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

Abstract

Abstract-Several solar gas rich lunar soils and breccias have trapped 40Ar/36Ar ratios >lo, although solar Ar is expected to yield a ratio of <0.01. Radiogenic 40Ar produced in the lunar crust from 40K decay was outgassed into the lunar atmosphere, ionized, accelerated in the electromagnetic field of the solar wind, and reimplanted into lunar surface material. The 40Ar loss rate depends on the decreasing abundance of 40K. In order to calibrate the time dependence of the 40Ar/36Ar ratio in lunar surface material, the period of reimplantation of lunar atmospheric ions and of solar wind Ar was determined using the 235U-136Xe dating method that relies on secondary cosmic-ray neutron-induced fission of 235U. 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 235U-136Xe time of implantation of 3.252::; Ga ago. On the basis of several cosmogenic noble gas signatures we calculate the duration of this near surface exposure of 393 k 45 Ma and an average shielding depth below the lunar surface of 73 k 7 g/cm*.A second, recent exposure to solar and cosmic-ray particles occurred after this soil was excavated from Shorty crater 17.2 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 40ArP6Ar ratio. A possible trend for the increase with time of the solar 3He/4He and 20NeP2Ne ratios of about 12%/Ga and about 2%/Ga, respectively, is also discussed.

Cosmic-ray exposure age and heliocentric distance of the parent bodies of enstatite chondrites ALH 85119 and MAC 88136

“…They vary among the Apollo samples (0.42-7.4) (Hintenberger and Weber 1973;Hintenberger et al 1974), which suggests that the Apollo samples consist of soils exposed to SW at various times (∼0-3 Ga) (cf. Eugster et al 2001). Thus, the ( 36 Ar S / 21 Ne C ) L ratio deduced from the Apollo samples can be taken as the long-term average flux of solar gases.…”

Solar wind and other gases in the regoliths of the Pesyanoe parent object and the moon

“…3 to 6 include the following published soil data: bulk and grain size separates of 10084, Apollo 12 fines, grain size separates of Apollo 15 fines, and Luna 16 fines; bulk, grain size, and ilmenite separates of 12001, bulk and grain size separates of Apollo 14, 15, and 17, bulk 67701, ilmenite separate of 71501, metal separate of 68501, bulk Apollo 17 deep drill core, and bulk 74261. The following lunar regolith breccia data are also included: 10046, 14301, 14318, 14313, 60019, 60275, 67455, 60255, and bulk and ilmenite separates of 79035 (Podosek et al 1971;Drozd et al 1972;Bogard and Nyquist 1972;Basford et al 1973;Behrmann et al 1973;Bogard et al 1974;Reynolds et al 1974;Bernatowicz et al 1978;Swindle et al 1985;Frick et al 1988;Pepin 1989, 1994;Wieler and Baur 1994;Eugster et al 2001). All Xe literature data are renormalized and corrected for spallation Xe components following the same procedure adopted for Pesyanoe.…”

40Ar/39Ar and cosmic ray exposure ages of plagioclase-rich lithic fragments from Apollo 17 regolith, 78461

“…During the early lunar evolution (>4 Ga), a greater amount of 40 K was available to decay to 40 Ar in the lunar crust and this radiogenic 40 Ar escaped to lunar atmosphere (e.g., Eugster et al 2001). A fraction of the escaped 40 Ar was ionized and accelerated by solar wind.…”