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The olivine alkali basalt flow field at Surtsey volcano consists of two pahoehoe lava shields largely made up of inflated sheet lobes and intercalated surface breakouts. Each sheet lobe exhibits a three-fold structural division into upper crust, core and lower crust, where the core corresponds to the liquid part of an active pahoehoe flow lobe sealed by the continually growing crusts. Segregations are common in Surtsey lavas and are confined to the core of individual lobes. Field relations and volume considerations indicate that segregation is initiated by generation of volatile-rich melt at or near the lower crust to core boundary via in-situ crystallization. Once buoyant, the segregated melt rose through the core during last stages of flow emplacement and accumulated at the base of the upper crust. The segregated melt is preserved as vesicular, coarse-grained material, of FeTi basalt composition, within vesicle cylinders, horizontal vesicle sheets and as floor-fill in megavesicles. Our results indicate that the segregated melt evolved from the host lava by 50–60% fractional crystallization of olivine, plagioclase and clinopyroxene. The eruption temperature of the host lavas was c. 1160 °C, whereas the segregated melt formed at c. 1130 °C and crystallized down to c. 750 °C. The oxygen fugacity during crystallization varied from approximately FMQ-buffer to values two order of magnitude lower at the time of final solidification. The reducing conditions were favourable for crystallization of olivine, which forms a near-continuous compositional trend from Fo85 to Fo13. Feldspar compositions range from An79Or0 to An2Or57 and clinopyroxene compositions range from diopside-rich augites to aegerine–augites. Other mineral phases are magnetite, ilmenite and apatite, along with trace amounts of aenigmatite and nepheline. Vesicle cylinders in the lava core also contain residual phonolite glass of variable peralkalinity. The segregations have a strikingly similar whole-rock composition to the FeTi basalts from Katla and other volcanoes in South Iceland. These basalts are of relatively evolved composition with high melt densities. Volatile induced liquid transfer, as we propose for the formation of the segregations, may play an important role during magma differentiation and may explain the abundance of FeTi basalts in Iceland. Finally, closed-system fractional crystallization in Surtsey lavas produced melts of composition very close to the agpaitic lujavrites from the Illimaussaq intrusion in South Greenland.

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