40Ar/39Ar dating of the UG-2 Vein of the Bushveld Complex (South Africa): The diversity of spectra.

The UG-2 vein (upper critical zone) is one of the prominent Pt-group enrichment zones in the Bushveld Complex (South Africa). Eleven individual phlogopite crystals were degassed incrementally with a CO2 laser in 30 to 40 steps each. The apparent age spectra display widely varying characteristics, ranging from essentially 100% concordant to strongly discordant (in some cases undulatory). All discordant spectra display common features; 1) apparent low age in the first 10 to 20% of the spectrum; 2) higher ³⁷Ar_Ca/³⁹Ar_K ratio (0.4 to 0.2). These spectra are good examples of the variable effects of ³⁹Ar recoil during irradiation in interlayer chlorite as observed in other studies of Fe-Mg micas (Lo and Onstott, 1989). In such spectra, plateau-like segments (more than 50% of the ³⁹Ar released) can be found even in highly discordant spectra and the more discordant spectra coincide with the oldest plateau "ages" (2106 +/- 12 Ma; analytical errors only). In such cases the integrated ages are better clustered than plateau ages. The least discordant spectra (90 to 100% of the ³⁹Ar released) yield integrated and plateau ages ranging between 2034 and 2047 Ma (based on Steiger and Jager (1977) decay constants and 28.02 Ma for the FCs standard; Renne et al., 1998), slightly younger than existing constraints from U/Pb dating (Ca. 2059 Ma, Buick et al., 2001). If the recently-inferred ca. 1% bias between currently used calibrations of the ⁴⁰Ar/³⁹Ar and U/Pb systems in valid, this implies a very rapid cooling rate for this part of the Bushveld Complex as proposed by Cawthorn and Walraven (1998). Nevertheless the possibility of subtle bias due to excess ⁴⁰Ar and unobviated recoil artifacts must be considered. In all cases, the heterogeneous age spectra reveal the likelihood for erroneous conclusions to be drawn from less complete data sets. The observed complexities also underscore the stringent requirements for data which can be used objectively to constrain the inter-system bias between U/Pb and ⁴⁰Ar/³⁹Ar.