Hf isotope evidence for selective mobility of high-field-strength elements in a subduction setting: South Sandwich Islands
176Hf/177Hf isotopes provide information about the behaviour of so-called immobile elements in subduction environments. Early studies of Hf isotopes in subduction zones reached different conclusions regarding the mobility of high-field-strength elements during subduction-related processes. To test the behaviour of Hf during subduction, we have examined the young, intra-oceanic South Sandwich subduction system. Combined 176Hf/177Hf and trace element ratios reveal that Hf may behave as both immobile and mobile, depending upon the exact spatial relationship of the arc volcano to the slab. Throughout most of the arc, magmas show no detectable Hf transfer from the slab to the wedge, perhaps because enrichment of the wedge took place by Hf-deficient, fluid-dominated processes. On the basis of ΔεNd values, which describe the Nd isotope deviation from a local MORB-OIB array, we can discern that northern volcanoes of the arc require a source enriched by fluids that originated from the oceanic crust, whereas southern arc volcanoes have a source modified by a higher proportion of sediment-derived fluids. However, close to the southern slab edge and in rear-arc settings, arc magmas were derived from a source that had undergone Hf addition; we attribute this to element transfer via partial melts from sediment. This implies that Hf mobility from the slab is possible where temperatures are sufficiently high to induce sediment melting rather than fluid generation alone. The implication of this work, for the majority of the arc, is that sediment-derived fluids contribute to magmatism and that sediment-derived melt does not.