East Scotia Ridge unsecured

J pearce r stern t elliott and m wilson are gratefully

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Unformatted text preview: bulk sediment: 206Pb/204Pb: 17·8 and 18·6; 0·05 and 10·3 ppm; 87Sr/86Sr: 0·7025 and 0·7080; 14·7, 170 and 440 ppm; MORB source + 0·4% bulk sediment–plume mantle: 206Pb/204Pb: 17·94 and 19·5; 0·05 and 0·15 ppm; 87Sr/86Sr: 0·7030 and 0·7035; 14·7 and 19·9 ppm. 1462 FRETZDORFF et al. PETROGENESIS OF EAST SCOTIA RIDGE Fig. 16. 208Pb/204Pb (a) and 207Pb/204Pb (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 (Dickey et al., 1977; Le Roex et al., 1985; Ito et al., 1987; Kurz et al., 1998), Pacific MORB (Castillo et al., 1998), bulk South Atlantic sediment (Barreiro, 1983; Ben Othman et al., 1989; Plank & Langmuir, 1998), and Bouvet Island (Sun, 1980; Kurz et al., 1998), 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 For samples DR.157 and DR.158, dredged on the flanks of the E2 segment, data are from D. Harrison (unpublished data, 2000; location sites shown in Fig. 2).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). Data used for mixing calculations in (a) MORB source–bulk sediment: 206 Pb/204Pb: 17·8 and 18·6; 0·05 and 10 ppm; 208Pb/204Pb: 37·4 and 38·6 ppm; in (b) 207Pb/204Pb: 15·45 and 15·62 ppm. water content and the Rb and Ba concentrations are slightly higher than those of N-MORB. Furthermore, the samples show the highest Na8 contents compared with the other segments and relatively high (Dy/Yb)N ratios. On the basis of the slightly elevated water contents and higher Ba/Th ratios compared with N-MORB, we 1463 JOURNAL OF PETROLOGY VOLUME 43 NUMBER 8 AUGUST 2002 Fig. 17. Sketch of the South Sandwich subdution zone showing the distribution of components contributing to the source of individual backarc segments. (See text for details.) suggest that there is a minor influence of a subduction component, probably an aqueous fluid, on an N-MORB mantle source in the central part of the ridge. Segment E8 Lavas from the southern part of segment E8 are distinctive in having an apparently depleted, low Nb/Yb and (Dy/ Yb)N source, and strong enrichments in LILE (Figs 7b, 8b and 10). The samples are similar to those of the South Sandwich island arc in many trace element ratios (Fig. 13). Their low Nb/Yb and (Dy/Yb)N ratios indicate that their mantle source was depleted relative to MORB owing to the extraction of a previous melt fraction. Although, in general, mantle has already been processed through the back-arc melting and then is depleted beneath arcs we see a clear involvement of such depleted mantle in the magma source of the lavas erupted in the southern part of segment E8 (Figs 8 and 13), which might be explained by high degrees of melting of a MORB source. The high Ba/Th ratios of the samples, evident in Figs 11 and 12, indicate that segment E8 was affected mainly by aqueous fluid rather than sediment from the subduction zone. Segment...
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