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.
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology