Solar‐like trapped noble gases in the Brenham pallasite

Abstract

The isotopic composition of trapped neon in the Brenham pallasite is solar (20Ne/22Ne=12.9±0.3), with contributions from a cosmogenic component. The carrier of the solar neon is an acid‐soluble minor phase of the meteorite that is inhomogeneously distributed. There are excesses at 4He over the cosmogenic component, presumably radiogenic; the highest measured 4He, however, is lower than what is expected from bulk uranium and thorium concentrations. Trapped Ar is more retentively sited than the solar Ne component. Trapped Kr is isotopically similar to that in the terrestrial atmosphere, to solar, or to ureilite Kr with contributions from spallation and (n, γ)‐capture reactions on Br. The trapped Xe in this meteorite is solar, with about 20% atmospheric Xe. The heavy Xe isotopic ratios are modified by in situ decay of 244Pu, and the light Xe isotopic ratios by low‐energy proton induced production of these isotopes from Te. The presence of this component is an indication that a low‐energy proton environment prevailed in the early solar system. More detailed analysis of Brenham samples, in particular, determining whether the excesses at light Xe isotopes are correlated with trapped Xe or with Te content may provide clues to the conditions under which the proton irradiation took place. Overabundances at 129Xe from now extinct 129I are marginal. Since the trapping of noble gases necessarily preceded the formation of the meteorite itself, it is suggested that the trapped Ne isotopic composition as measured in Brenham represents the Ne isotopic composition of ancient solar gases. Elemental ratios suggest noble gases to be a mixture of “planetary” and solar components. The Kr and Xe isotopic compositions of Springwater pallasite are markedly modified by contributions from a cosmogenic component, and the light noble gases are dominated by spallation products. This meteorite does not show any evidence for trapped Ne, solar or otherwise.