Pre-atmospheric depths and thermal histories of Canyon Diablo spheroids

Abstract

Despite having melted during formation, seven of eight Canyon Diablo spheroids weighing from 0.6 to 13 mg retain cosmic-ray-produced 38Ar (38Arcos) in concentrations [10^(-10) cm^3 STP/g] ranging from 0.35 to 68. The presence of 38Arcos is consistent with pre-atmospheric depths of <2.3 meters and most likely rules out an origin for the spheroids deep within the projectile, which had a radius of ~15 m. Low levels of 21Necos indicate gas loss from these spheroids. Relative to most Canyon Diablo meteorites, the spheroids contain lower concentrations of cosmogenic noble gases. The difference partly reflects diffusion losses from the spheroids, especially for 3He and 21Ne, but also suggests deeper locations on average for the precursor material, consistent with independent results from 59Ni.