Diamonds from Fort à la Corne – post-Archean formation in exceptionally cool and fertile lherzolitic substrates

Abstract

Diamonds from the Star kimberlite at Fort à la Corne formed in an unusual substrate: 94% of inclusion-bearing diamonds derive from refertilized cratonic lherzolites, with olivine Mg# [molar 100 Mg/(Mg + Fe)] centered around a mode at 88.7. In addition, there is a minor eclogitic suite (6%) and a single sublithospheric diamond is most likely linked to oceanic crust subducted into the lower mantle. In addition to low Mg#, the refertilized lherzolitic association is characterized by low Ni contents, elevated V and Ti at normal Na contents, garnet rare earth element patterns very similar to primitive mantle garnet, and positive Nb anomalies. These characteristics are best explained by refertilization through a kimberlitic low degree partial melt. Single clinopyroxene-based geothermobarometry for inclusions in diamond and kimberlite-derived concentrate yields identical cold geotherms (equivalent to ~ 37 mW/m² surface heat flow), which implies that diamond formation occurred in a steady state thermal environment that did not change measurably from the time of diamond formation (1.26 Ga) to the time of kimberlite emplacement (0.10 Ga). Consequently, the refertilization event affecting the lherzolitic diamond substrates must have predated diamond formation. A further unique signature of the lherzolitic diamond association is its carbon isotope composition, with 97% of diamonds having δ¹³C values between − 18.0 and − 14.6‰. This constitutes the first observation of a peridotitic diamond suite dominated by subducted carbon, originating as organic matter or biogenic carbonates, instead of mantle-like carbon.