47th Lunar and Planetary Science Conference (2016) 1689.pdf

ENHANCED COSMOGENIC NEON-21 AND HELIUM-3 IN HIBONITE-RICH CAIS L. Kööp1,2,4, P. R. Heck1,2,4, H. Busemann5, C. Maden5, R. Wieler5, and A. M. Davis1,2,3,4, 1Department of the Geophysical Sciences, 2Chicago Center for Cosmochemistry, 3Enrico Fermi Institute, Univ. of Chicago, Chicago, IL, USA 4Robert A. Pritzker Center for and Polar Studies, Field Museum of Natural History, Chicago, IL, USA (E-mail: [email protected]), 5Institute of Geochemistry and Petrology, ETH Zurich, Zurich, Switzerland.

Introduction: PLAty hibonite Crystals (PLACs) and troscopy (EDS; average composition of 1 wt% MgO, 89 isotopically related hibonite-rich Ca-Al-rich inclusions wt% Al2O3, 8 wt% CaO, 2 wt% TiO2) resemble proper- (e.g., non-platy crystals and aggregates, collectively re- ties of other PLAC-like CAIs [4]. The total mass of the ferred to as PLAC-like CAIs hereafter) are among the two grains (1.4×10–6 g) was calculated from volume es- most isotopically anomalous materials believed to have timates using scanning electron microscope (SEM) im- formed inside the Solar System [1,2]. Isotope anomalies ages taken at different stage rotation and tilt angles and in PLAC-like CAIs can be of similar magnitude as those an assumed density of 3.84 g cm–3. A conservative in (e.g., PLAC-like CAIs show a δ50Ti ~50% uncertainty was assumed for the volume determi- range of ~300‰ [1,2], which is comparable to many nation and propagated. mainstream silicon carbide grains [3]). However, several For noble gas analysis, the grains were first cleaned lines of evidence indicate a Solar System origin for with OptimaTM quality isopropanol and water, then dried PLAC-like CAIs. For example, with diameters up to and pressed into gold. Noble gases were extracted with a ~200 µm, PLAC-like CAIs are significantly larger than Nd-YAG 1064 nm laser and He and Ne isotopes were presolar grains (typically sub-µm-sized). Furthermore, measured with an ultra-high-sensitivity noble gas mass their Δ17O values fall within the range of Solar System spectrometer at ETH Zurich [7]. We used an analytical materials [2,4] and their mass-independent isotopic protocol developed specifically for analyses of very low characteristics strongly resemble those of FUN CAIs gas amounts [8]. SEM work after noble gas measure- (i.e., variations in Δ17O, large variations in the neutron- ments showed that the hibonites were fully melted, and rich isotopes 48Ca and 50Ti that tend to correlate in sign, Ca- and Al-enrichments were found in the gold in the a link between 26Al-depletions and presence of large- analysis region by EDS. scale anomalies [4,5 and references therein]). Results: The measured amount of 3He is ~9×10–13 The highly anomalous isotopic character of PLAC- cm3 STP (standard temperature and pressure; 1 cm3 STP like CAIs may indicate that these CAIs formed at an = 2.6868×1019 atoms) with an uncertainty on the order early stage in Solar System history, prior to a wide- of 3–4%, the 3He concentration is (6.0±2.8)×10–7 cm3 spread distribution of 26Al and large-scale dilution of STP g–1 (uncertainty dominated by volume estimate). nucleosynthetic effects. They could thus be among the The measured Ne isotopic composition plots close to a first materials that formed inside the Solar System. The mixing line between air [9] and the cosmogenic Ne iso- resulting long residence times inside the solar nebula topic composition produced within hibonite ([10]; Fig. make PLAC-like CAIs interesting samples to look for 1a). Assuming that the measured composition is a mix- effects that could have affected disk materials. Among ture of these two components, the cosmogenic amount of 21 21 –13 3 these is the possibility that nebular materials may have Ne ( Necos) is ~2×10 cm STP, with an uncertainty 21 been exposed to high fluxes of solar energetic particles, on the order of 3–4%. The concentrat