East Scotia Ridge unsecured

1462 fretzdorff et al petrogenesis of east scotia

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Unformatted text preview: at both aqueous fluid and subducted sediment components had contributed to the range of compositions. The mantle plume component is interpreted to have a high 206Pb/204Pb ratio, similar to Bouvet, as such compositions have been identified in a dredge from one flank of the central topographic high that dominates the segment. The segment contains some of the most evolved compositions from the East Scotia Ridge, as a result of extensive fractional crystallization, which is believed to have occurred in a seismically imaged magma chamber (Livermore et al., 1997). Fig. 13. Variation of Th/Yb vs Nb/Yb (a), Ta/Nd vs Th/Nb (b) and 143 Nd/144Nd vs Th/Nd (c) for the East Scotia Ridge lavas (E2: Leat et al., 2000), the South Sandwich island arc (Pearce et al., 1995), Bouvet Island (Sun, 1980; Weaver et al., 1987) and bulk South Atlantic sediment (Plank & Langmuir, 1998). For samples DR.157 and DR.158, dredged on the flanks of the E2 segment, data are from D. Harrison (unpublished data, 2000; location sites shown in Fig. 2). Calculated mixing trends (grey lines) between a MORB-like end-member and enriched source components (Bouvet Island: Trend A; bulk South Atlantic sediment: Trend B) are shown. It should be noted that, in (a) and (b), samples from the southern part of segment E8 tend towards the data field of the South Sandwich island arc, which is suggested to be generated from sub-arc mantle modified by subduction-related components from the downgoing slab. Segment E3 Most samples from this segment are close to N-MORB in composition, but there is a range in compositions to high LILE and low 143Nd/144Nd. The range in Nd isotopes may be modelled by addition of up to >0·7% sediment (Fig. 15). Lavas located at the southern end of this segment (E3–E4 overlapper) have high (H2O)8 contents, up to 1·6%. The segment is interpreted to have erupted magmas derived from an N-MORB source mantle that has been moderately enriched by a water-rich, LILEbearing subduction component. 1460 FRETZDORFF et al. PETROGENESIS OF EAST SCOTIA RIDGE Fig. 14. Variation of U8 (a) and Nb8 (b) vs (H2O)8 (wt %) content (see Fig. 6 for calculation procedure) for the East Scotia Ridge (ESR) volcanic glasses compared with (a) Mariana Trough lavas (Volpe et al., 1987; Stolper & Newman, 1994) and (b) South American–Antarctic Ridge lavas (Le Roex et al., 1985; Michael, 1995). It should be noted that two clear trends can be distinguished in both diagrams. Trend A is observed in segment E9 and South American–Antarctic Ridge lavas showing an increase of U8 and Nb8 at nearly constant water contents, whereas Trend B (samples from segments E2–E5) shows in (a) a positive correlation of H2O with U8 and in (b) constant Nb8 with increasing water content. Samples having MORB-like geochemical character are indicated. Water contents for dredged samples from segment E3 (D20; indicated with a ‘D’) are from Muenow et al. (1980); U ppm from Cohen & O’Nions (1982a) and Nb ppm from Saunders & Tarney (1979) and are recalculated to 8 wt % MgO (see Fig. 6 for calculation procedure). Se...
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This document was uploaded on 02/01/2014.

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