N-δ 13 C-inclusion profiles of cloudy diamonds from Koffiefontein: Evidence for formation by continuous Rayleigh fractionation and multiple fluids

Abstract

Six diamonds with a fibrous core, intermediate zone and monocrystalline outer zone (“cloudy diamonds”) from the Koffiefontein mine, South Africa, were investigated for N concentrations, carbon isotope compositions and micro-inclusion compositions along core to rim traverses. This study evaluates the nature of the change from fibrous to gem diamond growth and the relation between major element composition of high density fluid inclusions and N − δ¹³C in fibrous growth zones. Three diamonds contain saline to carbonatitic fluid micro-inclusions with constant or increasing carbon isotope values which are inferred to have formed by varying amounts of Rayleigh fractionation in a closed system of a carbonate-bearing fluid. Continuous N − δ¹³C fractionation trends from the fibrous to gem growth zone in two of the diamonds and equally low nitrogen aggregation states indicate formation of diamond shortly before kimberlite eruption from a single fluid without a time gap between fibrous and gem diamond growth. High major element/CO₃²⁻ ratios in the growth media resulted in a constant major element composition of the fluid inclusions found in the studied fibrous diamonds. The transition from fibrous to gem diamond growth is likely caused by the precipitation of diamond reducing the degree of oversaturation of carbon in the fluid and hence decreasing the rate of diamond growth. Two other diamonds have inclusions that change from silicate minerals in the inner fibrous growth zones towards pure saline fluid compositions in the outer fibrous growth zones. This decrease in Si, Mg and Ca and increase in K and Cl in the inclusions is accompanied by a decrease in δ¹³C values and N contents. These trends are suggested to be the result from gradually mixing in more saline fluids with lower δ¹³C values. One diamond with silicic inclusions has significant N aggregation into B-centres, suggesting this fluid is different and that diamond formation occurred significantly (1250 °C gives ≥10 Ma) before the kimberlite eruption.