Heavy rare gases in silicates from the Estherville mesosiderite

Abstract

Krypton in Estherville silicates is shown to be a mixture of primordial and spallation-produced components, and xenon a mixture of primordial, spallation, and fission components. Small amounts of radiogenic Xe129 may also be present. An average isotopic composition for Estherville spallation xenon is Xe124 : Xe126 : Xe128 : Xe130 : Xe131 : Xe132 = .34 : .62 : 1.00 : 1.05 : 4.80 : 3.3. This composition is compared with theoretical isobaric production rates calculated from a model of cosmic-ray spallation in meteoritic material. The relative abundance of Xe131 is found to be significantly higher than predicted: whether this implies either thermal neutron irradiation or a departure from a simple cosmic-ray irradiation history is at present uncertain. The possible presence of a spallation component in carbonaceous chondrite xenon is considered. Extinct radioactivity chronologies and the question of concordant formation intervals are examined for Estherville and for the Pasamonte achondrite. I-Xe and Pu-Xe formation intervals for Pasamonte are probably concordant at ∼220±40 million years. The isotopic composition of Pasamonte fission xenon, after correction for spallation contributions, is estimated as Xe131 : Xe132 : Xe134 : Xe136 = 0.18 : 0.91 : 0.97 : 1.00.