East Scotia Ridge unsecured

This may be related to plume mantle migrating around

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Unformatted text preview: gment E4 Segment E4 is compositionally diverse. Some samples are close to unmodified N-MORB, whereas lavas from the E3–E4 overlapper have very high LILE enrichments, especially Ba and Rb (Fig. 10). The latter have high Th/Yb, 87Sr/86Sr and 206Pb/204Pb ratios, highest water contents and low 143Nd/144Nd relative to all other East Scotia Ridge samples (Figs 6, 13 and 15). Sr–Nd–Pb isotope mixing calculations suggest that the samples can be modelled by addition of 2% sediment melt to a MORB-source mantle (Figs 15 and 16). These relatively extreme compositions are interpreted to have been generated by addition of a very water-rich subduction component that also transported Sr, Th, Nd, Pb, Ba and Rb. The extremely high water content of the samples makes it unlikely that all the water could have been transported by a sediment melt phase, which suggests either that two subduction components were involved (aqueous fluid and sediment melt; see section ‘Characterization of components contributing to the back-arc magma source’) or that an aqueous fluid transported these elements. Segments E5–E7 Lavas from the central part of the East Scotia Ridge are close to N-MORB with respect to trace element ratios such as Nb/Yb, (La/Sm)N, Th/Nb and Th/Yb. The 1461 JOURNAL OF PETROLOGY VOLUME 43 NUMBER 8 AUGUST 2002 Fig. 15. 143Nd/144Nd (a) and 87Sr/86Sr (b) vs 206Pb/204Pb for the East Scotia Ridge lavas (E2: Leat et al., 2000), South Sandwich island arc (Pearce et al., 1995), South American–Antarctic Ridge (SAAR; Dickey et al., 1977; Le Roex et al., 1985; Ito et al., 1987; Kurz et al., 1998), southern Pacific MORB (Castillo et al., 1998), bulk South Atlantic sediment (Ben Othman et al., 1989; Plank & Langmuir, 1998), Bouvet Island (Sun, 1980; Weaver et al., 1987), and Discovery [average composition from Douglass et al. (1999)]. For samples DR.157 and DR.158, dredged on the flanks of the E2 segment, data are from D. Harrison (unpublished data, 2000; location sites shown in Fig. 2). Data from additional E3 (D20) and E9 (D23) samples, indicated with a ‘P’, are from Pearce et al. (2001). Three end-members are suggested to be involved in the petrogenesis of the East Scotia Ridge lavas and calculated mixing trends are indicated. Literature data: ‘MORB source’ from McKenzie & O’Nions (1995) and Cohen & O’Nions (1982b); ‘plume mantle’ from Sun & McDonough (1989), McDonough & Sun (1995) and McKenzie & O’Nions (1995); ‘sediment melt’ from Ben Othman et al. (1989), Plank & Langmuir (1998) and Class et al. (2000). Data used for mixing calculations in (a) MORB source–bulk sediment: 206 Pb/204Pb: 17·8 and 18·6; 0·05, 10 and 3 ppm; 143Nd/144Nd: 0·5132 and 0·5125; 1·2, 34 and 90 ppm; MORB source–plume mantle: 206Pb/ 204 Pb: 17·8 and 19·5; 0·05 and 0·15 ppm; 143Nd/144Nd: 0·5132 and 0·5129; 1·2 and 1·25 ppm; MORB source + 0·4% bulk sediment–plume mantle: 206Pb/204Pb: 17·94 and 19·5; 0·06 and 0·15 ppm; 143Nd/144Nd: 0·5130 and 0·5129; 1·23 and 1·25 ppm; in (b) MORB source...
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