Studies of K-Ar dating and xenon from extinct radioactivities in breccia 14318; implications for early lunar history

Abstract

The rare gases argon and xenon were studied intensively in lunar breccia 14318, one of a family of three Apollo 14 breccias exhibiting similarities, including substantial amounts of ‘parentless’ xenon from the spontaneous fission of extinct 244Pu. We made stepwise heatings on both unirradiated and pile-irradiated specimens. The isotopic composition of the xenon from fission was determined by a new method which invokes a minimum of assumptions; it is shown to be from 244Pu and almost certainly parentless. For example, the fission component, although not appreciably fractionated with respect to the trapped component during stepwise heating, has a low temperature character so that, relatively speaking, it appears to be more surficial than xenon emanating from uranium sites in the irradiated sample. We demonstrate that this effect is not an artifact of the neutron irradiation. The breccia contains abundant trapped argon with a high 40Ar 36Ar ratio for lunar material—~14. Otherwise the argon is radiogenic and gives a convincing K-Ar age of 3.69 ± 0.09 b.y. by the stepwise 40Ar- 39Ar method, nearly in agreement with ages for other Apollo 14 breccias obtained in our laboratory and elsewhere. Rock 14301, another of the family of breccias and one which has been studied in other laboratories, contains similar trapped argon and parentless xenon. Unlike 14318 it also contains a conspicuous excess of 129Xe from the radioactive decay of extinct 129I. Implications of the parentless xenon from extinct sources, as seen in these different rocks, depend upon the model adopted for its evolution and storage. We present four different models, all of which are unsatisfactory in some respects, so that we are presently unable to narrow the question. We must stress that other Apollo 14 breccias, such as 14321, contain fission xenon from 244Pu which was apparently produced in situ.