Renewed melting at the abandoned Húnafloí Rift, northern Iceland, caused by plume pulsing

Abstract

The abandoned Húnafloí Rift (HR), near Skagi in northern Iceland, provides an opportunity to investigate how relocation of an oceanic spreading ridge influences melt generation. Melting recommenced beneath HR several million years after spreading ceased at 7–4 Ma. Both the older (pre-HR abandonment) and younger (post-abandonment) lavas are tholeiitic basalts with high Fe–Ti and enriched incompatible trace element compositions. The HR lavas are compositionally similar to modern lavas from central Iceland (above the Iceland Plume conduit) and the Eastern Volcanic Zone (a propagating rift). Although older and younger HR lavas are compositionally similar, geological and geophysical observations indicate different mantle melting conditions. For the older lavas, a plume migration reconstruction suggests that the combination of incompatible trace element enrichment and crustal thickness of ∼25 km can be explained by plume-driven upwelling, a scenario similar to central Iceland at present. The smaller volume of the younger HR lavas (<1 km melt thickness) and their greater distance from the plume centre argues against plume-driven upwelling to explain their enrichment. Rare earth element inversion modelling indicates that the younger lavas come from reactivation of the deepest part of the melting region beneath HR. We suggest that the base of the HR melting region was reactivated when a pulse of unusually hot asthenosphere, known to be producing unusually thick crust ∼700 km away from the plume centre today, passed laterally beneath the HR at ∼3 Ma. A thermal and kinematic ridge–plume interaction model indicates that this scenario can explain both the volume and composition of the younger Skagi lavas, provided that a small amount of decompression occurs. The decompression may result either from extension or from buoyancy-driven upwelling of hot mantle beneath the relatively thin lithosphere of the abandoned HR. The hypothesis that a hot plume pulse caused the younger HR lavas is significant not only as an example of interaction between mid-ocean ridge melting and time-dependent mantle convection, but also because this same hot pulse may have subsequently helped promote intensification of the Northern Hemisphere Glaciation when it uplifted the oceanic gateways either side of Iceland.