High resolution argon analysis of neutron-irradiated Apollo 16 rocks and separated mineral

Abstract

Ar analyses are reported for detailed thermal release studies on neutron-irradiated samples of mineral separates and a whole sample of breccia 65015 and rock fragments 67483,15-2 and 60503,3-3. The ³⁷Ar release patterns show a single release peak at ~800°C for 65015 plagioclase and a distinctive release peak at ~1200°C for 65015 pyroxene. 60% of the ³⁷Ar is released from the pyroxene above 1000°C, whereas only 15% of the 39 Ar is released above this temperature. From a graph of ⁴⁰Ar/³⁹Ar ages versus ³⁷Ar, the anomalously low ages of high-temperature releases, which are a common feature in ⁴⁰Ar/³⁹Ar age spectra of lunar samples, can be associated with the unique high-temperature release of ³⁷Ar from the pyroxene. The age spectrum of 65015 plagioclase plotted versus ³⁷Ar shows a plateau at 3.98 AE and then rises with a sharp ascent to a maximum of 4.47 AE for the last stages of gas release that correlates with the proportion of ancient relict plagioclase as determined by Rb-Sr analyses. This confirms the existence of ancient lithic fragments within this metaclastic rock and indicates that the protolith of 65015 contained plagioclase fragments older than 4.47 AE and the 65015 breccia was severely metamorphosed in an event at 3.98 AE. The time interval between this metamorphic event and the event dated by Luna 20 samples (neutron-irradiated with 65015) is 80 m.y. These events occurred at distinctly separate times and presumably represent the times of formation of the Nectaris and Crisium basins, respectively. The terminal lunar cataclysm involved at least two separate major impact events.