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.