Nucleosynthesis of ¹¹B-rich boron in the pre-solar cloud recorded in meteoritic chondrules

Abstract

MODELS of the chemical evolution of the Galaxy, in which most elements are created inside stars and distributed by stellar winds and supernovae, cannot produce the observed abundances of boron and beryllium¹. These elements have been produced continuously since the Big Bang by collisions between Galactic cosmic rays (very energetic protons and a-particles) and heavier elements, such as carbon and oxygen, in the interstellar medium^2–6. But models of chemical evolution that include these effects predict a boron isotope ratio (¹¹B/¹⁰B = 2.5, ref. 2) that is very different from that observed on Earth and in meteorites (¹¹B/¹⁰B≈4.0, refs 7–9). Here we present ion-probe measurements of the ¹¹B/¹⁰B ratio in meteoritic chondrules, which reveal significant variations (3.84–4.25) correlated with the beryllium and boron concentrations. These correlations can be explained by production of ¹¹B-rich boron in the pre-solar cloud, resulting from collisions between interstellar hydrogen (and helium) and low-energy cosmic rays¹⁰ such as the carbon and oxygen nuclei recently observed in the Orion star-forming complex¹¹. Our results also suggest that isotopic heterogeneities have been partially preserved during the process of chondrule formation.