Pre-atmospheric depths and thermal histories of Canyon Diablo spheroids

Despite having melted during formation, seven of eight Canyon Diablo spheroids weighing from 0.6 to 13 mg retain cosmic-ray-produced ³⁸Ar (³⁸ Ar_cos) in concentrations (10^-10 cm³ STP/g) ranging from 0.35 to 68. The presence of ³⁸ Ar_cos is consistent with pre-atmospheric depths of <2.3 m and most likely rules out an origin for the spheroids deep within the projectile, which had a radius of ∼15 m. Low levels of ²¹ Ne_cos 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.