Magmatic Evolution of Semisopochnoi Island, Alaska: Trace-Element and Isotopic Constraints

Semisopochnoi has erupted a tholeiitic suite of moderately evolved lavas ($$SiO_{2} = 50-65\%; Zr = 64-250 ppm; La = 10-60x chondrites$$), whose bulk-rock chemistry and phenocryst mineralogy are generally compatible with an important role for fractional crystallization in controlling the overall evolution of the suite. Trace-element data, however, are not consistent with crystal fractionation operating alone. Instead, a variety of plots involving incompatible elements (or ratios among them) demonstrate that simple mixing between basaltic and silicic components can reproduce the essential aspects of the trace-element variations and must have been operative as well. We are unable to distinguish between a silicic end-member represented by residual dacitic liquid like one of the analyzed lavas (therefore implying magma mixing) and one consisting of pre-existing silicic crustal rocks (therefore implying assimilation). Variations in Sr and Nd isotopic compositions are very limited: $$^{143}Nd/^{144}Nd = 0.51225-0.51227 (\epsilon_{Nd} = +8.1-8.5)$$ and $$^{87}Sr/^{86}Sr = 0.70320-0.70333$$. With the exception of the Sr composition of one sample, the isotopic data are also compatible with a combined mixing-fractionation model. Coupled variations of $$\epsilon_{Nd}$$ and chondrite-normalized Ba/La ratios imply derivation of parental magmas from a source that has been enriched in incompatible elements, relative to MORB-sediment mixtures, perhaps by dehydration of the subducted slab and resulting metasomatism of the overlying mantle.

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