Unformatted text preview: 7) composition of N-MORB. crystallization and diﬀerent degrees of partial melting for
elements of similar incompatibility to Nb. It also removes
the eﬀects of diﬀerent mantle compositions such as plume
vs MORB-source mantles. The LILE are ordered in Fig.
10 according to relative importance of the subduction
component to their abundances in arcs from Ba (highest)
to Sr (lowest) as determined by Pearce & Peate (1995).
Enrichment factors are [1 in nearly all cases. The
enrichment factors of <1 for Sr in some samples probably
reﬂect removal of Sr in plagioclase during fractional
crystallization. On the basis of these calculations (Fig.
10), the central part of the East Scotia Ridge (segments
E5–E7) shows only a minor inﬂuence of a subduction
component. Samples from segment E7 consistently have
the lowest enrichment factors, below two in most cases.
Samples from segments E2–E5, E8 and E9 show highly
variable subduction enrichment factors varying from one
to 10 and even up to 15 in segment E4, and tend to be
highest in the segments near the northern and southern
ends of the ridge. Samples with high enrichment factors
are interpreted to have very large contributions of LILE
from subducted material. There are diﬀerences in the
relative enrichment of LILE between samples within
segments and from segment to segment. In general, Ba,
Rb and Th are more enriched than K, U, Pb and Sr
(Fig. 10). Th is relatively more enriched in the northern
segments E2, E3 and E4 than the southern segments E8
and E9, suggesting regional variations in the importance
of diﬀerent processes or slab-derived components.
The relationships of LILE in the back-arc are shown
in Fig. 11. Samples from the South Sandwich Islands
form a negative correlation between Ba/Th and Th/ Nb. This was interpreted by Leat et al. (2000) as a result
of variations in the relative amounts of two diﬀerent
slab-derived components in the arc. The high Ba/Th
component was interpreted to be aqueous ﬂuid, and the
high Th/Nb component was suggested to be sediment.
Samples from the East Scotia Ridge are scattered between
MORB (and plume mantle) and the arc, and trend
away from MORB toward both arc components. An
implication of Fig. 11 is that the two components present
in the arc also inﬂuence compositions in the back-arc.
Some samples from E2, E4 and E8 plot toward the high
Th/Nb components, whereas most of the rest of the
samples trend toward the high Ba/Th component. In
this plot, it appears that the ﬂuid component is dominant
in most back-arc samples. If the identiﬁcation of the
origin of the components is correct, the few high Th/
Nb samples contain a signiﬁcant input from sediment or
sediment melt, whereas the majority of samples are
dominated by aqueous ﬂuid from the slab.
The behaviour of these two possible components is
further assessed in Fig. 12. Both Ba/Th and Th/Nb
correlate positively with H2O abundances. In the case of
Ba/Th, this is consistent with transport of Ba in an
aqueous ﬂuid. The positiv...
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