Dating igneous rocks using the Potassium-Argon Laser Experiment (KArLE) instrument: A case study for ~380 Ma basaltic rocks.

RATIONALE: We report new K-Ar isochron data for two ~380 Ma basaltic rocks, using an updated version of the Potassium-Argon Laser Experiment (KArLE), which is being developed for future in situ dating of planetary materials. These basalts have K contents comparable with those of lunar KREEP basalts or igneous lithologies found by Mars rovers, whereas previous proof-of-concept studies focused primarily on more K-rich rocks. We aim to measure these analogous samples to show the advancing capability of in situ K-Ar geochronology.

METHODS: Combining laser-induced breakdown spectroscopy (LIBS), mass spectrometry (MS), and microscopic analyses, we measured the abundance of K and 40 Ar from 23 spots on the basalt samples. We then constructed K-Ar isochron plots from these rocks. The breadboard instrument consists of flight-equivalent devices including a 30-mJ Nd:YAG laser and a quadrupole mass spectrometer.

RESULTS: Despite much lower K abundances than in previous studies, the isochron slopes yielded 380 ± 44 Ma and 398 ± 50 Ma for 380.7-Ma and 373.5-Ma rocks, respectively, indicating that accuracy better than 25 Ma (<7%) is achievable with our instrument. The isochron intercepts both yielded trapped 40 Ar approximately 1 × 10-6  cm3 STP/g.

CONCLUSIONS: Our experimental results demonstrate that accurate and precise measurements are possible using the KArLE approach on basaltic rocks, which are ubiquitous on planetary surfaces, and are useful in addressing a wide range of questions in planetary science.