44Ti in a Small SiC Grain from Murchison: Evidence for Deep Mixing of Supernova Ejecta

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Anomalies, Isotopic, Ion Probe, Meteorites, Murchison, Silicon Carbide, Supernova

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In a search for the isotopically highly anomalous silicon carbide grains of type X [1], some 6000 grains from the Murchison separates KJD and KJE (average sizes 0.81 micron, 1.14 micron) [2] have been analyzed with the University of Bern ion microprobe by ion imaging [3]. Image processing of the ^30Si/^28Si data yielded 43 X grains on which high mass resolution isotopic analyses of selected elements were subsequently made. Including data from earlier studies [4], isotopic data of 57 X grains from Murchison separates KJD and KJE are now available. On four grains, the isotopes 40Ca, 44Ca and 48Ti were analyzed along with the isotopes of Si and Mg-Al. One of these grains, designated X57, showed a strong enrichment in ^44Ca by a factor of about 20. Attributing this excess to the decay of ^44Ti, we obtain an initial ^44Ti/^48Ti ratio of 0.46. This value is by almost an order of magnitude larger than previously reported ones [1,5]. Also the Si-isotopic pattern is unique with delta-values well below the mixing line between solar Si and pure ^28Si (delta ^29Si = -750 per mil, delta ^30Si = -480 per mil). On the other hand, the initial ^26Al/^27Al = 0.18 in X57 falls within the range observed for the X grains [1]. The stellar sources of the SiC X grains are believed to be Type II supernovae [1]. Meyer et al. [6] report yields for a large number of isotopes in eight distinct zones in the ejecta from a 25 M solar supernova. Since the condensation of SiC requires C/O > 1, a significant contribution from the He/C zone is necessary. The depletion of the heavy Si isotopes typical for the X grains requires mixing of ^28Si-rich matter from the O/Si and/or Si/S zones. Titanium with ^44Ti/^48Ti = 1.91 is synthesized in the O/Si zone and a mixture of He/C and O/Si material in a 2:1 ratio would have C/O = 1, delta ^29Si= -710 per mil, delta ^30Si = -510 per mil, and ^44Ti/ 648Ti = 0.33, in satisfatory agreement with the composition of X57. Mixing of the O/Si and He/C zones without contamination by the intermediate O/Ne and O/C zones seems improbable, requiring rather specific conditions. However, the innermost Ni zone is extremely rich in Ti with ^44Ti/ 648Ti = 0.45. A mixture of the five inner zones (Ni, Si/S, O/Si, O/Ne, O/C) with the He/C zone in the ratio 1:1:1:1:1:20 would have C/O = 1, delta ^29Si = -550 per mil, delta ^30Si = -430 per mil, and ^44Ti/^48Ti = 0.35. Such a scenario seems more likely and would be roughly compatible with the measured composition of X57. References: [1] Amari S. et al. (1992) Astrophys. J. Lett., 394, L43. [2] Amari S. et al. (1994) GCA, 58, 459. [3] Hoppe P. et al. (1994) Meteoritics, 29, 474. [4] Hoppe P. et al. (1993) Meteoritics, 28, 363. [5] Nittler L. et al., this volume. [6] Meyer B. et al. (1994) Meteoritics, 30, 325.

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