Cosmogenic nuclides in the Brenham pallasite

Cosmic-ray-produced (cosmogenic) nuclides were studied in fragments of the Brenham pallasite, a large stony iron meteorite. The contents of light noble gases (He, Ne, and Ar) and long-lived radionuclides (Be-10, Al-26, Cl-36, and Mn-53), produced by nuclear reactions with cosmic rays, were measured in the separated metal and olivine phases from numerous samples representing a wide range of shielding conditions in the meteoroid. The distribution of cosmogenic nuclide concentrations in the metal follows patterns similar to that observed in large iron meteorites. Shielding effects were estimated from the relative proportions of low- and high-energy reaction products. The production rates varied, from surface to interior, by a factor of more than several hundred. The Cl-36-Ar-36 cosmic-ray exposure age of Brenham is 156 +/- 8 Myr. This determination is based on a multiple nuclide approach that utilizes cosmogenic nuclide pairs. This approach not only yields a "shielding independent" exposure age but also demonstrates that the production of cosmogenic nuclides occurred in a single stage. The depth profiles of Be-10 in the stone phase and Mn-53 in the metal phase are shown superimposed on corresponding profiles from the Apollo 15 long drill core. Surprisingly low abundances of lithophile elements, such as K, U, and Th, provided a unique opportunity to examine the production systematics of those nuclides whose inventories typically have significant contributions from non-cosmogenic sources, particularly radiogenic contributions. The U and Th contents of the olivine samples are extremely low, allowing detection of cosmogenic He-4 production from oxygen, magnesium, silicon, and iron.