Boron isotope ratios in meteorites and lunar rocks

The B-11/B-10 ratios of thirty-two meteorite falls and nine lunar rocks were measured as Cs2BO2+ using thermal ionization mass-spectrometry. The B-11/B-10 ratios of meteorites vary from 4.011 to 4.098, i.e., their delta(11)B values (relative to NIST SRM 951) range from -10.5 to +19.2 parts per thousand; however, excluding two outliers, Mokoia and Norton County, the range of most meteorites is smaller (-10.5 to +7.5). The average of two CI1 meteorites, Ivuna and Orgueil, is -3.3, in the middle of the range. The delta(11)B values of the lunar rocks vary less than those of meteorites, from -6.0 to 3.9. The average delta(11)B of CI1 chondrites is -3.3, similar to that of terrestrial fresh mid-ocean ridge basalts (-6.5 to -1.2) and to the estimated mantle value of +0.2 (Ishikawa and Nakamura, 1992), which is the best representative of the whole Earth. The similarity of delta(11)B values in meteorites, lunar rocks, and those parts of the Earth unaffected by water implies that the boron isotopic composition of the Solar System is rather homogeneous. Recently, Chaussidon and Robert (1995) reported larger variation of delta(11)B values in chondrules of three chondrites, from -50 to +40. This degree of heterogeneity is absent from bulk meteorites.