Multiple sulfur isotope evidence for surface-derived sulfur in the Bushveld Complex

The Rustenberg Layered Suite of the Bushveld Complex originated in the mantle, but is characterized by variably radiogenic strontium, neodymium, lead and osmium initial isotope ratios and more elevated δ18O values than expected from a partial melt of primitive mantle. It is not clear whether Bushveld magmas assimilated material from the crust, or whether they were instead generated from an eclogitic mantle source that contained crustal material. Here we report the Δ33S of fifteen whole-rock and sulfide separate samples from both the Main and Critical Zones of the Bushveld, including the UG2 Chromitite and the Merensky Reef. All samples have positive Δ33S values (0.11±0.02‰, 1σ) and δ34S values (2.2±0.6‰, 1σ). Non-zero Δ33S is fundamentally a surface process. Since high-temperature processes are not known to produce significant change in Δ33S, the positive Δ33S indicates that the Bushveld sulfur contains a sedimentary or surface-derived component. The homogeneity of Δ33S values for this suite of samples contrasts with the heterogeneity displayed by strontium, neodymium, lead and osmium initial isotope ratios of the Bushveld and requires that the sulfur contamination occurred prior to the emplacement of the Bushveld in the upper crust. Possible sources of contamination include lower crust and sub-continental lithospheric mantle. Limited multiple sulfur isotopic measurements of these reservoirs exist; more work needs to be done to constrain whether they contain surface-derived sulfur with non-zero Δ33S, and if they do, whether the concentrations and isotopic compositions of these reservoirs can explain the measured Δ33S of the Bushveld.

► Mafic Bushveld rocks have non-zero Δ33S indicating surface-derived sulfur.
► Δ33S is homogeneous, so sulfur incorporation occurred prior to emplacement.
► Composition of potential contaminant is constrained by mass-balance calculations.