Diamond-Bearing Metasediments Point to Thick, Cool Lithospheric Root Established by the Mesoarchean beneath Parts of the Slave Craton (Canada)

Abstract:

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, the minimum age of which can be constrained through U-Pb geochronology of accompanying detrital zircons, and whose mantle residence times can be broadly constrained through studies of their nitrogen content and aggregation state, can be used to address this argument. We report the recovery of 3 diamonds from gold-bearing conglomerates in the northern Slave craton, Canada, that allow us to provide firm constraints on the minimum age and thermal structure of part of its mantle root.

The three recovered diamonds (< 210 μm) have (cubo-)octahedral morphological features and have nitrogen contents ranging from <10 ppm (n=2) up to 1770 ppm N (Type IaA), the latter having positive δ¹⁵N (+3.1 to +4.4‰). Carbon isotope compositions of the 3 diamonds range from -3.5 to -0.3 ‰, similar to the heaviest values found in microdiamonds on the Slave Craton [1] and potentially indicating an oxidized source.

The Type IaA diamond contains a high-Mg olivine inclusion, representing a lithospheric origin. Single-crystal XRD applied to the olivine inclusion reveals a residual pressure exerted by the diamond host equal to 1.25-1.33 GPa. Assuming a mantle residence between 10 m.y. and 1 b.y., the diamond mantle residence temperature is 1059 to 1168 °C. When we apply equation of state constraints for olivine (Fo92 to 93) [2] and the above mantle residence temperature, we obtain a pressure of formation between 5.5 and 5.9 GPa (EosFit-Pinc software; [2]), implying mantle residence along a cool cratonic geotherm.

Parallel U-Pb studies on zircon recovered from the host metasediments (Vezinet et al., this meeting) show a restricted depositional age of ~2.95 Gy, suggesting sourcing from a small, proximal catchment basin. Whatever the size of the Archean cratonic nuclei acting as the source of the north Slave diamonds, our data indicate that the lithospheric mantle keel beneath this portion of the craton had already cooled to a geotherm equivalent to a surface heat flow of ~38 mW/m², analogous to the coolest of geotherms found beneath cratons today.

[1] Melton et al. (2013) Lithos 177, 110-119. [2] Angel et al. (2018) Phys Chem Miner 45, 95-113.