East Scotia Ridge unsecured

In general segments e2 and e8 are most strongly

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Unformatted text preview: ) are indicated by dashed lines. Series boundaries and nomenclature from Peccerillo & Taylor (1976). fractionation. Olivine, plagioclase, and subordinate chrome spinel can produce the trend between 9 and 7 wt % MgO in the E8 magmas, whereas below 7 wt % MgO the decrease in CaO in both E2 and E8 reflects the appearance of clinopyroxene in the fractionating assemblage. One of the most differentiated glass samples of segment E2 (WX33, Leat et al., 2000) could be produced from the assumed parental basalt (WX21, Leat et al., 2000) by a total of >50% fractional crystallization: 27·3% clinopyroxene, 20·8% plagioclase, 2·4% olivine, 0·5% chrome spinel (R2 = 0·24). The first step fractionation model for samples from segment E8 yielded >10% crystal fractionation from the assumed parental (WX58) to daughter magma composition (WX62): 5% plagioclase, 3·5% olivine, 0·8% chrome spinel (R2 = 0·33). During the second step fractionation, the most differentiated glass sample of segment E8 (WX60) could be produced by >40% fractional crystallization: 19·5% plagioclase, 17·5% clinopyroxene, 4·4% olivine, 0·1% chrome spinel (R2 = 0·11). It is important to note that elevated water contents increase the mineral–liquid Kd of plagioclase and suppress plagioclase crystallization (e.g. Eggler & Burnham, 1973; Michael & Chase, 1987; Sisson & Grove, 1993; Hergt & Farley, 1994). At present, these effects cannot be quantified in a rigorous way and so the calculations discussed above should only be taken as indicative of the fractional crystallization processes that may have affected the lavas. The East Scotia Ridge lavas have high Al2O3 for a given MgO content compared with the Mid-Atlantic Ridge (MAR) glasses (Fig. 4b). High Al2O3 concentrations compared with MORB have also been observed in other back-arc regions (e.g. Mariana Trough, Sumisu– Torishima back-arc rifts) and have been attributed to the greater H2O content in back-arc magmas during crystallization (Sinton & Fryer, 1987; Fryer et al., 1990). The differences in Al2O3 contents between segments shown in Fig. 4b may reflect either the effect of varying amounts of water on plagioclase crystallization or differences in Al2O3 contents of magmas in different segments. Experimental work (Gaetani & Grove, 1998) has shown that varying amounts of water in the source at the time of melting will not affect the Al content of the primary magma and so an investigation of the relationship between Al and H2O in the magmas should be able to distinguish between these two possibilities. Moreover, the measured H2O content of the glasses should reflect unperturbed magmatic values, as degassing during ascent and eruption has little effect on dissolved H2O contents unless eruption depths are less than a few hundred metres (e.g. Stolper & Newman, 1994). This is confirmed for the East Scotia Ridge by our observations that there is no correlation between eruption depth and measured water contents in the glasses. 145...
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