40Ar/39Ar age constraints on ore deposition and cooling of the Bushveld Complex, South Africa

The UG-2 chromitite layer in the upper critical zone of the Rustenburg Layered Suite is one of the prominent platinum-group element enrichment zones of the c. 2.06 Ga Bushveld Complex (South Africa). Eleven biotite crystals were selected from a production concentrate from the Impala Platinum mine and degassed incrementally with a CO2 laser. The apparent age spectra display widely varying characteristics, ranging from essentially 100% concordant to strongly discordant (in some cases undulatory) as a result of various degrees of chloritization of the grains. The least discordant spectra (seven grains, 90-100% 39Ar released) yield integrated and plateau ages ranging from 2037 to 2049 Ma (relative to the Fish Canyon sanidine standard at 28.02 Ma) and share a weighted mean age of 2042.4 +/- 3.2 Ma (2sigma, excluding systematic errors). This age is slightly younger than existing constraints from U-Pb dating (c. 2055-2061 Ma), which seem to indicate a cooling rate of around 20 degreesC/Ma. However, the recently inferred c. 1% bias between currently used calibrations of the 40Ar/39Ar and U-Pb systems implies a very rapid cooling rate for the Bushveld Complex between 700 and <500 degreesC. The possibility of subtle bias as a result of recoil artefacts must be considered. In all cases, the heterogeneous age spectra yielded older apparent plateau ages, revealing the likelihood for erroneous conclusions to be drawn from less complete data sets.