Charles M. Hohenberg

Xenon and other noble gases in shergottites

We have determined the isotopic composition of the xenon component trapped in EETA 79001 lithologies B and C, which we refer to as SPB-xenon. SPB-xenon is isotopically distinct from known xenon reservoirs, but differs in regular fashion. Normalized to 132Xe, the light isotope ratios are indistinguishable from air, the 129Xe/132Xe ratio is about 2.4, and 134Xe and 136Xe are enhanced relative to the terrestrial atmosphere or AVCC. The apparent heavy-isotope enrichments are not generated by in situ fission and there is no spectral evidence for the presence of 244Pu fission xenon. However, the xenon composition does match that of fractionated AVCC except at 129Xe, and consequently may be derived from or related to that component. ALHA 77005, Shergotty and EETA 79001 lithology A also have enhanced 129Xe/132Xe ratios in most temperature steps, and are seemingly consistent with varying mixtures of SPB-xenon and terrestrial xenon. Our results for neon and argon in EETA 79001 confirm earlier results on the exposure ages. We have also verified that the trapped 38Ar/36Ar ratio in lithology C is apparently substantially different from the terrestrial or meteoritic value. Krypton in EETA 79001,C is more fractionated with respect to AVCC than is terrestrial krypton and in the opposite direction as xenon. EETA 79001,C contains excess 80Kr (and perhaps 82Kr and 128Xe), presumably from neutron capture on bromine and iodine, but these neutron captures do not appear to have occurred by in situ processes.

Verification and interpretation of the I-Xe chronometer

Abstract Comparisons of I-Xe and Pb-Pb relative ages are made for phosphate and feldspar separates from 12 different meteorites. In all cases where I-Xe ages can be measured in phosphate, I-Xe and Pb-Pb chronometers agree within experimental uncertainty. No discordant phosphate samples are observed. With the exception of Allegan, I-Xe ages for feldspar separates generally agree with Pb-Pb ages for the corresponding phosphate. The general concordancy observed between I-Xe and Pb-Pb chronometers suggests that the I-Xe system is a reliable and interpretable chronometer when applied to single minerals systems. Allegan feldspar differs from the other feldspar separates, predating its phosphate by ∼17 Ma, with an I-Xe age indistinguishable from those of Orgueil and Murchison magnetites, among the oldest observed.

Origin and history of impact-melt rocks of enstatite chondrite parentage

We have conducted petrologic, chemical, and isotopic studies of two impact-produced rocks of enstatite chondrite parentage. Ilafegh 009 is a total impact-melt rock with no residual lithic clasts. Formation on the EL chondrite parent body is suggested by its mineralogy and mineral compositions. Cooling of the impact melt was rapid at melt temperatures and decreased at subsolidus temperatures. In contrast to previous studies, we show that Happy Canyon is not a new enstatite achondrite but an impactmelt breccia of enstatite chondrite (and not aubrite) parentage. This rock formed by impact melting and incorporation into the melt of clastic material (which resulted in relatively rapid cooling at all temperatures). Mineralogical and bulk compositional data (probably biased by the heterogeneous nature of this rock) do not allow unequivocal determination of its parent body (i.e., EL vs. EH), although some data such as bulk total Fe content seem to favor EL parentage. Both rocks were subjected to post-solidification shock, which was more severe for Ilafegh 009 than for Happy Canyon. It appears that both impact melt rocks could have formed by impact melting ∼4.57 Ga ago, as is indicated by the nearly identical IXe closure ages of 1.6 and 1.4 Ma before Bjurbole for Ilafegh 009 and Happy Canyon, respectively. An apparently younger 39Ar40Ar age of 4.53 Ga for Happy Canyon may be due to small biases in the intercalibration of the IXe and 39Ar40Ar chronometers, whereas the much younger 4.34–4.44 Ga age for Ilafegh 009 reflects thermal resetting during shock metamorphism. Shallowater, which was impact-derived from a different enstatite achondrite parent body, has an IXe closure age 0.4 Ma younger than that for Ilafegh 009 and an 39Ar40Ar age of 4.53 Ga. The ancient ages of these three rocks attest to the intense, early bombardment in this region of the solar system.

Xenon spallation systernatics in Angra dos Reis

We have resolved literature Xe data for the Angra dos Reis meteorite into constituent spallation, fission and trapped components. The spallation Xe compositions vary over a range wider than observed in any other samples, including lunear samples. These variations are due to the mixing of spallation Xe from Ba and rare earth element targets. It is possible to infer the Ba and rare earth spallation Xe compositions. Angra dos Reis spallation Xe compositions are systematically different from those observed in lunar samples, possibly because of differences in the irradiation conditions (geometry and shielding). Thus the Angra dos Reis data appear to be superior to lunar data for predicting spallation Xe compositions in other meteorites.

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.

Iodine-xenon studies of petrographically and chemically characterized Chainpur chondrules

Abstract We have performed petrographie, instrumental neutron activation analysis (INAA) and noble gas studies on samples of 18 chondrules and matrix from the Chainpur (LL3) meteorite to study variations in R0, the ratio of 129I to 129I at the time of xenon isotopic closure. R0 varies by more than a factor of 10 among the chondrules, corresponding to a span of more than 50 Ma in apparent I-Xe ages, including the latest apparent I-Xe ages ever observed for a chondritic sample. Variations are not closely related to any petrographie properties, although low values of R0 (late apparent ages) may be associated with high sulfide abundances and/or non-porphyritic textures. Similarly, R0 is not closely related to any chemical components, but does seem to correlate with abundance of refractory lithophile elements. Also, R0is correlated with 244 Pu 238 U and anticorrelated with trapped 129 Xe 132 Xe , as might be expected if the variations in iodine isotopic composition are dominated by decay of 129I. We have not found a completely satisfying explanation of the variations in R0. Models involving gas-dust mixing or nebular heterogeneity cannot satisfactorily explain the Chainpur data. However, there are also difficulties with explanations attributing the variations to differences in formation age, metamorphic age, or time of aqueous alteration. We believe the most plausible explanation is that the variations represent times of low-grade shock events.

Noble gas retention chronologies for the St Séverin meteorite

Abstract 40 Ar- 39 Ar and 129 Xe- 128 Xe analyses were performed on two lithologies (light and dark) of the St Severin (LL6) chondrite. For the light and dark fractions, respectively, we obtained 40 Ar retention ages of 4.38 and 4.42 AE and 129 Xe retention ages of 8.4 and 15.2 myr after Bjurbole. The two methods give age differences of opposite sense, and by both methods the differences are significant. Both the 40 Ar and the 129 Xe ages are interpreted as dating relaxation of metamorphic conditions. These two chronometers are decoupled, however, and do not date the same events. 40 Ar- 39 Ar reflect chondritic metamorphisrn on a 10 8 yr time scale. The 129 Xe- 128 Xe ages reflect isotopic closure at higher temperatures and earlier times.

I-Xe studies of the Acapulco meteorite: Absolute I-Xe ages of individual phosphate grains and the Bjurböle standard

We have measured Kr and Xe by laser volatilization of seventeen individual neutron-irradiated phosphate grains (nine apatite and eight merrillite) separated from the (reclassified) A-chondrite Acapulco. Radiogenic 129Xe from now-extinct 129I is only observed in the apatites, which all formed simultaneously (8.1 ± 1.2 Ma) after Xe closure in the Bjurbole (L4) standard. This relative closure time, when coupled with the Pb-Pb age of Acapulco phosphates (4.557 ± 0.002 Ga) provides an absolute I-Xe age for these apatites, for Bjurbole and for all samples previously referenced to the Bjurbole standard, subject to the assumptions implicit in I-Xe dating. We have also measured Kr and Xe by stepwise heating of irradiated and unirradiated whole-rock samples of Acapulco and of unirradiated phosphate concentrates. Iodine-derived xenon in the irradiated whole-rock sample does not yield an I-Xe isochron, consistent with the presence of multiple iodine host phases previously observed. Xenon from the spontaneous fission of now-extinct 244Pu is observed in the single phosphate grains and the inferred initial (244Pu/238U)0 ratios reflect fractionation of the actinides favoring plutonium in the merrillites and uranium in the apatites.

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.

Cosmic-ray exposure history at the Apollo 16 and other lunar sites. Lunar surface dynamics

Abstract Concordant 81 K-Kr exposure ages for four station 11 breccias indicate an age of 50.3 ± 0.8m.y. for North Ray Crater. Ray structures visible from orbital photography suggest that stations 8 and 9 should contain a substantial amount of South Ray ejecta. Concordant 81 Kr-Kr exposure ages at these sites indicate an age for South Ray Crater of 2.04 ± 0.08m.y. Surface effects (tracks, surface angularities, and micro-crater populations) show good general agreement with this young an age, but discrepancies on a sample-by-sample basis seem to indicate that extensive pre-surface irradiations must have occurred. A detailed pre-surface exposure history is derived for the parent boulder of samples 69935 and 69955. It is suggested that secondary impacts play a major role in near-surface regolithic stirring. Widespread pre-surface irradiation would in fact be expected if most of the newly excavated material had been transported to the surface by secondary impacts rather than by the South Ray event itself.