East Scotia Ridge unsecured

G stolper newman 1994 this is conrmed for the east

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Unformatted text preview: (Table 1). It was not possible to measure the H2O content of all the volcanic glasses along the back-arc, because of the presence of vesicles and microcrystals. We measured the H2O content of only two E2 samples from the centre of the segment (WX42, WX43; Table 1). Selected volcanic glass samples were analysed for trace elements and compositions are listed in Table 1. Comparison of the relative abundances of highly and moderately incompatible trace elements along the East Scotia Ridge is facilitated by using N-MORB-normalized (Hofmann, 1988) abundance profiles (Fig. 5). Relative to NMORB, the East Scotia Ridge volcanic glasses have higher abundances of large ion lithophile elements (LILE) such as K, Pb, Rb and Ba, but are less enriched than bulk South Atlantic sediment (from Plank & Langmuir, 1998). In contrast, the bulk sediment pattern has lower HREE abundances than the back-arc magmas. Trace element patterns of the South Sandwich Islands samples are sub-parallel to those of the back-arc glasses, although the island arc magmas tend to be more depleted in Nb, Ta, Zr and Hf. Isotopic data for selected East Scotia Ridge samples are given in Table 2. The back-arc magmas are less radiogenic in terms of Sr and Pb than those from the South Sandwich Islands (for discussion, see section on components contributing to the back-arc source and Figs 15 and 16) and in the case of segments E6 and E7 show isotopic ratios as low as an average MORB (from Cohen & O’Nions, 1982b). The Nd isotopic ratios of the backarc magmas are comparable with those from the island arc. Both island arc and back-arc are much less radiogenic in Sr and more radiogenic in Nd compared with bulk South Atlantic sediment. The Pb isotopes in the sediments are similar to those in the island-arc magmas and lie at the radiogenic extreme of the back-arc magma fields. MODELLING FRACTIONAL CRYSTALLIZATION The large range of major element compositions covered by the samples from segments E2 and E8 (see, for example, Figs 3 and 4) can be modelled at least partially as the product of simple crystal fractionation. Unfortunately, as these segments were sampled only with a wax corer, it is impossible to check our results against the petrography of the lavas. We have performed crystal fractionation calculations using the least-squares GPP program of Geist et al. (1989) and mineral compositions measured on phenocrysts from other segments. The calculated fractionation trends are shown in the CaO vs MgO diagram (Fig. 4a), together with the mineral contents of the cumulate. The trends can be modelled as the result of either one- (E2) or two-stage (E8) crystal 1449 JOURNAL OF PETROLOGY VOLUME 43 NUMBER 8 AUGUST 2002 Fig. 3. K2O vs SiO2 diagram for volcanic glasses from the East Scotia Ridge. Data for the E2 segment samples are, apart from WX42–WX44 (Table 1), from Leat et al. (2000). Generalized trends for the low-K tholeiite, tholeiite and calc-alkaline series from the South Sandwich island arc (Pearce et al., 1995...
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