The distribution of the short‐lived radionuclide26Al in the early solar system remains a major topic of investigation in planetary science. Thousands of analyses are now available but grossite‐bearing Ca‐, Al‐rich inclusions (
- Award ID(s):
- 1819550
- NSF-PAR ID:
- 10374698
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Meteoritics & Planetary Science
- Volume:
- 55
- Issue:
- 2
- ISSN:
- 1086-9379
- Page Range / eLocation ID:
- p. 352-375
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract CAI s) are underrepresented in the database. Recently found grossite‐bearing inclusions inCO 3 chondrites provide an opportunity to address this matter. We determined the oxygen and magnesium isotopic compositions of individual phases of 10 grossite‐bearingCAI s in the Dominion Range (DOM ) 08006 (CO 3.0) andDOM 08004 (CO 3.1) chondrites. All minerals inDOM 08006CAI s as well as hibonite, spinel, and pyroxene inDOM 08004 are uniformly16O‐rich (Δ17O = −25 to −20‰) but grossite and melilite inDOM 08004CAI s are not; Δ17O of grossite and melilite range from ~ −11 to ~0‰ and from ~ −23 up to ~0‰, respectively. Even within this small suite, in the two chondrites a bimodal distribution of the inferred initial26Al/27Al ratios (26Al/27Al)0is seen, with four having (26Al/27Al)0≤1.1 × 10−5and six having (26Al/27Al)0≥3.7 × 10−5. Five of the26Al‐richCAI s have (26Al/27Al)0within error of 4.5 × 10−5; these values can probably be considered indistinguishable from the “canonical” value of 5.2 × 10−5given the uncertainty in the relative sensitivity factor for grossite measured by secondary ion mass spectrometry. We infer that the26Al‐poorCAI s probably formed before the radionuclide was fully mixed into the solar nebula. All minerals in theDOM 08006CAI s, as well as spinel, hibonite, and Al‐diopside in theDOM 08004CAI s retained their initial oxygen isotopic compositions, indicating homogeneity of oxygen isotopic compositions in the nebular region where theCO grossite‐bearingCAI s originated. Oxygen isotopic heterogeneity inCAI s fromDOM 08004 resulted from exchange between the initially16O‐rich (Δ17O ~−24‰) melilite and grossite and16O‐poor (Δ17O ~0‰) fluid during hydrothermal alteration on theCO chondrite parent body; hibonite, spinel, and Al‐diopside avoided oxygen isotopic exchange during the alteration. Grossite and melilite that underwent oxygen isotopic exchange avoided redistribution of radiogenic26Mg and preserved undisturbed internal Al‐Mg isochrons. The Δ17O of the fluid can be inferred from O‐isotopic compositions of aqueously formed fayalite and magnetite that precipitated from the fluid on theCO parent asteroid. This and previous studies suggest that O‐isotope exchange during fluid–rock interaction affected mostCAI s in CO ≥3.1 chondrites. -
null (Ed.)Minor and trace elements in diamond-like carbon (DLC) are difficult to quantify using SIMS analysis because minor elemental and structural variations can result in major matrix effects even across individual, cm-sized samples. While this material is most commonly used for tribological coatings where minor element composition is not of critical importance, it is being increasingly used in electronic devices. However, it is a unique application that spurred this work: anhydrous, tetrahedrally-coordinated DLC (ta-C) was used as a solar wind (SW) collector material in the Genesis solar-wind sample return mission (NASA Discovery 5). So, for ∼15 years, we have been working on attaining accurate and precise measurement of minor and trace elements in the Genesis DLC using SIMS to achieve our mission goals. Specifically, we have learned to deal with relevant matrix effects in our samples, ion implants into ta-C. Our unknown element for quantification is SW Mg, a low-dose (1.67 × 10 12 at cm −2 ; ∼6 μg g −1 24 Mg), low-energy (∼24 keV average energy) implant; our standard is a high-dose (∼1 × 10 14 at cm −2 of both 25 Mg, 26 Mg) 75 keV laboratory implant for which the absolute 26 Mg/ 25 Mg ratio had been measured to account for variable instrumental mass fractionation. Analyses were performed using O 2 + primary ions having both a low impact energy and a current density of ∼2 × 10 14 ions per cm 2 . Although our unknown was solar wind, the method is applicable to many situations where minor elements in DLC need to be quantified. Recommendations are presented for modifying this data-reduction technique for other SIMS conditions.more » « less
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Abstract We report on the isotopic, chemical, and structural properties of four O‐rich presolar grains identified in situ in the Adelaide ungrouped C2, LaPaZ Icefield (
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Abstract Enstatite chondrites and aubrites are meteorites that show the closest similarities to the Earth in many isotope systems that undergo mass‐independent and mass‐dependent isotopic fractionations. Due to the analytical challenges to obtain high‐precision K isotopic compositions in the past, potential differences in K isotopic compositions between enstatite meteorites and the Earth remained uncertain. We report the first high‐precision K isotopic compositions of eight enstatite chondrites and four aubrites and find that there is a significant variation of K isotopic compositions among enstatite meteorites (from −2.34‰ to −0.18‰). However, K isotopic compositions of nearly all enstatite meteorites scatter around the bulk silicate earth (
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