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East Scotia Ridge unsecured

We interpret this trend to represent morbsubduction

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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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