Argon, krypton, and xenon in Angra dos Reis, Nuevo Laredo, and Norton County achondrites: The case for two types of fission xenon in achondrites

Abstract

This paper reports argon, krypton, and xenon analyses of the uranium rich achondrites Angra dos Reis and Nuevo Laredo. Results for the Norton County aubrite show good agreement with previously reported analyses from other laboratories. We obtain cosmic ray exposure ages of 68 m.y. and 16 m.y. for Angra dos Reis and Nuevo Laredo by comparing 38Ar/Ca ratios with Stannern whose cosmic ray exposure age has previously been determined by the 81Kr-78Kr method. Contrary to the earlier analysis of Nuevo Laredo by Reynolds and Lipson, both Angra dos Reis and Nuevo Laredo showed pronounced fission xenon anomalies. Failure of the fission xenon composition obtained by an Angra dos Reis-Stannern subtraction to match that of Pasamonte suggests there is more than one type of fission xenon in the calcium rich achondrites and at the some time invalidates the subtraction method. After spallation component and trapped component corrections, we obtain the following xenon composition for Angra dos Reis (Nuevo Laredo); 131Xe, 132Xe, 134Xe, and136Xe/0.81 (0.54), ≡ 1.00 (≡ 1.00), 0.96 (1.13), and 1.06 (1.27). In this case inclusion of errors makes the ratios slightly overlap, but the difference again suggests more than one type of fission xenon. If we treat other previously reported calcium rich achondrites the same way, we find a spread in the composition of the fission xenon components with the 134Xe132Xe ratios varying in an approximately linear way with the 136Xe132Xe ratios. This linear spread suggests that there are two types of fission xenon in the calcium rich achondrites. Either xenon from spontaneous fission of 238U or the carbonaceous chondrite type fission xenon recently reported by Pepin would provide the end member with high 134Xe132Xeand136Xe132Xe ratios, but the data indicate an additional type of fission xenon with low 134Xe132Xeand136Xe132Xe ratios. Spontaneous fission of 238U, which of all uranium fissions we expect to make the largest contribution to xenon, is insufficient by factors of 34 and 18 in providing for the amount of fission 136Xe in Angra dos Reis and Nuevo Laredo. Using the previously reported krypton and xenon data for Stannern, we can make an additional argument against uranium fission in Angra dos Reis. The purity of the spallation krypton spectrum in Stannern allows an upper limit to be placed on the 86Kr from fission in Angra dos Reis which is 6% of the 136Xe due to fission and a factor of two less than expected for spontaneous fission of 238U. This argument also more strongly rules out neutron fission of 235U as a major source of fission xenon in Stannern.