East Scotia Ridge unsecured

The mantle plume component is interpreted to have a

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Unformatted text preview: g negative correlation between Th/Nd and 143Nd/144Nd. We interpret this trend to represent MORB–subduction component mixing. Trend A is defined mostly by samples from segments E2 and E9 and therefore occurs symmetrically at the northern and southern ends of the spreading centre. This supports proposals that plumeinfluenced mantle is migrating into the back-arc around the lateral edges of the subducting plate (Livermore et al., 1997; Leat et al., 2000). The subduction contribution (Trend B) is seen in the high Th/Yb and Th/Nb of samples from segments E2, E3, E4 and E8. Lavas from the southern part of E8 samples are distinctive in that they have low (Dy/Yb)N ratios (Fig. 8) and low Nb/Yb and Ta/Nd ratios (Fig. 13). Their mantle source is therefore interpreted to have experienced previous partial melt extraction, and hence depletion in incompatible elements. This is the only place in the spreading centre where previously depleted mantle appears to have been a magma source. Such depleted mantle is also inferred to have formed the source of all the South Sandwich arc lavas (Pearce et al., 1995). Samples from segments E2, E3 and E4 are enriched in the sediment component, trending to high Th/Nb and Th/Nd ratios, at Nb/Yb and Ta/Nd ratios that are higher than MORB. We interpret these compositions to have been formed by addition of the sediment component to an N-MORB-like source mantle. Some samples of segment E2, especially the low-Na8 group, as well as several lavas dredged on the flanks of the axial high, plot between the two mixing lines (Fig. 13). The two trends are also clearly seen in plots of U8 and Nb8 vs (H2O)8 (Fig. 14). Trend A shows strong increase in U8 and Nb8 with moderate increase in (H2O)8. This trend is defined by most samples from segment E9 as well as the South American–Antarctic Ridge (note: we have no water contents for the most plume-influenced E2 samples, and so they do not appear in Figs 12 and 14). The South American–Antarctic Ridge is not influenced by the South Sandwich subduction system, but it has been suggested that the ambient MORB-source mantle beneath the ridge has been modified by mantle migrating westward from the Bouvet mantle plume (Le Roex et al., 1985; Kurz et al., 1998). Trend B in Fig. 14 shows constant, MORB-like Nb8 and increasing U8 with increasing (H2O)8 content, implying that magmas of the segments concerned (E2–E5, E9) were produced from sources that experienced addition of water in a subduction component. To place quantitative constraints on the contribution of the various source components to the different ridge segments, we examined the Sr–Nd–Pb isotope systematics. Figures 15 and 16 show our preferred mixing models between the three end-members involved in the genesis of the East Scotia Ridge lavas: (1) N-MORBsource mantle forming the ambient asthenosphere (Cohen & O’Nions, 1982b); (2) OIB-source mantles 1458 FRETZDORFF et al. PETROGENESIS OF EAST SCOTIA RIDGE Fig. 12. Variation of...
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This document was uploaded on 02/01/2014.

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