Evaluating mechanisms for eclogitic diamond growth: An example from Zimmi Neoproterozoic diamonds (West African craton)

Abstract Here we present SIMS data for a suite of Zimmi sulphide-bearing diamonds that allow us to evaluate the origin and redox-controlled speciation of diamond-forming fluids for these Neoproterozoic eclogitic diamonds. Low δ 13 C values below −15‰ in three diamonds result from fluids that originated as carbon in the oceanic crust, and was recycled into the diamond-stable subcratonic lithospheric mantle beneath Zimmi during subduction. δ 13 C values between −6.7 and −8.3‰ in two diamonds are within the range for mantle-derived carbon and could reflect input from mantle fluids, serpentinised peridotite, or homogenised abiogenic and/or biogenic carbon (low δ 13 C values) and carbonates (high δ 13 C values) in the oceanic crust. Diamond formation processes in eclogitic assemblages are not well constrained and could occur through redox exchange reactions with the host rock, cooling/depressurisation of CHO fluids or during H 2 O-loss from CHO fluids. In one Zimmi diamond studied here, a core to rim trend of decreasing δ 13 C (−23.4 to −24.5‰) and decreasing [N] is indicative of formation from reduced CH 4 -bearing fluids. Unlike mixed CH 4 -CO 2 fluids near the water maximum, isochemical diamond precipitation from such reduced CHO fluids will only occur during depressurisation (ascent) and should not produce coherent fractionation trends in single diamonds that reside at constant depth (pressure). Furthermore, due to a low relative proportion of the total carbon in the fluid being precipitated, measurable carbon isotopic variations in diamond are not predicted in this model and therefore cannot be reconciled with the 1‰ internal core-to- rim variation. Consequently, this Zimmi eclogitic diamond showing a coherent trend in δ 13 C and [N] likely formed through oxidation of methane by the host eclogite, although the mineralogical evidence for this process is currently lacking.