F.A. Podosek

40Ar-39Ar ages and cosmic ray exposure ages of Apollo 14 samples†

Abstract We have used the 40 Ar- 39 Ar dating technique on eight samples of Apollo 14 rocks (14053, 14310), breccia fragments (14321), and soil fragments (14001, 14167). The large basalt fragments give reasonable 40 Ar/ 39 Ar release patterns and yield well defined crystallization ages between 3.89–3.95 aeons. Correlation of the 40 Ar/ 39 Ar release patterns with 39 Ar/ 37 Ar patterns showed that the low temperature fractions with high radiogenic argon loss came from K-rich phases. A highly shocked sample and fragments included in the breccia yield complex release patterns with a low temperature peak. The total argon age of these fragments is 3.95 AE. Cosmic ray exposure ages on these samples are obtained from the ratio of spallogenic 38 Ar to reactor induced 37 Ar and show a distinct grouping of low exposure ages of ∼ 26 my correlated with Cone crater. Other samples have exposure ages of more than 260 my and identify material with a more complex integrated cosmic age exposure history.

Ages, Irradiation History, and Chemical Composition of Lunar Rocks from the Sea of Tranquillity

The 87Rb-87Sr internal isochrons for five rocks yield an age of 3.65 �0.05 x 109 years which presumably dates the formation of the Sea of Tranquillity. Potassium-argon ages are consistent with this result. The soil has a model age of 4.5 x109 years, which is best regarded as the time of initial differentiation of the lunar crust. A peculiar rock fragment from the soil gave a model age of 4.44 x 109 years. Relative abundances of alkalis do not suggest differential volatilization. The irradiation history of lunar rocks is inferred from isotopic measurements of gadolinium, vanadium, and cosmogenic rare gases. Spallation xenon spectra exhibit a high and variable 131Xe/126Xe ratio. No evidence for 129I was found. The isotopic composition of solar-wind xenon is distinct from that of the atmosphere and of the average for carbonaceous chondrites, but the krypton composition appears similar to average carbonaceous chondrite krypton.

Isotopic composition of xenon and krypton in the lunar soil and in the solar wind

Xe and Kr analyses of three lunar samples, the Murray meteorite and Xe from the terrestrial atmosphere are presented. The isotopic compositions of surface-correlated (solar wind?) and lunar soil spallation xenon and krypton are derived from the lunar soil data alone. The lunar soil spallation Xe is similar to that in lunar rocks and meteorites, but the lunar soil spallation Kr has higher (84Kr/83Kr) and82Kr/83Kr). We have no adequate explanation for this Kr spectrum, although independent evidence for such a component comes from stepwise heating data. The surface-correlated Xe (SUCOR) is distinct from both AVCC and terrestrial Xe. However, SUCOR Xe cannot be directly identified with the solar wind, but may contain an admixture of gases from the lunar atmosphere implanted on the grain surfaces by ion pumping processes. The general fractionation trend in SUCOR Xe relative to the atmosphere presumably reflects the solar wind composition. SUCOR Kr appears to be totally ascribable to the solar wind. Solar wind and terrestrial Kr are related by fractionation, but opposite to that of Xe.

129I and244Pu abundances in white inclusions of the Allende meteorite

Abstract Data are presented for thermal-release xenon analysis of a neutron-irradiated sample of white inclusions from the carbonaceous chondrite Allende. The best-fit high-temperature 129 Xe/ 132 Xe vs. 128 Xe/ 132 Xe isochron defines an iodine-xenon age 2.4 ± 1.3 my before that of the chondrite Bjurbole, well within the previously observed range of meteoritic iodine-xenon ages. The high-temperature data are not strictly consistent with the linear isochron model, however, and are best interpreted as defining a spread of about 4 my, corresponding either to slow cooling or to separate formation of phases before aggregation. The high-temperature data also define a good correlation between 130 Xe/ 132 Xe and 134 Xe/ 132 Xe, determining a ratio 244 Pu/ 238 U = 0.087 ± 0.011 at the time the sample began to retain xenon. This ratio is nearly six times higher than that found in the chondrite St. Severin. Comparison with the iodine chronology indicates that this result cannot be interpreted as very early gas retention, but must be ascribed to chemical fractionation between Pu and U in the formation of these inclusions. Results are also presented for a partial thermal-release xenon analysis of the neutron-irradiated carbonaceous chondrite Karoonda. Agreement of these results with those of a previous experiment is significant because Karoonda is the first case in which it has been possible to test the reproducibility of the I-Xe method by comparison of duplicate analyses.

Thermal history of the nakhlites by the40Ar-39Ar method

Abstract This paper reports the results of thermal-release argon analyses of neutron-irradiated samples of the two nakhlite meteorites, Lafayette and Nakhla. The initiation of retention of radiogenic 40 Ar in Lafayette appears to have been a reasonably well-defined event which occurred (1.33 ± 0.03) × 10 9 yr ago, as determined by the 40 Ar- 39 Ar method. Nakhla also appears to have been retaining argon no longer than 1.3 × 10 9 yr, but its gas-retention age cannot be considered well-defined because its apparently most-retentive sites have nominal gas-retention ages shorter than those of the less-retentive sites which contain most of its potassium.

Argon 40-argon 39 chronology of four calcium-rich achondrites☆

Abstract Results are presented from thermal-release argon 40-argon 39 dating experiments on four calcium-rich achondrites. Pasamonte shows an apparent age of 4.1 AE over 70 per cent of its gas release indicating a degassing event at this age. At higher temperatures, the apparent age for Pasamonte rises to 4.51 AE, defining a lower limit gas retention age which is identical to that of the chondrites St. Severin and Guarena. Petersburg has a high-temperature gas retention age of 4.40 ± 0.03AE, 100 million years younger than St. Severin, in agreement with previously reported Pu-Xe and K-Ar gas retention ages and the absence of Xe 129 from the decay of I 129 . Stannern has a complicated pattern of apparent age with gas release which cannot be accounted for in detail by simple diffusive loss following a well-defined initiation of gas retention. In broad scale, however, the Stannern pattern is relatively flat and confined to the range 3.5–3.9 AE, indicating that this meteorite was strongly degassed in this interval. The nakhlite Lafayette has an exceedingly young gas-retention age in the range 1.4–1.7 AE. These results indicate that as a group the calcium-rich achondrites have experienced a more diverse and extensive thermal history than have the chondritic meteorites.

Gas retention chronology of Petersburg and other meteorites

Abstract Argon and xenon data are presented for a thermal release study on a neutron-irradiated sample of the eucrite meteorite Petersburg. Xenon spallation corrections are made by the method of correlation systematics, and the relationship of lunar systematics to the systematics derived for the Angra dos Reis meteorite is discussed. Correlation systematics are also used in reevaluation of neutron-activation xenon data for other meteorites in which spallation effects are prominent. Petersburg has no excess Xe129 attributable to in situ decay of I129, and a Pu 244 U 238 ratio corresponding to onset of xenon retention 146 ± 14 million years after the chondrite St. Severin. The argon data show substantial loss of radiogenic Ar40 and do not define an Ar40-Ar39 plateau, establishing a lower limit K-Ar age of 4.35 × 109 yr, relative to an assumed age of 4.60 × 109 yr for St. Severin. Comparison with strontium data for other eucrites and the chondrite Guarena suggests an interval of 220 million years between fractionation from a rubidium-rich reservoir and the final cooling of Petersburg. The calcium-rich achondrite Lafayette has no detectable decay products of either I129 or Pu244, indicating a gas-retention formation time at least 350 million years after St. Severin. The current best value of the Pu 244 U 238 ratio in the chondrite St. Severin at the time of its formation is 0.0154 ± 0.0014, 21 per cent higher than previously reported.

Argon in Apollo 15 green glass spherules (15426):40Ar39Ar age and trapped argon

Abstract We report the results of thermal-release argon analyses of neutron-irradiated green glass spherules separated from lunar sample 15426. The gas-retention age, as determined by the 40 Ar 39 Ar method, is (3.38 ± 0.06) X 10 9 yr . This age is similar to those of local mare basalts and distinct from the ages of Appenine Front samples recovered from the same region as 15426. Trapped argon is present in near-surface regions of the spherules, and can be resolved into at least two components requiring separate origins, a shallow (0.1 μ) component with 40 Ar/ 39 Ar > 30, and a deeper (2 μ) component with 40 Ar/ 36 Ar= 2.9 . The ratio of trapped 40 Ar to 36 Ar is higher than found in any lunar soil and suggests that the trapped gas was implanted early in the spherules history. The cosmic-ray exposure age is 300 my.

Gas retention and cosmic-ray exposure ages of a basalt fragment from Mare Fecunditatis.

Abstract An40Ar-39Ar gas retention age and an38Ar-37Ar cosmic ray exposure age have been determined on a total rock sample of the basalt fragment B-1 returned from Mare Fecunditatis by the Luna 16 mission. This sample shows large low-temperature loss of radiogenic40Ar but defines a reasonably good high-temperature plateau at 3.45 ± 0.04 AE. This is presumed to date a period of igneous activity in Mare Fecunditatis. This activity is later than that at Mare Tranquillitatis but earlier than at Oceanus Procellarum and Mare Imbrium. The cosmic ray exposure age is 475 m.y.

Rare gas studies of the galactic cosmic ray irradiation history of lunar rocks

Abstract Eight analyses of all five noble gases in whole rock samples and mineral separates from lunar rocks 10017, 10044 and 10069, in conjunction with available literature data, permit qualitative conclusions concerning average irradiation depths and enable internally consistent exposure ages for Tranquillity Base rocks to be calculated. Correlated variations in He 3 Ne 21 and Ar 38 Ne 21 reflect shielding differences. Diffusion losses cannot explain this correlation, even though evidence for gas loss is obtained from mineral separate data. A qualitative shielding sequence can be constructed which agrees reasonably well with the sequence based on Gd isotopic data. There are no systematic differences between meteoritic and lunar He, Ne and Ar spallation spectra, with the possible exception of Ar 38 Ne 21 . Observed variations in meteoritic and lunar Xe spallation spectra, except at mass 131, are due primarily to differences in chemical composition. The relative Xe126 production rates from Ba and rare earths can be derived. Resonance neutron capture on Ba130 is a plausible source for the anomalous Xe131 observed in lunar rocks; however, the required fluxes are quite large. Surface production rates from measurements of radioactive spallation products in lunar samples give more concordant exposure ages than those obtained from meteoritic production rates. This concordance implies that the average irradiation depths for these rocks were small (≲150 g/cm2). Surface production rates for Xe126 and Ne21 yielded exposure ages for fourteen Tranquillity Base rocks which are in reasonable agreement for most rocks. Five low-K rocks have exposure ages around 100 m.y. in what may be a significant grouping. At least four major impacts are required to produce the fourteen rocks.