doi:10.1016/S0016-7037(03)00261-8 Presolar spinel grains from the Murray and Murchison carbonaceous chondrites ERNST ZINNER, 1, *SACHIKO AMARI, 1 ROBERT GUINNESS, 1 ANN NGUYEN, 1 FRANK J. STADERMANN, 1 ROBERT M. WALKER, 1 and ROY S. LEWIS 2 1 Laboratory for Space Sciences and the Physics Department, Washington University, St. Louis, MO 63130, USA 2 Enrico Fermi Institute, University of Chicago, 5630 Ellis Avenue, Chicago, IL 60637, USA (Received December 23, 2002; revised 9 April 2003; accepted in revised form April 9, 2003) Abstract—With a new type of ion microprobe, the NanoSIMS, we determined the oxygen isotopic compo- sitions of small (1m) oxide grains in chemical separates from two CM2 carbonaceous meteorites, Murray and Murchison. Among 628 grains from Murray separate CF (mean diameter 0.15 m) we discovered 15 presolar spinel and 3 presolar corundum grains, among 753 grains from Murray separate CG (mean diameter 0.45 m) 9 presolar spinel grains, and among 473 grains from Murchison separate KIE (mean diameter 0.5 m) 2 presolar spinel and 4 presolar corundum grains. The abundance of presolar spinel is highest (2.4%) in the smallest size fraction. The total abundance in the whole meteorite is at least 1 ppm, which makes spinel the third-most abundant presolar grain species after nanodiamonds (if indeed a significant fraction of them are presolar) and silicon carbide. The O-isotopic distribution of the spinel grains is very similar to that of presolar corundum, the only statistically significant difference being that there is a larger fraction of corundum grains with large 17 O excesses ( 17 O/ 16 O  1.5  10 -3 ), which indicates parent stars with masses between 1.8 and 4.5 M J . Copyright © 2003 Elsevier Ltd 1. INTRODUCTION In 1987 Tang and Anders (Tang and Anders, 1988; Tang et al., 1988) prepared, by physical and chemical separation, a residue from the CM2 carbonaceous chondrite Murray. This residue, Murray CF (Fig. 1) contains high concentra- tions of the noble gas components Ne-E(H) and Xe-S (Tang and Anders, 1988). Subsequent analysis of CF by transmis- sion electron microscopy, Raman laser microprobe spectros- copy and ion microprobe analysis led to the first identifica- tion of presolar silicon carbide (Bernatowicz et al., 1987; Zinner et al., 1987). However, SiC constitutes  10% of Murray CF and most of the residue consists of spinel grains. Zinner and Tang (1988) measured O isotopic ratios in the CF residue with the Cameca IMS 3f ion microprobe at Washington University. Oxygen isotopic measurements require high mass resolution that allows the separation of the 16 OH – ion signal from that of 17 O – . In the IMS 3f ion probe, this leads to a large reduction in secondary ion transmission. Because of the small grain size of CF (aver- age grain diameter of 0.15 m), measurements could not be made on single grains but were made on aggregates of thou- sands of grains. The results are shown in Figure 2. All CF analyses show excesses in 17 O relative to the isotopic compo- sitions of spinel grains in the Murchison carbonaceous chon- drite (Clayton and Mayeda, 1984). Although this result hinted at the presence of presolar spinel grains, such a presence could not be clearly established and it took several years before the existence of presolar oxide grains in primitive meteorites was unambiguously demonstrated (Hutcheon et al., 1994; Nittler et al., 1994). Most presolar oxide grains have large 17 O excesses (Nittler et al., 1997) and if the O isotopic compositions of the putative presolar grains in CF were the same as those of larger oxide grains we estimate that 2% of the CF spinel grains should be of presolar origin. The arrival of a new type of ion microprobe, the Cameca NanoSIMS (Stadermann et al., 1999a, 1999b, 2000), in our laboratory has opened the possibility of measuring the O isotopic composition of in- dividual grains as small as those in Murray CF. The Nano- SIMS has several novel features that make it the instrument of choice for the isotopic analysis of very small grains. First, at a mass resolving power of 5000, necessary for O iso- topic analysis, the secondary ion transmission is  30 times higher than that of the Cameca IMS 3f. Second, the Nano- SIMS has miniaturized electron multipliers (Slodzian et al., 2003), four of which can be moved along the focal plane of the secondary ions. This means that O isotopic ratios can be measured simultaneously (“multidetection”). This not only increases the overall sensitivity for such measurements, but any change in the secondary ion signal, which, because of the small size of the grains, is unavoidable, does not affect the measured isotopic ratios as it would if magnetic peak jumping and just one detector were employed. Third, the NanoSIMS features a very small primary beam size. For Cs + primary ions, beam diameters  50 nm have been achieved and a beam diameter of 100 nm is routine. The reason for this is that the primary ions are incident normal to the sample surface, along the same axis secondary ions are extracted. As a consequence, the immersion lens can be placed very close to the sample surface, resulting in a large demagnification of the primary beam diameter. We took advantage of these new instrumental capabilities to measure O isotopic ratios of individual grains from Murray CF (average grain diameter 0.15 m). In addition, we measured the O isotopic ratios of grains from separates Murray CG (average grain size 0.45 m) and Murchison KIE (average grains size 0.5 m) (Fig. 1). * Author to whom correspondence should be addressed (ekz@ wuphys.wustl.edu). Pergamon Geochimica et Cosmochimica Acta, Vol. 67, No. 24, pp. 5083–5095, 2003 Copyright © 2003 Elsevier Ltd Printed in the USA. All rights reserved 0016-7037/03 $30.00 + .00 5083