion HAL. Geophys. Res. Lett. 7, 493–496; Ireland T. R., Zinner E. K., Fahey A. J. and Esat T. M. (1992) Evidence for distillation in the formation of HAL and related hibonite inclusions. Geochim. Cosmochim. Acta 56, 2503–2520; Ushikubo T., Hiyagon H. and Sugiura N. (2007) A FUN-like hibonite inclusion with a large 26Mg-excess. Earth Planet. Sci. Lett. 254, 115–126]. The suite of mass-fractionated hibonites exhibit a range of isotopic properties, including 26Al/27Al ratios from below detection to a “canonical” level and oxygen and titanium anomalies that are not exceptional by PLAC standards. This suggests that F (fractionation) processes—evaporation under (oxidizing) conditions—are not necessarily associated with sampling a special isotopic reservoir. [Copyright &y& Elsevier]