Halogens of Bushveld Complex, South Africa: δ37Cl and Cl/F evidence for hydration melting of the source region in a back-arc setting

The Bushveld Complex is one of only a few layered intrusions anomalously enriched in chlorine. This study presents the first chlorine stable isotope analyses of Bushveld and associated rocks designed to constrain the source of Bushveld Cl. amples overlap the range of values for pristine MORB and altered seafloor samples (δ37Cl ∼+4‰ relative to sea water). This contrasts with distal metasedimentary floor rocks (δ37Cl ∼+0‰), with little evidence of contamination away from the marginal rocks. The Cl isotope composition and high Cl/F ratio is therefore a fundamental characteristic of the Lower and Critical Zone magmas and not derived from infiltration from the country rock. These results, the high Cl/F ratio and the boninitic arc-like character of Bushveld magma, are consistent with hydration partial melting of a depleted source in a subduction-related setting with Cl supplied either by the dehydration of a subducted slab with a heavy Cl signature or an overridden “wet” mantle hotspot.