ISOTOPE GEOCHEMISTRY OF BORON IN MANTLE R OCKS, TEKTITES AND METEORITES

Recent ion microprobe studies of fresh oceanic basalt glasses and chon drules from primitive meteorites give an overview of the distribution of boron isotopes in the mantle and in extra-terrestrial rocks. After removal of secondary boron isotope variations due to interactions betw een mantle melts and the oceanic crust, the primitive mantle is found to have a constant delta(11)B value of -10 +/- 2 parts per thousand, s imilar to that of the bulk continental crust. In contrast, large isoto pic Variations between -50 and +40 parts per thousand are present at t he micron scale in meteoritic chondrules which are among the most prim itive objects of the solar system. These isotopic variations imply tha t a significant part of the boron of the solar system was synthesized in the presolar cloud, likely by spallation reactions between low-ener gy cosmic rays and nebular hydrogen. These heterogeneities were partly preserved in chondrules which formed early in the evolution of the so lar system but are not observed for the silicate Earth implying an eff icient mixing just before or during the accretion of the Earth.