Identification of molybdenite in diamond-hosted sulphide inclusions: Implications for Re-Os radiometric dating

Sulphide inclusions are common features of natural diamonds. They can provide an insight into the nature of diamond-forming reactions and are especially important for Re-Os dating of diamond formation. A discrete molybdenite (MoS2) phase has been identified for the first time by Raman spectroscopy in 73 out of 80 syngenetic sulphide inclusions in 7 eclogitic diamonds from the Mir kimberlite (Yakutia, Russia). The sulphide inclusions were chemically and texturally characterised by electron probe microanalyses (EPMA), focused ion-beam scanning electron microscopy (FIB-SEM) and synchrotron-based X-ray tomographic microscopy (SXRTM). Our observations suggest the molybdenite has unmixed from an original sulphide melt or monosulphide solid solution. It occurs as sub-micron sized grains, commonly in association with the chalcopyrite rims of the inclusions and sometimes, within surrounding decompression cracks. Molybdenite has also been identified by Raman spectroscopy in at least 50% of sulphide inclusions in preliminary studies of eclogitic diamonds from Argyle (NW Australia), Orapa, Letlhakane, Damtshaa (Botswana) and Dachine (French Guiana), and peridotitic diamond-hosted inclusions from Udachnaya (Yakutia, Russia) and Murowa (Zimbabwe). We have modelled the effects that different amounts of Re loss - through its segregation into an unrecovered molybdenite phase - could have on the radiometric ages of diamonds dated using the Re-Os system. In general Re loss through this process will lead to isochron ages older than the true age, and variable degrees of Re loss will lead to increased scatter around the apparent isochron. For model age calculations, the effects would depend on the 187Re/188Os ratio of the inclusions (if their compositions evolved above or below that of the chondritic mantle evolution curve) but Re loss could generate unrealistically old or future ages, particularly in eclogitic inclusions.