Noble gas geochemistry of fluid inclusions in South African diamonds: implications for the origin of diamond-forming fluids

Fibrous diamond growth zones often contain abundant high-density fluid (HDF) inclusions and these provide the most direct information on diamond-forming fluids. Noble gases are incompatible elements and particularly useful in evaluating large-scale mantle processes. This study further constrains the evolution and origin of the HDFs by combining noble gas systematics with C-13, N concentrations, and fluid inclusion compositions for 21 individual growth zones in 13 diamonds from the Finsch (n=3), DeBeers Pool (n=7), and Koffiefontein (n=3) mines on the Kaapvaal Craton. C isotope compositions range from -2.8 to -8.6 parts per thousand and N contents vary between 268 and 867 at.ppm, except for one diamond with contents of <30 at.ppmN. Nine of the thirteen studied diamonds contained saline HDF inclusions, but the other four diamonds had carbonatitic or silicic HDF inclusions. Carbonatitic and silicic HDFs yielded low He concentrations, R/Ra (He-3/He-4(sample)/He-3/He-4(air)) values of 3.2-6.7, and low Ar-40/Ar-36 ratios of 390-1940. Noble gas characteristics of carbonatitic-silicic HDFs appear consistent with a subducted sediment origin and interaction with eclogite. Saline HDFs are characterised by high He concentrations, with R/Ra mostly between 3.9 and 5.7, and a wide range in Ar-40/Ar-36 ratios (389-30,200). The saline HDFs likely originated from subducted oceanic crust with low He but moderate Ar contents. Subsequent interaction of these saline HDFs with mantle peridotite could explain the increase in He concentrations and mantle-like He isotope composition, with the range in low to high Ar-40/Ar-36 ratios dependent on the initial Ar-36 content and extent of lithosphere interaction. The observed negative correlation between He-4 contents and R/Ra values in saline HDFs indicates significant in situ radiogenic He-4 production.