Condensation of fallout glasses in the Hiroshima nuclear fireball resulting in oxygen mass-independent fractionation

Abstract

A new kind of Hiroshima nuclear fallout, the Hiroshima glasses, was discovered around the Hiroshima Bay. Here, the chemical compositions and the silicon and oxygen triple isotope compositions were analyzed to understand the formation process of these new fallouts. The chemical analysis shows four different families of glasses: the melilitic glasses, the anorthositic glasses, the soda-lime glasses, and the silica glass. The silicon isotopic compositions show wide variations in the glasses, with δ30Si varying between -23.0 ± 1.8 ‰ and -1.5 ± 1.1 ‰. The oxygen isotopic compositions indicate the presence of mass-independent fractionation on ≈38 % of the analyses, reaching a Δ17O of -3.1 ± 0.6 ‰. The chemical and silicon isotopic compositions of the Hiroshima glasses show that these glasses were formed by condensation within the nuclear fireball. Our scenario for the Hiroshima glasses formation, tested by modeling (GGchem code), considers a rapid condensation (1.7–5.5 s) in the nuclear fireball (3200–1000 K) at atmospheric pressure with a gas resulting from a mixing between air, and vaporized water and city materials. Chemical reactions during the Hiroshima glasses condensation are the most probable source for the oxygen mass-independent fractionation. The formation of the Hiroshima glasses by condensation implies that they may be an analog to the first condensates in the solar system: Calcium-Aluminum-rich Inclusions (CAIs), which are found in chondrites. The Hiroshima glasses exhibit similarities with CAIs in their chemical and isotopic compositions (Si and O).