• A study of Mg and K isotopes in Allende CAIs: Implications to the time scale for the multiple heating processes

      Ito, M.; Nagasawa, H.; Yurimoto, H. (The Meteoritical Society, 2006-01-01)
      The measurements of magnesium and potassium isotopic compositions of refractory minerals in Allende calcium-aluminum-rich inclusions (CAIs), 7R-19-1, HN3-1, and EGG3 were taken by secondary ion mass spectrometry (SIMS). The 7R-19-1 contains 16O-rich and 16O-poor melilite grains and define a single isochron corresponding to an initial 26Al/27Al ratio of (6.6 +/- 1.3) x 10^(-5). The Al-Mg isochron, O isotope measurements and petrography of melilite in 7R-19-1 indicate that 16O-poor melilite crystallized within 0.4 Myr after crystallization of 16O-rich melilite, suggesting that oxygen isotopic composition of the CAI-forming region changed from 16O-rich to 16O-poor within this time interval. The 16O-poor melilite is highly depleted in K compared to the adjacent 16Orich melilite, indicating evaporation during remelting of 7R-19-1. We determined the isochron for 41Ca-41K isotopic systematics in EGG3 pyroxene with (4.1 +/- 2.0) x 10^(-9) (2-sigma) as an initial ratio of 41Ca/40Ca, which is at least two times smaller than the previous result (Sahijipal et al. 2000). The ratio of 41Ca/40Ca in the EGG3 pyroxene grain agrees within error with the value obtained by Hutcheon et al. (1984). No evidence for the presence of 41K excess (decay product of a short-lived radionuclide 41Ca) was found in 7R-19-1 and HN3-1. We infer that the CAI had at least an order of magnitude lower than canonical 41Ca/40Ca ratio at the time of the CAI formation.
    • Carbon investigation of two Stardust particles: A TEM, NanoSIMS, and XANES study

      Matrajt, G.; Ito, M.; Wirick, S.; Messenger, S.; Brownlee, D. E.; Joswiak, D.; Flynn, G.; Sandford, S.; Snead, C.; Westphal, A. (The Meteoritical Society, 2008-01-01)
      In this work we present the results of a systematic search for cometary organics in 14 Stardust particles (particles from comet 81P/Wild 2, captured by NASAs Stardust mission) by TEM and multidisciplinary studies (XANES and NanoSIMS) of Febo and Ada, two of the organic-bearing particles identified. The combination of the three analytical techniques has established the presence of organic, cometary degrees C in both particles. Using energy-filtered and high-resolution imaging it was shown that the degrees C is amorphous and rare, given that it is found in grains less than or equal to 200 nm in size that are not abundant throughout the particles. The XANES maps and spectra of the carbonaceous areas identified with the TEM have shown that the carbonaceous material is organic due to the presence of carbonyl (C=O) functional groups and the overlapping of degrees C and N on the same grains. In addition, several different C-XANES spectra were obtained from the same particle, suggesting that there is diversity in the types of carbonaceous phases present in these particles, as well as a heterogeneous distribution of the carbonaceous phases within these particles. The C-XANES spectra obtained are different from C-XANES spectra of carbonaceous chondrites and IDPs. In the particle Febo we found five spots showing a pronounced enrichment in the isotope 15N (delta-15N from 420 to 639 +/- 20 to 70 ppm, 1-sigma) that were clearly associated with the C-rich regions. The carbonaceous material has approximately solar C and D/H isotopic compositions, and the bulk O isotopic composition was found to be delta-17O = -18 +/- 13 ppm and delta-18O = -37 +/- 12 ppm (1-sigma). In the particle Ada we found a C-rich phase with enrichments in the isotope 15N (delta-15N = 550 +/- 70 ppm, 1-sigma) and the isotope D (delta-D = 610 +/- 254 ppm, 1-sigma). The C isotopic composition at this phase is solar (delta-13C = -4 +/-29 ppm, 1 sigma). The bulk O isotopic composition of Ada was found to be delta-17O = 9 +/- 14.6 ppm and delta-18O = -7.3 +/- 8.1 ppm (2-sigma).