Nucleogenic noble gas components in the Cape York iron meteorite

Abstract

We report data on neutron capture products of the secondary cosmic ray component, the inferred proton and neutron fluences, and the identification of double beta decay of ⁸²Se in heavily shielded samples of the Cape York iron meteorite. One purpose of this study is to develop a new chronometer for cosmic ray exposure, based on the nuclides ¹²⁹I (16 My half-life) and ¹²⁹Xe from low energy cosmic ray reactions on Te. The abundance ratio of these two nuclides permits the determination of an (effective) exposure age of 93 ±16 My, which represents the first exposure age datum of Cape York. The very small concentrations of spallogenic ³⁸Ar = 6.5 × 10^-10 cm³ STP/g in the metal and troilite (per g Fe) document the heavily shielded locations of our sample. An excess of ¹²⁹Xe in the troilite is shown to be entirely due to the decay of cosmic-ray-produced ¹²⁹I. On the other hand, an inclusion in the troilite reveals the presence of ¹²⁹Xe from extinct ¹²⁹I and documents its ~4.5 Gy formation age. Mono-isotopic excess of ⁸²Kr is identified as due to ββ-decay of ⁸²Se with an inferred half-life of 1.0 × 10¹⁰ y. This represents the first ββ-decay product observed in a meteorite.