SURFACE-DERIVED SULFUR IN THE SUB-CONTINENTAL LITHOSPHERIC MANTLE: WHOLE ROCK MULTIPLE SULFUR ANALYSIS OF PERIDOTITE AND ECLOGITE XENOLITHS FROM THE PREMIER KIMBERLITE, SOUTH AFRICA

We report whole rock sulfur isotope analysis of sulfides with variable Δ³³S in peridotite and eclogite xenoliths from the Premier Kimberlite, South Africa. Previous studies have shown that sulfide inclusions in eclogitic diamonds from the Kaapvaal-Zimbabwe Craton can contain mass-independent fractionated (MIF; Δ³³S) sulfur ranging from -0.5‰ up to 1.3‰, which differs from the expected mantle value of Δ³³S=0. Most of the peridotite xenoliths in this study yielded variable amounts of a surface-derived component (average Δ³³S: 0.042±0.039‰, 1 s.d.), with values up to 0.158‰, the highest among all samples analyzed in this study. The δ³⁴S values are mostly positive, from -0.68‰ to +9.29‰, which could reflect heterogeneities in the mantle, resulting from processes such as recycling of sedimentary material, or metasomatism.

The eclogite xenoliths yield negative δ³⁴S ranging from -2.80‰ to -1.17‰, and average Δ³³S of 0.031±0.014‰ (1 s.d.), defining a compositional field that differs from the peridotites. The kimberlite matrix material analyzed has a similar Δ³³S signature to the peridotite xenoliths, with an average of 0.055±0.015‰ (1 s.d.) but different δ³⁴S values, ranging from -0.57‰ to +3.08‰. Kimberlite magmas erupt rapidly and it is possible that the non-zero Δ³³S composition does not come solely from assimilation of crustal material, or that assimilation of crustal sulfur within the kimberlite magma does not severely affect the composition of the xenoliths. It is also possible that sulfides within the xenolith matrix have interacted with the kimberlite in an open-system exchange.

These results show the presence of a MIF signature in the Premier Kimberlite and in the sub-continental lithospheric mantle underneath the Bushveld Complex in the Kaapvaal Craton, which points toward the presence of a surface-derived component in the mantle. Since MIF is chemically conservative, the most efficient way to change it is by mixing of materials with different composition, which would allow for its conservation in the mantle for long periods of time. That, and the similarity of the signatures between peridotites and kimberlites, but different eclogite signature, suggests the possibility that the source of the kimberlite had a similar sulfur signature to the peridotite xenoliths.