Spallation and fissiogenic xenon and krypton from stepwise heating of the Pasamonte achondrite; The case for extinct plutonium 244 in meteorites; Relative ages of chondrites and achondrites

Abstract

Following the discovery by Rowe and Kuroda that there are striking excesses of the xenon ratios 134/132 and 136/132 in the Pasamonte achondrite, we performed stepwise heating experiments with this meteorite. Xenon results from three experimental systems were concordant. The ratio 124/130 is linearly correlated with the ratio 126/130, showing a (cosmogenic) spallation component. Ratios reported by other laboratories for total xenon from various achondrites and a mesosiderite fall on this correlation line. The ratio 134/132 is linearly correlated with the ratio 136/132, showing a component presumed to be from fission. The line so defined is a gross but accurate extrapolation of the ‘Pepin line,’ which is now seen to be applicable to both chondrites and achondrites. Our Pasamonte data, taken alone, permit calculation of the isotopic composition of the fission and spallation components which are: fission, 131/132/134/136 = 0.285/1.00/1.06/1.13; spallation, 124/126/130/131/132 (assumed zero) = 0.285/0.995/1.00/4.48/0. Our Pasamonte data combined with Berne data for the highly cosmogenic Stannern achondrite provide an alternative but similar set of values: fission, 131/132/134/136 = 0.31/1.00/0.98/1.02; spallation, 124/126/128/130/131/132 = 0.58/1.00/1.34/0.73/2.76/0.08. Krypton data from the stepwise heating experiment provide ratios 80/84, 82/84, and 83/84 that are linearly correlated, showing a spallation component. Results from other laboratories for total krypton from achondrites and a mesosiderite fall on the correlation line. A preferred composition of the spallation krypton results from Berne measurements on Stannern and our measurements on Pasamonte: 80/82/83/84 = 0.52/0.75/1.00/0.42. The ratio 86/84 does not correlate with the others, suggesting detectable fission krypton in Pasamonte, but background interference makes this a weak conclusion at present. Results for the amounts and compositions of the spallation components agree generally well with calculations based on a formula due to Rudstam and a cosmic-ray spectrum given by Arnold and co-workers. The case for 82-m.y. Pu244 as an extinct radioactivity in meteorites rests on the following evidence: (1) the fission component in xenon differs isotopically from any possible fission component from uranium; (2) the concentration of fission xenon exceeds that attributable to uranium in Pasamonte by at least a factor of 15; (3) arguments are given that rule out other sources of the fission component such as primitive irradiations by slow neutrons, fast neutrons, or γ rays. Previous experiments on ordinary and carbonaceous chondrites can be reexamined in light of the Pu244 hypothesis. At face value, the fission components for chondrites are very large, suggesting that ordinary chondrites cooled at least 168 m.y. earlier than achondrites, and carbonaceous chondrites cooled at least 416 m.y. earlier than achondrites. An initial Pu/U ratio in carbonaceous chondrites would have to exceed 0.26, a result that would require that there was extensive local nucleosynthesis of the transbismuth elements shortly before the origin of the solar system, conflicting with present theories about the evolution of uranium in the solar system. We propose an experiment to test possible correlation of the fission component at retentive sites in the meteorite with fission xenon produced at uranium sites by pile neutrons.