Investigating metasomatic effects on the 187Os isotopic signature: A case study on micrometric base metal sulphides in metasomatised peridotite from the Letlhakane kimberlite (Botswana)

The TRD ages at the single BMS grain scale show a much larger range than their respective whole-rock TRD age, most notably from BMS in the two metasomatic end members: the cryptically-metasomatised peridotite and the most modally overprinted peridotite containing BMS 1 to 2.5 Ga older than their whole-rock. Base metal sulphides are thus better time capsules than the whole-rock, regardless of the silicate metasomatism the whole-rocks have experienced. Furthermore, the ancient ages are obtained from interstitial Ni-rich BMS indicating that all possible hosts of Os, not only BMS which appear to be 'primary' (i.e. Fe-rich BMS isolated inclusions in olivine) should be investigated. Single BMS grain Re-Os dating should then be considered as the only reliable dating approach, across all tectonic contexts (orogenic, ophiolitic, cratonic), which have experienced extensive metasomatic overprinting or refertilisation.