The antiquity indicator argon‐40/argon‐36 for lunar surface samples calibrated by uranium‐235‐xenon‐136 dating

Several solar gas rich lunar soils and breccias have trapped 40 Ar/ 36 Ar ratios >10, although solar Ar is expected to yield a ratio of <0.01. Radiogenic 40 Ar produced in the lunar crust from 40 K decay was outgassed into the lunar atmosphere, ionized, accelerated in the electromagnetic field of the solar wind, and reimplanted into lunar surface material. The 40 Ar loss rate depends on the decreasing abundance of 40 K. In order to calibrate the time dependence of the 40 Ar/ 36 Ar ratio in lunar surface material, the period of reimplantation of lunar atmospheric ions and of solar wind Ar was determined using the 235 U‐ 136 Xe dating method that relies on secondary cosmic‐ray neutron‐induced fission of 235 U. We identified the trapped, fissiogenic, and cosmogenic noble gases in lunar breccia 14307 and lunar soils 70001‐8, 70181, 74261, and 75081. Uranium and Th concentrations were determined in the 74261 soil for which we obtain the 235 U‐ 136 Xe time of implantation of 3.25 +0.38 ‐0.60 Ga ago. On the basis of several cosmogenic noble gas signatures we calculate the duration of this near surface exposure of 393 ± 45 Ma and an average shielding depth below the lunar surface of 73 ± 7 g/cm 2 . A second, recent exposure to solar and cosmic‐ray particles occurred after this soil was excavated from Shorty crater 17.2 ± 1.4 Ma ago. Using a compilation of all lunar data with reliable trapped Ar isotopic ratios and pre‐exposure times we infer a calibration curve of implantation times, based on the trapped 40 Ar/ 36 Ar ratio. A possible trend for the increase with time of the solar 3 He/ 4 He and 20 Ne/ 22 Ne ratios of about 12%/Ga and about 2%/Ga, respectively, is also discussed.