GLASSES AND MELT FRAGMENTS POSSIBLY RELATED TO THE HIROSHIMA NUCLEAR EXPLOSION AS IMPACT GLASS ANALOGS

Explosions of nuclear devices are, in terms of temperature and pressure, the closest available analog to asteroid/meteorite impacts. Best known is trinitite, which is glassy material found on the desert floor after the plutonium-based Trinity nuclear bomb test on July 16, 1945, near Alamogordo, New Mexico. The usually light green glass is primarily composed of melts derived from arkosic sand composed of quartz grains and feldspar (e.g., [1]). No melts have so far been described from the Hiroshima blast site. During a visit to Hiroshima Bay, in August 2015, some beach sand was sampled, with the aim to analyze the mineral and bioclastic content of these sediments. Preliminary microscopic observations surprisingly established a high concentration of what appear to be melt products, ranging in size from fine gravel to silt. Subsequent blind sediment sampling at 7 separate beaches around the Motoujina Peninsula resulted in all 13 samples yielding melt products. A variety of particle types were selected for detailed electron microscopic and microprobe investigations. Overall particle sizes range from ~ 0.4-2 mm. In terms of morphology, texture and modal composition, the samples comprise a wide range including different types of glass objects (compact spherules, vesicular spherules, hollow spherules, filaments), objects with partly to fully recrystallized domains and /or skeletal crystals, and fluffy objects with delicate internal textures. WDS-analyses performed on the glassy constituents of 53 particles reveal a large compositional variation (within one object as well as between different particles), e.g., 31.7-74.8 SiO₂. Several glass objects contain silica-rich areas related to individual inclusions and/or to areas exhibiting diffuse boundaries to the adjacent glass matrix. In places, the chemical composition of these areas corresponds to that of pure silica. However, from the present investigation it cannot yet be firmly concluded if these silica inclusions represent incompletely dissolved/digested quartz grains or silica glass (lechatelierite), which would confirm the high-T (=nuclear blast) origin. Some glasses contain small inclusions (average size <10 µm) of “droplets” consisting of (Fe, P, Ca)-alloys.

[1] Eby G.N. et al., Am. Mineral., v. 100, p. 427–441, 2015.