Chalcophile element processing beneath a continental arc stratovolcano

Abstract

The chalcophile elements are important both in terms of their economic value and as potential tracers of magmatic processes at convergent margins. However, because of analytical difficulties, comprehensive datasets of chalcophile element concentrations for volcanic rocks are rare. Here, we present analyses of a near complete suite of chalcophile elements (S, Cu, Ag, Se, As, Sb, Sn, W, Mo, Pb, Bi, Tl, Zn, Ga, Co) for volcanic rock samples collected from a typical continental arc stratovolcano in southern Chile (Antuco). Enrichment in Pb, Bi, W, Tl, Sb and As relative to Parental-MORB indicates that these elements have been mobilised from the subducting slab into the sub-arc mantle wedge, in contrast to Cu and Ag. Very low Se concentrations suggest that Se, like S, was lost during co-eruptive degassing of the Antuco magmas. Previous studies on oceanic arcs have demonstrated that as higher fO2 subduction-related magmas ascend through the overlying lithosphere, magnetite fractionation may trigger sulfide fractionation during crystallisation. If such a process is extensive and has a sharp onset, this would result in a plummet in the Cu, Se and Ag contents of the residual melt. At Antuco, although a decrease in the Fe2O3(T) and TiO2 concentrations at ∼55 wt.% SiO2 (∼3 wt.% MgO) indicates magnetite fractionation, this is not associated with a corresponding drop in Cu contents. Instead, we observe a general decrease in Cu and a decrease in Cu/Ag with increasing SiO2 and decreasing MgO. Furthermore, Cu/Ag in the most primitive Antuco rocks are lower than the global MORB array, indicating that the melts were sulfide saturated at an early stage in their crustal evolution. Through modelling fractional crystallisation, we show that only a minor volume (0.5–0.6 vol.%) of fractionating sulfide is needed to produce divergent trends in Cu and Ag, as observed in the Antuco samples. Our results show that sulfide fractionation occurred from an early stage during the crustal evolution of Antuco's magmas. We infer that this was promoted by stalling in the lower crust, which for oxidised magmas at depths >20 km is within the sulfide stability field. However, elevated DyN/YbN of the Antuco magmas compared to oceanic island arc magmas provides an additional, or alternate mechanism to inducing sulfide fractionation in the lower crust prior to ascent, through initial garnet fractionation. Fractional crystallisation within this depth range meant that later magnetite fractionation had only a minor impact on the partitioning behaviour of the chalcophile elements. In contrast, arc magmas transiting thinner crust may not experience sulfide saturation until a later stage in their evolution, induced by magnetite fractionation. Our results imply that convergent margin crustal thickness, and therefore the depth range of magmatic differentiation, determines the dominant control on initial magmatic sulfide saturation and therefore the primary distribution of chalcophile elements. This implies that secondary processes are required to explain the transport and concentration of sulfides and chalcophile elements at shallower crustal levels.