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Unformatted text preview: which is shallow. The high Nb/Yb ratios
in segment E2 are not the result of low degrees of partial
melting of MORB-source mantle, because low degrees
of partial melting would be characterized by both high
Nb/Yb and high Na8. This is not observed (Fig. 7).
Because Nb is not likely to have been introduced from
the subducting slab (Pearce & Peate, 1995), segments E2,
E9 and part of E4 seem to have been supplied by a more
enriched, higher Nb/Yb mantle source than MORB.
The southern part of segment E8 contains low Na8 lavas
that also have Nb/Yb ratios signiﬁcantly below the NMORB ratio. These characteristics are consistent with
relatively high degrees of melting of a MORB or subarc depleted mantle source.
Variations in REE along the ridge are shown in Fig.
8. Light REE (LREE) abundances can be increased by
input from the subducting slab as well as aﬀected by
mantle composition and degree of partial melting (Pearce
& Peate, 1995). HREE abundances are not aﬀected by
additions from the subducting slab. There are signiﬁcant
variations in REE along the East Scotia Ridge. All
samples from segments E2 and E9 as well as samples
from the overlapping region of segments E3 and E4
(hereafter E3–E4 overlapper) are enriched in LREE.
Segments in the central part of the back-arc have low
(La/Sm)N ratios comparable with N-MORB. The alongridge variations in the (La/Sm)N ratio closely follow those
in Nb/Yb (Figs 7 and 8), consistent with diﬀerent mantle sources underlying diﬀerent parts of the ridge. However,
variable inputs from the subducting slab may also be
present, especially in segments at the ends of the ridge.
The (Dy/Yb)N ratios of samples from the ridge closely
follow variations in Na8, and are lowest in segment E8,
where they are similar to N-MORB (Figs 7 and 8). (Dy/
Yb)N ratios are high in segment E9, moderately high in
the central segments E5–E7 and relatively low in segment
E2. The low (Dy/Yb)N ratios in segments E2 and E8
indicate that these segments had higher degrees of partial
melting than other segments of the ridge, consistent with
the low Na8 values of segments E2 and E8 (Figs 7 and
The positive correlation of (La/Sm)N with 87Sr/86Sr
(Fig. 9) reinforces our conclusion that the trace element
variations cannot be the result of partial melting alone.
The increase in (La/Sm)N with increasing 87Sr/86Sr could
be a result either of addition of radiogenic Sr with LREE
via subducted sediments, or the presence of an enriched
(OIB) mantle component, such as Bouvet mantle plume,
under segments E2, E3, E4 and E9. In the following
sections, we distinguish the separate mantle and slabderived components in the back-arc source. Characterization of components
contributing to the back-arc magma source
It has been argued that the characteristic enrichments
in volcanic arc rocks of most highly incompatible trace 1453 JOURNAL OF PETROLOGY VOLUME 43 NUMBER 8 AUGUST 2002 Fig. 7. (a) Na8 composition [calculated after Klein & Langmuir (1987); see also Fig. 5] and (b) Nb/Yb ratios and...
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This document was uploaded on 02/01/2014.
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