Application of 36/40Ar Versus 39Ar/40Ar Correlation diagrams to the 40Ar/39Ar spectra of phlogopites from Southern African kimberlites

40Ar/39Ar incremental heating analyses were performed on phlogopite mica from the DK‐7 kimberlite pipe in southern Botswana. No age plateau was obtained, as the spectrum yielded a step‐wise increase in age with increasing temperature. The ages appear to be anomalously high, suggesting the presence of excess radiogenic argon components that have been partially released by diffusion during cooling of the kimberlite.The pattern of data points on an 39Ar/40Ar vs. 39Ar/40Ar correlation plot, however, delineates two types of excess argon, one which is radiogenic‐rich, and one with a 40Ar/36Ar ratio of 366. The latter component may reflect the contamination of the kimberlite xenocrysts by fluid originating from an undegassed mantle source.