R. J. Drozd

Cosmic ray exposure ages of features and events at the Apollo landing sites

Cosmic ray exposure ages of lunar samples have been used to date surface features related to impact cratering and downslope movement of material. Only when multiple samples related to a feature have the same rare gas exposure age, or when a single sample has the same81Kr-Kr and track exposure age can a feature be considered reliably dated. Because any single lunar sample is likely to have had a complex exposure history, assignment of ages to features based upon only one determination by any method should be avoided. Based on the above criteria, there are only five well-dated lunar features: Cone Crater (Apollo 14) 26 m.y., North Ray Crater (Apollo 16) 50 m.y., South Ray Crater (Apollo 16) 2 m.y., the emplacement of the Station 6 boulders (Apollo 17) 22 m.y., and the emplacement of the Station 7 boulder (Apollo 17) 28 m.y. Other features are tentatively dated or have limits set on their ages: Bench Crater (Apollo 12) ⩽99 m.y., Baby Ray Crater (Apollo 16) ⩽2 m.y., Shorty Crater (Apollo 17) ≈ 30 m.y., Camelot Crater (Apollo 17) ⩽140 m.y., the emplacement of the Station 2 boulder 1 (Apollo 17) 45–55 m.y., and the slide which generated the light mantle (Apollo 17) ⩾50 m.y.

Horizontal transport of the regolith, modification of features, and erosion rates on the lunar surface

New lunar soils, freshly deposited as impact ejecta, evolve into more mature soils by a complex set of processes involving both near-surface effects and mixing. Poor vertical mixing statistics and interregional exchange by impact ejection complicate the interpretation of soil maturization. Impact ejecta systematics are developed for the smaller cratering events which, with cumulative crater populations observed in young mare regions and on Copernicus ejecta fields, yield rates and a range distribution for the horizontal transport of material by impact processes. The deposition rate for material originating more than 1 m away is found to be about 8 mm m.y.−1 Material from 10 km away accumulates at a rate of about 0.08 mm m.y.−1, providing a steady influx of foreign material. From the degradation of boulder tracks, a rate of 5±3 cm m.y.−1 is computed for the filling of shallow lunar depressions on slopes. Mass wastage and downslope movement of bedrock outcroppings on Hadley Rille seems to be proceeding at a rate of about 8 mm m.y.−1 The Camelot profile is suggestive of a secondary impact feature.

Fission xenon from extinct 244Pu in 14301

Xenon extracted in step-wise heating of lunar breccia 14 301 contains a fission-like component in excess of that attributable to uranium decay during the age of the solar system. There seems to be no adequate source for this component other than 244Pu. Verification that this component is in fact due to the spontaneous fission of extinct 244Pu comes from the derived spectrum which is similar to that observed from artificially produced 244Pu. It thus appears that 244Pu was extant at the time lunar crustal material cooled sufficiently to arrest the thermal diffusion of xenon. Subsequent history has apparently maintained the isotopic integrity of plutonium fission xenon. Of major importance are details of the storage itself. Either the fission component is the result of in situ fission of 244Pu and subsequent storage in 14 301 material, or the fission xenon was stored in an intermediate reservoir before incorporation into 14301. In the former case accurate dating of this material is possible which would place formation of lunar crustal material early in primitive solar system history (nearly simultaneous with meteorite parent-body formation). In the latter case accurate dating is not possible, but the implied re-implantation from a source rich in 244fission xenon still demands that the moon was once primitive enough to contain large amounts of now extinct 244Pu and at the same time cool enough to prevent xenon isotopic mixing.

Primordial129Xe in meteorites

Abstract Xenon isotopic analyses by stepwise heating are presented for two neutron-irradiated chondrites, Arapahoe (L5) and Bjurbole (L4). The iodine-xenon formation age of Arapahoe is the oldest yet observed, 9.9 ± 0.8 m.y. before that of Bjurbole. It is thus unlikely that younger ages found in carbonaceous chondrite magnetite record the condensation of the solar nebula. The composition of trapped xenon in Arapahoe is normal except for a deficiency of129Xe, where we infer 129/Xe132Xe= 0.56 ∓ 0.04, well below the apparent primordial solar system value. This need not conflict with higher values in other metamorphosed meteorites since growth of129Xe from decay of129I in xenon-depleted environments can be substantial. The contrast with apparent average solar system composition cannot be easily explained, however, since there is no way to generate one composition from the other. The simplest way to achieve low129Xe seems to be to suppose that before decay to129Xe r-process production at mass 129 condensed into dust as129I, and that Arapahoes parent body formed in a region of the solar system substantially depleted of this dust before any isotopic homogenization by vaporization of the remaining dust. Arapahoe is not unique in having trapped129Xe-deficient xenon, nor in any other respect yet observed, so some such history evidently characterizes major groups of meteorites.

Extinct lunar radioactivities: Xenon from 244Pu and 129I in Apollo 14 breccias

Two Apollo 14 breccias have been found to contain xenon from the spontaneous fission of 82 my 244 Pu. A third contains 60 times as much fission xenon as local uranium can account for and is probably of similar character. One of the breccias shows a 129 Xe excess most likely due to the decay of 17 my 129 I. That these components can be separated and identified at all implies that complete isotopic homogenization has not occurred over a period which encompasses both extinction of these radionuclides and compaction of the breccias into their final form. In this sense isotope pre-history has been preserved in some lunar breccias providing information that pre-dates the formation of the rock itself (as determined by conventional techniques). The gases are not due to in situ decay. They appear to have been implanted in the surface crustal layer of the primitive Moon nearly simultaneously with production by 244 Pu and 129 I decay in the lunar interior. The combination of these two radionuclides, whose relative abundance is strongly time dependent, form the basis of a new, extremely sensitive, dating scheme that establishes the time of implantation.

Heavy rare gases from Rabbit Lake (Canada) and the Oklo mine (Gabon): natural spontaneous chain reactions in old uranium deposits

Xenon isotopic studies confirm natural spontaneous chain reactions at the Oklo mine site (Republic of Gabon) with an integrated neutron flux of approximately 4 × 1020n/cm2 and a duration between 14,000 and 70,000 years. Similar studies for the Rabbit Lake deposit (Saskatchewan), a slightly younger site with some evidence of continued geologic activity, shows no evidence of self-supporting reactions.