East Scotia Ridge unsecured

Data for dredged samples from segments e3 d20 and e9

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Unformatted text preview: 0 FRETZDORFF et al. PETROGENESIS OF EAST SCOTIA RIDGE Fig. 4. Variation of CaO and Al2O3 (wt %) vs wt % MgO for samples from the East Scotia Ridge. In (a), the continuous and dashed lines represent calculated liquid compositions from crystal fractionation models determined with the GPP program of Geist et al. (1985). In (b), the continuous lines with arrows show the crystal fractionation trends of the E2 (Leat et al., 2000), E3, E7 and E8 segments, and Mid-Atlantic Ridge lavas (MAR: Schilling et al., 1983). Each segment forms a distinct trend, parallel to those from the other segments and to the Mid-Atlantic Ridge. The dashed line with an arrow is perpendicular to the fractionation trends and may reflect either the effect of varying amounts of water on plagioclase crystallization or different Al2O3 contents in primary magmas of each segment (see text for details). First, however, we must remove the effects of lowpressure crystal fractionation on Al2O3 and H2O in the erupted magmas. For Al2O3, this involves calculating Al8 (Al2O3 content of the magma recalculated to 8 wt % MgO along the fractionation vector for each segment) in a manner similar to the Fe8 or Na8 values of Klein & Langmuir (1987). We chose samples for H2O determinations that are relatively primitive (>6% MgO), and so do not expect the effects of crystal fractionation on water contents to be large. We used the quantified fractionation assemblages calculated above and the Rayleigh distillation equation (see, e.g. Cox et al., 1979) to work out the effects of fractionation on magmatic water contents, assuming complete incompatibility of water in the cumulate assemblage. Figure 6 shows the relationship between (H2O)8 and Al8. We see a positive correlation 1451 JOURNAL OF PETROLOGY VOLUME 43 NUMBER 8 AUGUST 2002 Fig. 5. N-MORB-normalized (Hofmann, 1988) trace element patterns for selected volcanic glasses from the East Scotia Ridge. Data for the E2 segment (apart from WX42–WX44) are from Leat et al. (2000) and the shaded area represents the range of trace element abundances in South Sandwich island arc lavas (Pearce et al., 1995). The thick continuous line represents the composition for bulk South Atlantic sediment (n = 15) from Plank & Langmuir (1998). between these two parameters which, in view of the possibilities outlined above, we interpret as reflecting the effect of magmatic water content on plagioclase solubility in the melt. Michael & Chase (1987) have shown that the MORB crystallization sequence is olivine → olivine + plagioclase → olivine + plagioclase + clinopyroxene. Our interpretation of the correlation between (H2O)8 and Al8 is that magmas with higher water contents progress further along the olivine-only fractionation path (shown in Fig. 4b) and so reach higher Al2O3 contents before plagioclase starts to precipitate. In general, segments E2 and E8 are most strongly affected by fractional crystallization processes, with differentiated samples reaching MgO contents as low as 3 wt %. Interestin...
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