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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
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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