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

Sammanfattning: 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 crustal xenoliths in the Sæfell tuff-cone suggests that there is a downward migration of explosion foci in tuff cone forming eruptions, and that a diatreme (at least 820 meters deep) formed during the eruption. This is also supported by the steep dip (35-45°) measured for ring-fractures at the crater-rim of the tuff cone, which creates sharp unconformities between early and late stage tuff-deposits. A zone of graded tuff-breccias (5-20 m thick) marks the transition from phreatomagmatic to subaerial activity. The subaerial stage is characterized by effusive lava emplacement and the formation of scoria cones and lava ponds inside the tuff-cones. The most common type of lava flows found on Heimaey are inflated pahoehoe (e.g. the Helgafell lava field). Based on measurements of lava inflation features (i.e. the thickness of the upper-vesicular crust of flow-lobes) in the Helgafell lava field an eruption duration was estimated for that eruption (11-12 months). This estimate seems reasonable as it yields an averaged volumetric effusion rate of 0.05-0.06 km3/month, which is similar to the 1963-1967 Surtsey eruption (0.02 km3/month) and the 1973 Eldfell eruption (0.04 km3/month) considering that the estimate does not account for early spatter-fed flows. The oldest units on Heimaey (i.e. the six units comprising the Norðurklettar formation) formed over a short time span following the deglaciation of southern Iceland 10-9.3 ka ago. The Stórhöfði eruption occurred sometime between 9-6 ka, Sæfell and Helgafell to approximately 6 ka, and finally the Eldfell eruption, which occurred in 1973. Heimaey is believed to represent the early stage of forming a central volcano in the Vestmannaeyjar system, mainly because (1) the average erupted volumes for a single eruption are twice (0.33 km3 DRE) that previously reported as an average for Vestmannaeyjar, (2) there have been close to 1 eruption/km2 on Heimaey, compared to 0.1 eruption/km2 as average for the Vestmannaeyjar system, and (3) the occurrence of more evolved lavas, such as the hawaiites of Dalfjallshryggur and Eldfell, suggesting that a crustal magma chamber is developing under the island.