Boron in presolar silicon carbide grains from supernovae

Eleven presolar silicon carbide grains of type X separated from the Murchis on meteorite have been analyzed for boron abundances and isotopic compositi ons by secondary ion mass spectrometry. Boron concentrations are low with t ypical B/Si ratios of approximate to1 x 10(-5). The average B-11/B-10 ratio of 3.46 +/- 1.39 is compatible with the solar system value but might be af fected by contaminating boron of laboratory origin. These data are compared with theoretical predictions for Type II supernovae, the most likely paren t stars of X grains. The B/Si ratios of X grains are much lower (more than an order of magnitude on average) than expected from Type II supernova shel l-mixing of matter from the C- and Si-rich zones, contrary to other element al ratios such as Al/Si and Ti/Si. Condensation calculations show that with C/O > 1 in the ejecta, boron and aluminum will readily condense as BN and AlN, respectively, into silicon carbide, and the B/Al ratio is expected to remain constant. The nitrogen, aluminum, and titanium abundances in SiC X g rains are well reproduced by the condensation calculations. Given the simil arity of the boron and aluminum condensation chemistry and the generally ex pected high B/Al ratios (relative to solar) in Type II supernova mixtures w ith C/O > 1, the observed difference between measured and predicted B/Al ra tios must be considered a serious problem. Possible solutions include (1) l ower than predicted boron production from Type II supernovae, (2) complex m ixing scenarios in supernova ejecta involving only sublayers of the C-rich zones, and (3) formation of silicon carbide under conditions with C/O < 0.1 .