Normal Zirconium Isotopic Composition in Murchison Hibonite 13-13

Details Normal Zirconium Isotopic Composition in Murchison Hibonite 13-13 Normal Zirconium Isotopic Composition in Murchison Hibonite 13-13

Jul 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..370i&link_type=abstract

Meteoritics, vol. 28, no. 3, volume 28, page 370

Astronomy and Astrophysics

Astronomy

Hibonite, Ion Probe, Isotopic Anomalies, Nucleosynthesis

Scientific paper

Murchison hibonite 13-13 has a special place in isotopic astronomy because it has the largest isotopic anomalies in Ca and Ti as yet measured with delta ^48Ca at +105 per mil and delta ^50Ti at +273 per mil [1]. It has been suggested that these anomalies are the result of nucleosynthesis in neutron-rich supernova ejecta [2] and Ca and Ti in this hibonite grain have been the least diluted by normal solar system material. It would be expected that the isotopic compositions of the other Fe peak elements would also show the effects of the same nucleosynthetic process(es), but the abundances of the other elements are quite low. This is because of the refractory composition of hibonite and the relatively high volatilities of the other Fe peak elements. Zirconium is another refractory element that has recently been examined for its isotopic composition. Harper and coworkers [3] found a small ^96Zr enhancement (~2 epsilon) in Allende CAIs, and it was suggested that the ^96Zr anomaly was correlated with the ^50Ti enhancements (~10 epsilon) in the same inclusions and that both isotopes were produced by the same process [4]. The purpose of this report is to test whether a ^96Zr anomaly can be resolved in 13-13. If present at the same level as in the Allende CAIs relative to the ^50Ti anomaly, a ^96Zr enhancement of the order of 60 per mil should be observed. Zirconium isotopic compositions were measured by ion microprobe mass spectrometry. A mass resolution of 8000 (1% valley) was found to be sufficient to resolve molecular interferences as well as hydrides. However, in some hibonite analyses, tailing from ^40Ca(sub)2^160^+ became unacceptably large. Terrestrial zircon and Zr metal standards were also measured, and good agreement was found with terrestrial Zr isotopic compositions [5] after normalization to the ^94Zr/^90Zr ratio. The Zr isotopic composition of 13-13 is normal within expenmental uncertainty and the maximum anomaly at the 2- sigma level is well below 10 per mil (Fig. 1). No ^96Zr anomaly is associated with the large ^50Ti anomaly in this Murchison hibonite, and so it is unlikely that the nucleosynthetic model of Harper and coworkers for coproduction of ^96Zr and ^50Ti is valid. References: [1] Ireland T. R. (1990) GCA, 54, 3219-3237. [2] Hartmann D. et al.(1985) Astrophys. J., 297, 837-845. [3] Harper C. L. et al. (1990) Meteoritics, 25, 369. [4] Harper C. L. et al. (1991) LPSC XXII, 517-518. [5] Minster J. and Ricard L. P. (1981) Int. J. Mass Spec. Ion Phys., 37, 259-272. Fig. 1 appears in the hard copy here.

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