Quantitative laser-induced breakdown spectroscopy of potassium for in-situ geochronology on Mars

Laser-induced breakdown spectroscopy is explored for the development of an in-situ K–Ar geochronology instrument for Mars. Potassium concentrations in standard basaltic glasses and equivalent rock samples in their natural form are quantified using the potassium doublet at 766.49 and 769.90 nm. Measurement precision varies from 0.5 to 5.5 (% RSD) over the 3.63% to 0.025% potassium by weight for the standard samples, and little additional precision is achieved above 20 laser shots at 5 locations. For the glass standards, the quantification limits are 920 and 66 ppm for non-weighted and weighted calibration methods, respectively. For the basaltic rocks, the quantification limits are 2650 and 328 ppm for the non-weighted and weighted calibration methods, respectively. The heterogeneity of the rock samples leads to larger variations in potassium signal; however, normalizing the potassium peak by base area at 25 locations on the rock improved calibration accuracy. Including only errors in LIBS measurements, estimated age errors for the glasses range from approximately ± 30 Ma for 3000 Ma samples to ± 2 Ma for 100 Ma samples. For the basaltic rocks, the age errors are approximately ± 120 Ma for 3000 Ma samples and ± 8 Ma for 100 Ma samples.