Geochemical Evidence for Subduction Fluxes, Mantle Melting and Fractional Crystallization Beneath the South Sandwich Island Arc

Abstract

Abstract The volcanoes of the South Sandwich island arc follow three distinct series: low-K tholeiitic (followed by Zavodovski, Candlemas, Vindication, Montagu and Bristol), tholeiitic (followed by Visokoi, Saunders and Bellinghausen) and calcalkaline (followed by Leskov, Freehand and part of Cook and Thule). Flux calculations indicate that the percentage contribution of the subduction component to the mantle source of all three series varies from undetectable (e.g. Zr) through small (e.g. Nd=20%) and moderate (e.g. La, Ce, Sr=50–80%) to dominant (e.g. Pb, K, Ba, Rb, Cs >90%) with little change along the arc. Isotope systematics (Pb, Nd, Sr) show that this subduction component obtains a greater contribution from altered oceanic crust than from pelagic sediment. Elements for which the subduction contribution is small show that the mantle is already depleted relative to N-MORB mantle (equivalent to loss of an ∼2⋅5% melt fraction) before melting beneath the arc. After addition of the subduction component, dynamic melting of this depleted mantle then causes the variations in K that distinguish the three series. The estimated degree of partial melting (∼20%) is slightly greater than that beneath ocean ridges, though geothermometry suggests that the primary magma temperature (∼1225°C) is similar to that of primary MORB. About half of the melting may be attributed to volatile addition, and half to decompression. Dynamic melting involving three-dimensional, two-phase flow may be needed to explain fully the inter-island variations.