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

摘要

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 30mJ and 15mJ energy lasers under a vacuum condition (10(-3)Pa) 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 777nm and continuum emission from the laser-induced plasma. Experimental results indicate that when K2O<1.1wt%, a calibration curve using the intensity of the K emission line at 769nm normalized with that of the oxygen line yields the best results for the 30mJ laser energy, with a detection limit of 88ppm and 20% of error at 2400ppm of K2O. Futhermore, the calibration curve based on the K 769nm line intensity normalized with continuum emission yielded the best result for the 15mJ laser, giving a detection limit of 140ppm and 20% error at 3400ppm K2O. Error assessments using obtained calibration models indicate that a 4 Ga rock with 3000ppm K2O would be measured with 8% (30mJ) and 10% (15mJ) of precision in age when combined with mass spectrometry of Ar-40 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.