Wataru Fujiya

Evidence for the late formation of hydrous asteroids from young meteoritic carbonates

The accretion of small bodies in the Solar System is a fundamental process that was followed by planet formation. Chronological information of meteorites can constrain when asteroids formed. Secondary carbonates show extremely old (53)Mn-(53)Cr radiometric ages, indicating that some hydrous asteroids accreted rapidly. However, previous studies have failed to define accurate Mn/Cr ratios; hence, these old ages could be artefacts. Here we develop a new method for accurate Mn/Cr determination, and report a reliable age of 4,563.4+0.4/-0.5 million years ago for carbonates in carbonaceous chondrites. We find that these carbonates have identical ages, which are younger than those previously estimated. This result suggests the late onset of aqueous activities in the Solar System. The young carbonate age cannot be explained if the parent asteroid accreted within 3 million years after the birth of the Solar System. Thus, we conclude that hydrous asteroids accreted later than differentiated and metamorphosed asteroids.

SULFUR MOLECULE CHEMISTRY IN SUPERNOVA EJECTA RECORDED BY SILICON CARBIDE STARDUST

We studied about 3400 presolar silicon carbide (SiC) grains from the Murchison CM2 meteorite for C- and Si-isotopic compositions. Among these grains we identified 7 unusual or type C SiC (U/C) grains, characterized by isotopically heavy Si, and 36 supernova type X SiC grains, characterized by isotopically light Si. Selected U/C and X grains were also measured for S-, Mg-Al-, and Ca-Ti-isotopic compositions. We show that the U/C grains incorporated radioactive 44Ti, which is evidence that they formed in the ejecta of Type II supernova (SNII) explosions. Abundances of radioactive 26Al and 44Ti are compatible with those observed in X grains. U/C and X grains carry light S with enrichments in 32S of up to a factor of 2.7. The combination of heavy Si and light S observed in U/C grains is not consistent with abundance predictions of simple supernova models. The isotope data suggest preferential trapping of S from the innermost supernova zones, the production site of radioactive 44Ti, by the growing silicon carbide particles. A way to achieve this is by sulfur molecule chemistry in the still unmixed ejecta. This confirms model predictions of molecule formation in SNII ejecta and shows that sulfur molecule chemistry operates in the harsh and hot environments of stellar explosions.

Evidence for Radiogenic Sulfur-32 in Type AB Presolar Silicon Carbide Grains?

We report C, Si, and S isotope measurements on 34 presolar silicon carbide grains of Type AB, characterized by 12C/13C < 10. Nitrogen, Mg-Al-, and Ca-Ti-isotopic compositions were measured on a subset of these grains. Three grains show large 32S excesses, a signature that has been previously observed for grains from supernovae (SNe). Enrichments in 32S may be due to contributions from the Si/S zone and the result of S molecule chemistry in still unmixed SN ejecta or due to incorporation of radioactive 32Si from C-rich explosive He shell ejecta. However, a SN origin remains unlikely for the three AB grains considered here, because of missing evidence for 44Ti, relatively low 26Al/27Al ratios (a few times 10–3), and radiogenic 32S along with low 12C/13C ratios. Instead, we show that born-again asymptotic giant branch (AGB) stars that have undergone a very-late thermal pulse (VLTP), known to have low 12C/13C ratios and enhanced abundances of the light s-process elements, can produce 32Si, which makes such stars attractive sources for AB grains with 32S excesses. This lends support to the proposal that at least some AB grains originate from born-again AGB stars, although uncertainties in the born-again AGB star models and possible variations of initial S-isotopic compositions in the parent stars of AB grains make it difficult to draw a definitive conclusion.

Hints for neutrino-process boron in presolar silicon carbide grains from supernovae

We have studied more than 1000 presolar silicon carbide (SiC) grains from the Murchison CM2 chondrite for Cand Si-isotopic compositions. Twelve SiC X grains, characterized by strong enrichments in 28 Si and believed to originate from Type II Supernovae (SNeII), were also measured for Li- and B-isotopic compositions. None of these grains show resolvable isotope anomalies in Li or B. For the seven X grains without Li and B contributions from nearby or attached SiC grains of distinct origins we find on average 7 Li/ 6 Li = 11.83 ± 0.29 (solar system: 12.06) and 11 B/ 10 B = 4.68 ± 0.31 (solar system: 4.03). The average 7 Li/ 6 Li is compatible with the solar system ratio and the lithium in the X grains is likely largely dominated by contaminating Li of laboratory or meteoritic origin. Also, most of the boron in X grains appears to be contamination but the small 11 B excess of ∼16%, significant at the 2σ level, can be considered a hint for the presence of boron produced by the neutrino process in the parent SNeII. Despite this finding, a quantitative comparison of the B isotope and abundance data of X grains with model predictions reveals deficiencies in our current understanding of the details of B production in SNeII as well as on B chemistry and condensation in SNII ejecta.

Solar wind boron in Ilmenite grains from lunar soil 71501

Introduction: Baddeleyite (monoclinc-ZrO2) is a widely occurring accessory phase reported from an array of terrestrial mafic and ultra-mafic rocks [e.g. 1] as well as within shergottites [2,3], Lunar meteorites and Apollo samples [e.g. 4,5], asteroidal achondrites [6] and ordinary chondrites [7]. As an established U-Pb geochronometer [8], baddeleyite has the potential to resolve the timing of Solar System crystallization events for a number of low-Si lithologies where zircon (ZrSiO4) is absent. However, the exposure of these grains to shock metamorphism induces partial to complete loss of lead, resetting the U-Pb chronometer and complicating their interpretation. Recent work has focused on coupling isotopic analysis with microstructural observations, linking the extent of amorphisation and recrystallisation to the severity of lead-loss for the first time [i.e. 2, 9]. This approach allows for the targeting of pristine or deformed crystals in an attempt to differentiate the timings of igneous and impact events. However, given a discrepancy between the severity of lead diffusion within natural [2, 9] and experimental [10] shock conditions, our fundamental understanding of U-Pb age resetting in this potentially key planetary chronometer is poorly constrained. The application of atom probe tomography (APT) to zircon has proven exceptionally useful in distinguishing the response of Pb to both post-crystallization annealing [11] and deformation [12]. However, this approach has never been applied to heavily shock loaded material. Here we present the first insights into the atomic-scale shock response of lead cations within baddeleyite, coupling these observations with detailed EBSD analysis to produce a first order insight into the mechanisms of U-Pb age resetting in baddeleyite.