Deep to Shallow Sulfide Saturation at Nisyros Active Volcano

Abstract

AbstractWe report new petrographic and geochemical observations on magmatic sulfides occurring in different enclave types, including hornblende and/or clinopyroxene‐rich cumulates, and in the host lavas, in Nisyros (South Aegean volcanic arc). We discuss our findings in the context of pre‐existing petrological and thermobarometry studies. Our results indicate that sulfides become less abundant and Cu‐richer (Cu median values) with magma differentiation at progressively lower pressure/P‐temperature/T‐depth conditions. Starting with high T‐P sulfide‐free pillow lavas and wehrilitic cumulates still representing a sulfide undersaturated system, passing to high T and lower P, deep‐forming sulfide‐rich and hornblende‐rich gabbroic enclaves (SiO2 = 53–55 wt.%, 4–5 × 10−5 area %, Cu = 260 μg/g) at the base of the crust (∼25–30 km), then to clinopyroxene‐rich gabbroic micro‐cumulates (2–3 × 10−5 area %, Cu = 570 μg/g) forming in shallower crustal levels (∼10 km), then to more evolved sulfide‐poor hybrid enclaves (SiO2 = 56–70 wt.%, 1 × 10−5 area %, Cu = 602 μg/g), and finally to even more sulfide‐poor rhyodacitic host lavas (SiO2 = 66–76 wt.%, <0.5 × 10−5 area %, Cu = 6.4 wt.%) differentiating at even shallower crustal levels (∼7 km). Sulfide‐free quenched basaltic andesitic enclaves differentiating near surface levels, carry no textural evidence of pre‐existing magmatic sulfides suggesting that the system returned to a sulfide‐undersaturated state. Finally, we point out two important processes for sulfide evolution, a reaction replacement of clinopyroxene by amphibole observed in the deep‐forming hornblende‐rich gabbroic enclaves triggering the onset of sulfide saturation, and an increased mafic input followed by magma mingling and enclave disaggregation leading to sulfide dissolution and Cu‐enrichment of the magmas.