PRIMITIVE XENON IN DIOGENITES AND PLUTONIUM-244-FISSION XENON AGES OFA DIOGENITE, A HOWARDITE, AND EUCRITES

The Pu-244-fission-Xe-136 retention ages of howardites, eucrites, and diogenites (HEDs) show that these meteorites have retained Xe since th ey were formed about 4500 Ma ago. For the Garland diogenite and the Mi llbilillie eucrite, we obtain fission Xe ages of 4525 +/- 40 Ma and 44 86 +/- 40 Ma, respectively. If Xe isotope data reported by other worke rs are also considered, we conclude that the monomict equilibrated euc rites Camel Donga, Juvinas, and Millbillillie formed about 40 Ma later than Pasamonte, a polymict unequilibrated eucrite. Stannern, a monomi ct equilibrated brecciated eucrite, yields a Pu-244-Xe-136 age of 4442 Ma. The K-40-Ar-40 retention ages fall, for most HEDs, into the 1000- 4000 Ma age range, indicating that Ar-40 is generally not well retaine d. The good retentivity for Xe of HEDs allows us to study primordial t rapped Xe in these meteorites. Except for Shalka, in which other autho rs 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, Ibbenbuhren, Shalka, and Tatahouine. We sh ow that Tatahouine contains two types of trapped Xe: a terrestrial con tamination acquired by an irreversible adsorption process and released at pyrolysis temperatures up to 800-degrees-C, and indigenous primord ial Xe released primarily between 800-degrees-C and 1200-degrees-C. Th e isotopic composition of this primordial Xe is identical to that prop osed earlier to be present in primitive achondrites and termed U-Xe or ''primitive'' Xe, but it has not been directly observed in achondrite s 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 outgas sing of the Earth) is related to it by a mass dependent fractionation favoring the heavier Xe isotopes. This primitive Xe is isotopically ve ry similar to solar Xe except for Xe-134 and Xe-136. Solar Xe appears to contain an enrichment of unknown origin for these isotopes relative to the primitive Xe.