Lawrence Berkeley National Laboratory

fsLIBS setup and molecular emisson (CaF and CaCl) used in this study. Halogens, including fluorine (F) and chlorine (Cl), play essential roles in magmatic, hydrothermal, and ore-forming processes. Determining their geochemical behaviors is challenging due to their low abundance and volatility. Apatite containing halogens serves as an ideal recorder of Earth's and planetary processes. This study presents a quantitative method using molecular emission spectra of calcium fluoride (CaF) and calcium chloride (CaCl) to measure F and Cl concentrations in apatite through femtosecond laser-induced breakdown spectroscopy (fs-LIBS). Reference materials with varying F and Cl levels were employed to optimize conditions and develop calibration curves for F (0.55–3.75 wt%) and Cl (0.45–4.26 wt%), with detection limits of 0.22 wt% and 0.38 wt%, respectively. The calibration curves achieved R 2 values of 0.995 for F and 0.992 for Cl, with RMSRE of 8% for F and 13% for Cl. Validation using natural apatite samples confirmed accuracy within ± 0.10 wt% compared to EPMA data. The fs-LIBS technique offers potential for accurate and precise measurement of F and Cl in minerals when reference materials are available.