Primitive xenon in diogenites and plutonium-244-fission xenon ages of a diogenite,a howardite,and eucrites |
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| Authors: | Th Michel O Eugster |
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| Abstract: | Abstract The 244Pu-fission-136Xe retention ages of howardites, eucrites, and diogenites (HEDs) show that these meteorites have retained Xe since they were formed about 4500 Ma ago. For the Garland diogenite and the Millbillillie eucrite, we obtain fission Xe ages of 4525 ± 40 Ma and 4486 ± 40 Ma, respectively. If Xe isotope data reported by other workers are also considered, we conclude that the monomict equilibrated eucrites Camel Donga, Juvinas, and Millbillillie formed about 40 Ma later than Pasamonte, a polymict unequilibrated eucrite. Stannern, a monomict equilibrated brecciated eucrite, yields a 244Pu-136Xe age of 4442 Ma. The 40K-40Ar retention ages fall, for most HEDs, into the 1000–4000 Ma age range, indicating that 40Ar is generally not well retained. The good retentivity for Xe of HEDs allows us to study primordial trapped Xe in these meteorites. Except for Shalka, in which other authors found Kr and Xe from terrestrial atmospheric contamination only, we present for the first time Kr and Xe isotopic data for diogenites. We studied Ellemeet, Garland, Ibbenbühren, Shalka, and Tatahouine. We show that Tatahouine contains two types of trapped Xe: a terrestrial contamination acquired by an irreversible adsorption process and released at pyrolysis temperatures up to 800 °C, and indigenous primordial Xe released primarily between 800 °C and 1200 °C. The isotopic composition of this primordial Xe is identical to that proposed earlier to be present in primitive achondrites and termed U-Xe or “primitive” Xe, but it has not been directly observed in achondrites until now. This type of primitive Xe is important for understanding the evolution of other Xe reservoirs in the Solar System. Terrestrial atmospheric Xe (corrected for fission Xe and radiogenic Xe from outgassing of the Earth) is related to it by a mass dependent fractionation favoring the heavier Xe isotopes. This primitive Xe is isotopically very similar to solar Xe except for 134Xe and 136Xe. Solar Xe appears to contain an enrichment of unknown origin for these isotopes relative to the primitive Xe. |
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