Argon Stable Isotope Concentrations in Lunar Regolith

We performed stepwise heating experiments for determination of the two stable isotope ratios of argon fractions and total concentrations of the three stable isotopes ³⁶Ar, ³⁸Ar and ⁴⁰Ar in lunar regolith acquired from the Apollo 11, Apollo 12 and Apollo16 missions. Also the concentration of in situ formed radiogenic ⁴⁰Ar was estimated on the basis of known ages and potassium concentrations determined by isotope dilution method. The observed excess of ⁴⁰Ar concentration is interpreted to be due to variable (over geological time) flux of solar energetic particles which were implanted into the material at the Moon surface.