Bruce S. Hudson

Xenon spallation systernatics in Angra dos Reis

We have resolved literature Xe data for the Angra dos Reis meteorite into constituent spallation, fission and trapped components. The spallation Xe compositions vary over a range wider than observed in any other samples, including lunear samples. These variations are due to the mixing of spallation Xe from Ba and rare earth element targets. It is possible to infer the Ba and rare earth spallation Xe compositions. Angra dos Reis spallation Xe compositions are systematically different from those observed in lunar samples, possibly because of differences in the irradiation conditions (geometry and shielding). Thus the Angra dos Reis data appear to be superior to lunar data for predicting spallation Xe compositions in other meteorites.

Noble gas retention chronologies for the St Séverin meteorite

Abstract 40 Ar- 39 Ar and 129 Xe- 128 Xe analyses were performed on two lithologies (light and dark) of the St Severin (LL6) chondrite. For the light and dark fractions, respectively, we obtained 40 Ar retention ages of 4.38 and 4.42 AE and 129 Xe retention ages of 8.4 and 15.2 myr after Bjurbole. The two methods give age differences of opposite sense, and by both methods the differences are significant. Both the 40 Ar and the 129 Xe ages are interpreted as dating relaxation of metamorphic conditions. These two chronometers are decoupled, however, and do not date the same events. 40 Ar- 39 Ar reflect chondritic metamorphisrn on a 10 8 yr time scale. The 129 Xe- 128 Xe ages reflect isotopic closure at higher temperatures and earlier times.

129I/127I variations among enstatite chondrites

Abstract This paper presents the 129I/127I composition for 11 different enstatite chondrites. There is a well-defined hiatus in iodine isotopic compositions separating the enstatite chondrites. The hiatus correlates with a similar hiatus defined by variations in chemical composition (both major and minor elements). EH(4,5) chondrites have 129I/127I ratios ∼20% higher (∼ 4 m.y. older) than EL(6) chondrites. The enstatite chondrites are the third meteorite group, joining silicates in IAB irons and C3V carbonaceous chondrites, in which 129I/127I correlates with chemical composition. Within each enstatite chondrite group, the variations in initial iodine isotopic composition is less than ~ ±4%, comparable to the experimental resolution. The ~20% difference between EH and EL may indicate a maximum iodine isotopic heterogeneity between the EH and EL nebular reservoirs. Alternatively, if EH and EL chondrites evolved from a homogeneous iodine isotopic reservoir, then the EL group must have evolved ~ 4.0 m.y. longer than the EH group and either in an environment with a much lower I/Xe ratio than presently observed or one which was open to the loss of radiogenic 129Xe. The latter case may imply a nebular environment and would therefore be consistent with the open system evolution required to produce the chemical hiatus. The duration of subsequent evolution in the high I/Xe environment must have been ≲1 m.y. and probably represents thermal metamorphism in two different parent bodies.