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East Scotia Ridge unsecured

East Scotia Ridge unsecured - JOURNAL OF PETROLOGY VOLUME...

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JOURNAL OF PETROLOGY VOLUME 43 NUMBER 8 PAGES 1435–1467 2002 Petrogenesis of the Back-arc East Scotia Ridge, South Atlantic Ocean S. FRETZDORFF 1 , R. A. LIVERMORE 2 , C. W. DEVEY 3 , P. T. LEAT 2 AND P. STOFFERS 1 1 INSTITUTE OF GEOSCIENCES, UNIVERSITY OF KIEL, OLSHAUSENSTRASSE 40, 24118 KIEL, GERMANY 2 BRITISH ANTARCTIC SURVEY, HIGH CROSS, MADINGLEY ROAD, CAMBRIDGE CB3 0ET, UK 3 FACHBEREICH 5—GEOWISSENSCHAFTEN, UNIVERSITY OF BREMEN, POSTFACH 330 440, 28334 BREMEN, GERMANY RECEIVED JANUARY 3, 2001; REVISED TYPESCRIPT ACCEPTED JANUARY 22, 2002 The East Scotia Ridge is an active back-arc spreading centre located petrogenesis, e.g. basalts erupted in the Mariana Trough and the Lau Basin (e.g. Hawkins, 1976; Hawkins & to the west of the South Sandwich island arc in the South Atlantic Melchior, 1985; Volpe et al. , 1987; Stern et al. , 1990; Ocean, consisting of nine main segments, E1 (north) to E9 (south). Gribble et al. , 1998; Turner & Hawkesworth, 1998; Peate Major and trace element and Sr–Nd–Pb isotope compositions are et al. , 2001). These characteristics provide evidence for presented, together with water contents, for lavas sampled along the prior depletion of the mantle source and enrichment in active ridge axis. Magmatism along the East Scotia Ridge is H 2 O and elements such as Ba, Th, U and Pb, thought to chemically heterogeneous, but there is a common mid-ocean ridge be transported into the mantle wedge from the subducting basalt (MORB)-type source component for all the magmas. An slab via sediment melts or aqueous fluids (Stern et al. , almost unmodified MORB-source mantle appears to underlie the 1990; Saunders et al. , 1991; Woodhead et al. , 1993; central part of the back-arc. Subduction components are found at Stolper & Newman, 1994; Pearce et al. , 1995; Gribble et the northern and southern ends of the ridge, and there is a marked al. , 1998). Back-arc basins can therefore be used to model sediment melt input of up to 2% in segment E4. Enriched (plume) the transport of slab components into the mantle wedge mantle is present beneath segment E2 at the northern end of the over much wider areas in subduction zone environments ridge, suggesting that plume mantle is flowing westward around the than can be achieved by studying volcanic arcs alone, and edges of the subducting slab. The southern part of segment E8 is can be used to test models of mantle wedge convection. unique in that its magma source is similar to sub-arc depleted Furthermore, the transitional character of back-arc bas- mantle. alts from MORB to arc-like provides critical evidence of the relative roles of decompression vs volatile fluxed melting of mantle (Gribble et al. , 1998). KEY WORDS: geochemistry; petrogenesis; volcanism; back-arc; subduction The East Scotia Ridge, an active back-arc spreading centre behind the South Sandwich island arc in the South Atlantic Ocean (Fig. 1), is an interesting place to study subduction-related processes for several reasons: (1) its INTRODUCTION simple tectonic setting as part of an intra-oceanic arc, Fractional melting of mantle peridotite is the primary without the influence of continental crust; (2) its close
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