Cool transient lithospheric thermal conditions driven by subduction recorded by Neoarchean diamonds from Knee Lake, northwestern Superior Craton

Abstract

The Oxford Lake—Knee Lake greenstone belt, northwestern Superior Craton, Canada hosts an unconventional diamond occurrence within Neoarchean ultramafic volcaniclastic rocks. Unusual features of the diamond occurrence include its ancient emplacement age (2.73 Ga), nature of the host rocks and high microdiamond counts (> 11,500 recovered from 1800 kg of drill core). The diamonds show unusual physical characteristics relative to typical kimberlite-hosted diamonds, having small dimensions, a restricted size range, prevalent cuboid primary shapes and low levels of nitrogen aggregation. These characteristics, together with a statistically significant correlation between shape and nitrogen aggregation, point to a single, rapid growth event in a cool mantle substrate for most of the Knee Lake diamonds, driven by C-oversaturation of the growth medium. Pressure–temperature estimates of diamond formation from a non-touching garnet-olivine inclusion pair indicate the lithospheric nature of the diamonds and formation at conditions equivalent to a modern-day reference model geotherm of 38–39 mW/m². This lithospheric column also contained eclogite, indicated by an omphacite inclusion in one diamond. The diamonds resided for a short time in a cool lithospheric mantle environment, possibly lower than 1050 °C at 5.5 GPa. These cool P–T conditions are difficult to achieve within > 150 km thick lithosphere without invoking slab stacking, a process supported by the C isotope composition of some of the diamonds. The Knee Lake diamond suite likely records cool transient thermal conditions inherited from subduction stacking of lithospheric slabs to form the lithosphere of the western Superior Craton.