NUCLEOSYNTHESIS OF B-11-RICH BORON IN THE PRE-SOLAR CLOUD RECORDED INMETEORITIC CHONDRULES

MODELS Of the chemical evolution of the Galaxy, in which most elements are created inside stars and distributed by stellar winds and superno vae, cannot produce the observed abundances of boron and beryllium(1). These elements have been produced continuously since the Big Bang by collisions between Galactic cosmic rays (very energetic protons and al pha-particles) and heavier elements, such as carbon and oxygen, in the interstellar medium(2-6). But models of chemical evolution that inclu de these effects predict a boron isotope ratio (B-11/B-10 = 2.5, ref. 2) that is very different from that observed on Earth and in meteorite s (B-11/B-10 approximate to 4.0, refs 7-9). Here we present ion-probe measurements of the B-11/B-10 ratio in meteoritic chondrules, which re veal significant variations (3.84-4.25) correlated with the beryllium and boron concentrations. These correlations can be explained by produ ction of B-11-rich boron in the pre-solar cloud, resulting from collis ions between interstellar hydrogen (and helium) and low-energy cosmic rays(10) such as the carbon and oxygen nuclei recently observed in the Orion star-forming complex(11). Our results also suggest that isotopi c heterogeneities have been partially preserved during the process of chondrule formation.