Chemical Constraints on the Petrogenesis of Mildly Alkaline Lavas from Vestmannaeyjar, Iceland - the Eldfell (1973) and Surtsey (1963-1967) Eruptions

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Contributions to Mineralogy and Petrology
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The Vestmannaeyjar archipelago is composed of alkalic lavas erupted at the southern end of the active, southward propagating, Eastern Volcanic Zone. Recent eruptions include the most primitive (Surtsey) and most evolved (Eldfell) compositions found in this area. We studied time-stratigraphic sample suites from both eruptions to characterize the magmatic environment of Vestmannaeyjar. All samples are nearly homogeneous in radiogenic isotopic ratios (Sr-87/Sr-86 0.70304 to 0.70327; Nd-143/Nd-144 0.51301 to 0.50307; Pb-208/Pb-204 18.96 to 19.18; Pb-207/Pb-204 15.50 to 15.53; Pb-208/Pb-204 38.47 to 38.76; KH Park and A Zindler, in preparation). Compositional trends of lavas from the two eruptions are not consistent with fractionation in a near-surface environment, but indicate rather moderate pressure evolution of small magma batches. At Eldfell, mugearite lavas can be modeled by 30% closed-system fractional crystallization of olivine + plagioclase + clinopyroxene + Fe-Ti oxides from parental hawaiite. The phase proportions are consistent with an experimentally determined moderate pressure (8 kbar) cotectic in mildly alkaline systems (Mahood and Baker 1986). Compositional variations of Surtsey lavas can be modeled by crystallization of clinopyroxene + olivine + plagioclase + minor Fe-Ti oxides. The presence of sodic plagioclase megacrysts and clinopyroxene with approximately 8 wt% Al2O3 in xenoliths from Surtsey lavas are consistent with a moderate pressure fractionation event. Based on major-element and REE data the most primitive Surtsey lavas formed by small degrees of melting of a lherzolite source. The alkaline nature of Vestmannaeyjar lavas is not the result of assimilation of lower crustal melts (cf. Oskarsson et al. 1985; Stein-thorsson et al. 1985.


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