Primordial noble gases in separated meteoritic minerals—I

Abstract

Magnetite and a layer-lattice silicate from the Orgueil carbonaceous chondrite show strikingly different noble gas patterns. Magnetite contains He, Ne and Ar in essentially solar proportions, with some excess Kr and Xe. Isotopic ratios are typically solar: Ne 20 Ne 22 = 12.5 , He 3 He 4 = (3.8 ± 0.4) × 10 −4 ; but Xe 129 Xe 132 is quite high, 2.0 to 6.6. He 4 Ne 20 is 730, which is probably close to the true solar ratio. Silicate contains planetary gas, with Ne 20 Ne 22 = 8.2 , He 3 He 4 = (1.4 ± 0.4) × 10 −4 , and Xe 129 Xe 132 = 1.018 ± 0.003 . The Ne 21 radiation age of the magnetite, 10.7 m.y., is higher than that of the bulk meteorite, 4.5 m.y., which implies substantial loss of cosmogonic Ne 21 from silicate. Presumably the radiation ages of other Cl chondrites are too low by similar factors. The primordial component in magnetite is loosely bound, and may represent trapped solar wind. The gas in silicate is more tightly held and was probably absorbed from the solar nebula at 350 ± 50° K. Data on low-temperature solubilities of He, Ne and Ar suggest that the observed amounts and elemental fractionations of planetary gas can probably be explained by equilibrium solubility. The isotopic features of planetary gas require an additional process, such as production of Na 22 by charged-particle irradiation.