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Unformatted text preview: water depth as a function of
latitude along the East Scotia Ridge. In (a) the dashed line represents the average Na8 composition of South Atlantic MORB (Klein & Langmuir,
1987) and in (b) the N-MORB average of Hofmann (1988). E2 segment data are, apart from WX42–WX44 (Table 1), from Leat et al. (2000). elements (relative to MORB) result from the contribution
of at least two components: sediment or sediment melt
and aqueous ﬂuid (Ellam & Hawkesworth, 1988; Ryan
et al., 1995; Turner et al., 1996; Elliot et al., 1997; Turner
& Hawkesworth, 1997; Class et al., 2000). The sediment
component is characterized by high La/Yb, La/Sm and
Th/Nb, and is variously regarded as addition of bulk
sediment or partial melt of subducted sedimentary rocks.
The slab-derived ﬂuid has high B, B/Be, U/Th and Ba/ Th, and may consist of variable proportions derived from
dehydration of basaltic crust and from dewatering of
sediments (Ishikawa & Tera, 1999; Class et al., 2000).
Cross-arc geochemical traverses indicate that the ﬂuid
component dominates at the volcanic front, where hydration of the mantle causes the greatest amount of
melting of the mantle wedge (Ryan et al., 1995). This
suggests that subduction components in back-arc spreading centres may be poor in the ﬂuid component. 1454 FRETZDORFF et al. PETROGENESIS OF EAST SCOTIA RIDGE Fig. 8. (a), (b) Chondrite-normalized (Sun & McDonough, 1989) REE ratios and water depth as a function of latitude along the East Scotia
Ridge. The vertical lines mark the boundaries of the main ridge segments. In (a) and (b), dashed lines show the N-MORB average of Hofmann
(1988). (For E2 segment data, see Fig. 7.) The addition of subduction components to a mantle
source can be assessed by comparing abundances of trace
elements in lavas to a reference composition. Pearce &
Peate (1995) compared arc lavas with a global MORB
array and estimated the percentage contribution from
subducted material for various elements. They found
that LILE such as Ba, Rb, K, Pb, Th, U and Sr are
dominated by subducted material, having a subduction
zone contribution of >80%. Class et al. (2000) normalized to a local composition to isolate particular components.
In Fig. 10, we have compared lavas from segments E2–E9
with sample WX48, which has N-MORB-like values
of (La/Sm)N, Nb/Yb, H2O and 87Sr/86Sr (0·58, 0·75,
0·18 wt % and 0·702511, respectively) (Tables 1 and 2).
Enrichment factors were calculated by dividing measured
element/Yb ratios by the element/Yb ratio of sample
WX48, normalized to the Nb/Yb ratio of the sample.
This method eﬀectively removes the eﬀects of fractional 1455 JOURNAL OF PETROLOGY VOLUME 43 NUMBER 8 AUGUST 2002 Fig. 9. Variation of (La/Sm)N vs 87Sr/86Sr for lavas from the East Scotia Ridge (E2: Leat et al., 2000), South American–Antarctic Ridge (Dickey
et al., 1977; Le Roex et al., 1985), South Sandwich island arc (Pearce et al., 1995), Bouvet Island (Le Roex & Erlank, 1982; Kurz et al., 1998),
and the (La/Sm)N (Hofmann, 1988) and 87Sr/86Sr (Ito et al., 198...
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