⁴⁰Ar/³⁹Ar measurements in Apollo 16 and 17 samples and the chronology of metamorphic and volcanic activity in the Taurus-Littrow region

Abstract

Argon analyses are reported for neutron irradiated whole samples and mineral separates of Apollo 16 crystalline rock 68415, Apollo 17 basaltic rock 75055 and fragment 75083,3,3, breccia 76055, soil 74241 and orange glass from soil 74220, giving gas retention and exposure ages of these samples as well as information on the systematic variations of all Ar isotopes necessary to evaluate these ages. An age of 3.97 ±0.04 AE is determined for the metamorphosed breccia 76055 which probably dates the time of extensive metamorphism in the impact forming the Serenitatis basin. The moon was evidently bombarded by many large bodies about 3.95 AE ago, forming most of the major mare basins and smaller multi-ring basins over a period of less than 10⁸ years. A well-defined age of 3.78±0.04 AE obtained on a plagioclase separate of basalt 75055 and a less precise age of 3.70±0.09 for the basaltic fragment 75083,3,3 are evidence of flooding by basalt lava in the Taurus-Littrow region 3.7-3.8 AE ago. These ages are similar to the ages of some Mare Tranquilitatis basalts and do not extend the 3.1-3.9 AE range of lunar magmatic activity previously determined. The isotopic variations in Ar released from 74220 were complex, but separable into two distinctly different arrays at intermediate and high temperatures. A gas retention age of 3.54±0.05 AE was determined over the intermediate release containing 47% of the total ³⁹Ar*. The correspondence of this age with the period of lunar magmatism is suggestive evidence for a volcanic origin of the orange glass and possibly a significant fraction of the glass particles observed at other sites. The apparent age of 68415 is not well determined by the whole sample Ar release. A maximum apparent age of 3.86 AE in the intermediate release is similar to a precise Rb-Sr age of 3.84 AE. The apparent age plateau at 4.09 AE in the intermediate release from the plagioclase is atypically higher than the Rb-Sr age. Anomalous Ar isotopic variations are observed in the Ar release from both 68415 plagioclase and the high temperature release from the 74220 glass. Both have high Cl contents, and in both apparent ages are higher than anticipated from other criteria, suggesting inherited ⁴⁰Ar. The possibility of a complex multistage origin for 68415 must be investigated. Formation times of 95 m.y. ago for Camelot Crater and 30 m.y. ago for Shorty Crater are inferred from the cosmic-ray exposure ages of the associated samples 75055 and 74220, respectively. The exposure age for the soil fragment 75083,3,3 is 310 m.y., for the breccia 76055 is 140 m.y., and for the soil 74241 is 300 m.y. No well-defined exposure age could be determined for 68415.

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