Volcanism at the tip of a propagating rift : the Heimaey volcanic centre, south Iceland

Primary magmas are generated by 4-6% partial melting near the garnet-spinel stability fields beneath Heimaey (i.e. 80-65 km depth). The magmas fractionate 31% olivine and clinopyroxene en-route to ponding at the base of the crust (and forming a parental Vestmannaeyjar magma). Abundant normally zoned phenocrysts of olivine and plagioclase, in combination with curvilinear trends in major element variation diagrams and heterogeneous isotope ratios (similar to MORB) suggests that the rock suite evolved by fractional crystallization and that crustal contamination and/or magma-mixing was insignificant. Although Heimaey rocks are devoid of clinopyroxene as a phenocryst phase, decreasing ratios of CaO/Al2O3 and Sc/Y with increasing degree of fractionation suggest that clinopyroxene was present as a fractionating phase. The most evolved lava on Heimaey (Eldfell) can be successfully related to the most primitive (Stórhöfði) through 73% fractional crystallization of an olivine + clinopyroxene + plagioclase + titanomagnetite assemblage. Individual magma batches were emplaced into different levels of the crust where they evolved separately from each other prior to eruption. The lack of equilibrium phenocryst assemblages in the Heimaey rocks suggests that the residence times in crustal magma chambers were short. Upon eruption, the pathways used by the rising magma are probably zones of weaknesses associated with extensional stress exerted by the propagating Eastern Volcanic Zone (as indicated by the preferentially aligned eruptive fissures). When magmas erupt in a shallow marine environment, tuff-cones are formed as a result of explosive interactions between magma and water/sediment mixtures. Tuffs comprise about 65% of total erupted volume on Heimaey. The tuff cones deposits consists of normally graded planar air-fall deposits and undulating cross-bedded base-surge deposits, with an increase in frequency of base-surge deposits closer to the vent regions. The distribution of crus