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

Pearce, Julian A., Baker, Peter E., Harvey, Peter K. and Luff, Ian W. 1995. Geochemical Evidence for Subduction Fluxes, Mantle Melting and Fractional Crystallization Beneath the South Sandwich Island Arc. Journal of Petrology 36 , pp. 1073-1109. 10.1093/petrology/36.4.1073

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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, Sounders and Bellinghausen) and calcalkaline (followed by Ltskov, Freezland 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 variationsin 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.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Earth and Ocean Sciences
Subjects: Q Science > QE Geology
Uncontrolled Keywords: geochemistry, petrology, fluxes, melting, subduction
Last Modified: 04 Jun 2017 02:05
URI: http://orca-mwe.cf.ac.uk/id/eprint/8604

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