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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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