East Scotia Ridge unsecured

London george allen unwin 445 pp dickey j s frey a

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Unformatted text preview: E9 Segment E9 lavas are variable in composition, with progressive north–south trends (Figs 7 and 8). Samples from the southern part of the segment are characterized by high Na8, high Nb/Yb and (La/Sm)N ratios, and high Nb8 and U8 abundances relative to the northern part (Figs 7 and 14). The relatively high magma production and hence positive topography of this segment suggests a high degree of mantle partial melting. This may be related to plume mantle migrating around the southern end of the slab, or a relatively large influence of subducted water in the source (Bruguier & Livermore, 2001). However, segment E9 lavas have relatively high Na8 values, implying small degrees of mantle partial melting. They are also relatively poor in geochemical components derived from the subducting slab. The chemical similarity of E9 lavas with lavas from the South American–Antarctic Ridge suggests similar mantle sources, but radiogenic isotopes indicate that the component that sourced segment E9 was unlikely to have been the uncontaminated HIMU Bouvet component. CONCLUSIONS (1) Sampling of segments E2–E9 of the East Scotia Ridge has shown that there are striking variations in sources of magmas erupted along the ridge. Some of these variations appear to be related to position within the back-arc basin, and tectonic relationship to the subducting slab. (2) Segments E2 and E8, both near the ends of the ridge, erupt significantly more evolved magmas than the others. The most evolved magmas are the products of >50% fractional crystallization for the most mafic observed compositions. The evolved lavas were erupted 1464 FRETZDORFF et al. PETROGENESIS OF EAST SCOTIA RIDGE on topographic highs along the axes of segments E2 and E8, and the fractionation is interpreted to have occurred in shallow magma chambers. In the case of Segment E2, such an axial magma chamber has been seismically imaged (Livermore et al., 1997). (3) The central segments of the ridge are sourced from MORB-source mantle that was influenced only to a minor degree by components from the subducting slab. (4) The northern segment E2 is strongly influenced by plume mantle, which is interpreted to be flowing westward into the back-arc around the edge of the subducting slab (Livermore et al., 1997; Leat et al., 2000; Pearce et al., 2001). The mantle plume component is chemically similar to the Bouvet mantle plume, consistent with evidence for westward flow of mantle from Bouvet, which has been traced geochemically along the South American–Antarctic Ridge (Le Roex et al., 1985; Kurz et al., 1998). Evidence for a mantle plume component in segment E9 at the southern end of the East Scotia Ridge is equivocal. (5) Subduction components derived from the slab are present in relatively large amounts at the ends of the ridge, and in small amounts in the central segments. Contributions from the slab are chemically heterogeneous, and at least two different subduction components appear to be present,...
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