Eleven presolar silicon carbide grains of type X separated from the Murchison meteorite have been analyzed for boron abundances and isotopic compositions by secondary ion mass spectrometry. Boron concentrations are low with typical B/Si ratios of ≈ 1 × 10-5. The average 11B/10B ratio of 3.46 ± 1.39 is compatible with the solar system value but might be affected by contaminating boron of laboratory origin. These data are compared with theoretical predictions for Type II supernovae, the most likely parent 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 shell-mixing of matter from the C- and Si-rich zones, contrary to other elemental 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 grains are well reproduced by the condensation calculations. Given the similarity of the boron and aluminum condensation chemistry and the generally expected high B/Al ratios (relative to solar) in Type II supernova mixtures with C/O 1, the observed difference between measured and predicted B/Al ratios must be considered a serious problem. Possible solutions include (1) lower than predicted boron production from Type II supernovae, (2) complex mixing 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.
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