ABSTRACT

Noble gas and oxygen isotopic compositions were studied for the Towada meteorite, a recent fall in Aomori Prefecture, Japan. The meteorite was classified as a H6 chondrite based on oxygen isotope ratios (δ¹⁷O = 2.82‰, δ¹⁸O = 4.03‰) and concentrations and elemental ratios of the heavy noble gases. The U, Th-He and K-Ar gas retention ages, calculated as 0.38 b.y. and 1.3 b.y., respectively, suggest preferential loss of ⁴He compared with ⁴⁰Ar. Cosmic-ray exposure ages using cosmogenic ³He, ²¹Ne and ³⁸Ar concentrations were 7.67, 31.1 and 18.6 m.y., respectively. The ³He and ⁴He loss observed for this meteorite may be due to heating by solar radiation during orbital motion around the Sun. The short nominal exposure age based on ³⁸Ar may be due to ³⁸Ar loss during weathering or due to chemical heterogeneity of the meteorite. Existence of excess fissiogenic ^131-136Xe from extinct ²⁴⁴Pu is consistent with the presence of ¹²⁹Xe produced from extinct ¹²⁹I.