Victor diamonds – replacement of carbon and nitrogen in the central Superior Craton mantle root associated with the Midcontinent Rift event

Abstract

We studied the δ¹³C-δ¹⁵N-[N] (with [N] = N concentration) characteristics of 129 diamonds from the Victor Mine, Canada, to constrain the mode of their formation and the source of their C and N. In addition, we provide new inclusion chemistry data (n = 22) principally from sulfide-bearing diamonds (n = 19). High Ni (> 18 wt%) and detectable Cr (> 0.06 wt%) clearly distinguish peridotitic from eclogitic sulfides in Victor diamonds. Coexisting silicate inclusions confirm the lherzolitic affinity of peridotitic sulfide-bearing diamonds. The C isotope characteristics of Victor peridotitic diamonds mostly fall within the typical mantle range (–5.0 ± 2‰) and N isotopes, despite a large range, also have a mantle-like mode (–7.2‰). Eclogitic diamonds are more variable in their δ¹³C and δ¹⁵N values and account for the most extreme values (δ¹³C down to –23‰; δ¹⁵N up to + 3.2‰) of the distribution. Core-rim trends of decreasing N content associated with variations in δ¹³C and δ¹⁵N are observed in a number of plates cut from peridotitic diamonds. These covariations can be modelled as diamond precipitation in a fluid-limited system, associated with Rayleigh fractionation, from both reducing (CH₄-bearing) and oxidizing (CO₂ or CO₃²⁻-bearing) fluids. Variations in δ¹³C, however, are typically close to analytical precision and the modelled fractionation trends suggest the unexpected operation of two distinct diamond-fluid N isotope fractionation factors (positive and negative). While our models are permissive of fractionation processes, they cannot provide proof for their operation and alternative interpretations, involving fluid evolution for reasons unrelated to diamond formation, cannot be excluded.