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East Scotia Ridge unsecured

Some samples are close to unmodied n morb whereas

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Unformatted text preview: (a) Ba/Th and (b) Th/Nb against the (H2O)8 (wt %) content (see Fig. 6 for calculation procedure) for the East Scotia Ridge glasses. (McDonough & Sun, 1995) similar to the source of Bouvet Island magmas; (3) sediment components based on South Atlantic bulk sediment (Barreiro, 1983; Ben Othman et al., 1989; Plank & Langmuir, 1998) using two different estimates of the trace element abundances of sediment melt (Class et al., 2000). We also show the composition of the Discovery mantle plume, which is situated at about 44°S, close to the Mid-Atlantic Ridge (Douglass et al., 1999), and which is a similar distance from the East Scotia Ridge to the Bouvet mantle plume. In Figs 15 and 16, the East Scotia Ridge samples plot close to the MORB end-member and between the MORB end-member and the sediment and mantle plume compositions. This indicates that the MORB component is the most widespread magma source for the East Scotia Ridge, with additional contributions from variable amounts of the sediment and mantle plume components, consistent with interpretations of trace element compositions (Figs 11, 13 and 14). The East Scotia Ridge samples have similar 143Nd/144Nd to, but lower 87Sr/86Sr than, the South Sandwich island arc. Apart from four samples from the flanks of segment E2, all the East Scotia Ridge samples have lower 206Pb/204Pb than the South Sandwich arc. Mixing curves show that the Sr, Nd and Pb isotopic compositions of all the East Scotia Ridge samples (apart from the high 206Pb/204Pb samples from segment E2) can be modelled by addition of up to >2% sediment melt to MORB-source mantle (Figs 15 and 16). Most samples from the central segments E3–E7 can be modelled by addition of <0·4% sediment melt to MORB-source mantle. A group of samples from the flanks of segment E2 shows a clear involvement of a component having 206Pb/204Pb ratios of >19·3, believed to have migrated from the Bouvet mantle plume (Leat 1459 JOURNAL OF PETROLOGY VOLUME 43 NUMBER 8 AUGUST 2002 et al., 2000; Pearce et al., 2001). However, it should be noted that it is difficult to distinguish between mixing of sediment and mixing of plume mantle components with a MORB-source mantle using Pb isotope systematics (Fig. 16). DISTRIBUTION OF MANTLE COMPONENTS The East Scotia Ridge is tectonically simple, and there are some features of the distribution of magma types within it that appear to be systematic (Fig. 17). The central segments of the ridge are sourced from MORBsource mantle that was influenced only to a minor degree by components from the slab. Departures from MORBlike compositions are strongest close to both northern and southern ends of the ridge. The range in water contents of the magmas, correlating with the range in trace element abundances from MORB-like to arc-like, suggests that decompression melting dominates melt production in the central part of the ridge, and that volatilefluxed melting is more important at the ends of the ridge. Segment E2 This segment was discussed by Leat et al. (2000), who suggested that the segment tapped both N-MORB and plume mantle sources, and th...
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