Quantitative Potassium Measurements with Laser-Induced Breakdown Spectroscopy Using Low-Energy Lasers: Application to In Situ K–Ar Geochronology for Planetary Exploration

Abstract

In situ radiogenic isotope measurements to obtain the absolute age of geologic events on planets are of great scientific value. In particular, K–Ar isochrons are useful because of their relatively high technical readiness and high accuracy. Because this isochron method involves spot-by-spot K measurements using laser-induced breakdown spectroscopy (LIBS) and simultaneous Ar measurements with mass spectrometry, LIBS measurements are conducted under a high vacuum condition in which emission intensity decreases significantly. Furthermore, using a laser power used in previous planetary missions is preferable to examine the technical feasibility of this approach. However, there have been few LIBS measurements for K under such conditions. In this study, we measured K contents in rock samples using 30 mJ and 15 mJ energy lasers under a vacuum condition (10^–3Pa) to assess the feasibility of in situ K–Ar dating with lasers comparable to those used in NASA’s Curiosity and Mars 2020 missions. We obtained various calibration curves for K using internal normalization with the oxygen line at 777 nm and continuum emission from the laser-induced plasma. Experimental results indicate that when K₂O < 1.1 wt%, a calibration curve using the intensity of the K emission line at 769 nm normalized with that of the oxygen line yields the best results for the 30 mJ laser energy, with a detection limit of 88 ppm and 20% of error at 2400 ppm of K₂O. Futhermore, the calibration curve based on the K 769 nm line intensity normalized with continuum emission yielded the best result for the 15 mJ laser, giving a detection limit of 140 ppm and 20% error at 3400 ppm K₂O. Error assessments using obtained calibration models indicate that a 4 Ga rock with 3000 ppm K₂O would be measured with 8% (30 mJ) and 10% (15 mJ) of precision in age when combined with mass spectrometry of ⁴⁰Ar with 10% of uncertainty. These results strongly suggest that high precision in situ isochron K–Ar dating is feasible with a laser used in previous and upcoming Mars rover missions.