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East Scotia Ridge unsecured

7 a na8 composition calculated after klein langmuir

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Unformatted text preview: gly, the samples with lowest MgO concentrations erupt near the topographic highs at the centres of these two segments. Surprisingly, lavas of segment E9, which shows a very pronounced topographic high, are not strongly fractionated. Geophysical investigations of segment E2 (Livermore et al., 1997) showed that there is a melt lens situated directly beneath the topographic high at the segment centre, >3 km beneath the sea floor. Leat et al. (2000) suggested that the highly fractionated magmas observed on the high probably formed within the seismically imaged melt lens. If this is the case, then we might expect further melt lenses to exist beneath the topographic highs on segments E8 and E9. The absence of highly fractionated magmas on E9, despite the existence of a pronounced axial high (Bruguier & Livermore, 2001), suggests that sea-floor topography is not always a reliable indicator of the presence of a persistent magma chamber. CHEMICAL VARIATIONS AND CHARACTERIZATION OF COMPONENTS ALONG THE RIDGE Chemical variations along the ridge There are systematic variations in composition of lavas along the East Scotia Ridge. Na8 values, which indicate degree of mantle partial melting (Klein & Langmuir, 1987), are roughly positively correlated with axial depth along the ridge, in that the highest Na8 samples are generally from deeper parts of the ridge (Fig. 7a), and shallow segments E2 and E8 have low Na8 values. Most samples from the central segments E4–E7 have relatively high and roughly constant Na8. Segment E9 also has high Na8, although it is as shallow as segment E2. The relationships between segments E2–E8 indicate that axial morphology may have been grossly controlled by degree of partial melting. Nb/Yb, a ratio unaffected by additions from the subducting slab, but sensitive to the degree of partial melting, is approximately positively correlated with axial depth (Fig. 7b). Lavas with generally low, NMORB-like, Nb/Yb ratios dominate in rift-like segments E5–E7 near the centre of the ridge, whereas high Nb/ Yb lavas occur in the shallow, inflated segments E2 and E9. However, high Nb/Yb samples have also erupted in segment E4, which is rift-like and deep, but not in 1452 FRETZDORFF et al. PETROGENESIS OF EAST SCOTIA RIDGE Fig. 6. Fractionation corrected Al8 (wt %) concentration vs (H2O)8 (wt %) content for the East Scotia Ridge glasses. The Al2O3 content of the magma has been recalculated to 8 wt % MgO along the fractionation vector for each segment in a manner similar to Fe8 or Na8 of Klein & Langmuir (1987). Quantified fractionation assemblages (see fractional crystallization calculation in text and in Fig. 4a) and the Rayleigh distillation equation (see, e.g. Cox et al., 1979) have been used to work out the effects of fractionation on magmatic water contents assuming complete incompatibility of water in the cumulate assemblage. Data for dredged samples from segments E3 (D20) and E9 (D23) (see Fig. 2) are from Muenow et al. (1980) and are indicated with a ‘D’. segment E8,...
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