When, and to what extent, Earth’s cratons became underpinned by thick lithospheric mantle roots capable of hosting diamonds is one of the most controversial aspects of Archean geology and geodynamics. Alluvial diamonds from Archean sediments can be used to address this argument. We report the recovery of 3 diamonds from gold-bearing Mesoarchean conglomerates in the Northern Slave craton, Canada, that allow us to provide constraints on the minimum age and thermal structure of its mantle root.
Accompanying zircons in the host metasediments show a restricted depositional U-Pb age of ~2.95 Ga and suggest sourcing from a small, proximal catchment basin and indicate a diamond age of >2.95 Ga. Nitrogen contents in the diamonds range from <10 up to 1770 atomic ppm N (Type IaA). The nitrogen systematics and a high-Mg olivine inclusion in one of the diamonds show these diamonds resided at 1060-1170°C and a pressure 5.5-5.9 GPa in the mantle along a cold geotherm of ~38 mW/m2. Thus, our data indicate that the lithospheric mantle keel beneath this portion of the craton had already cooled 3 billion years ago to a geotherm that is similar to the coldest geotherms beneath cratons today.