Vesteris Seamount - Magma plumbing dynamics and petrological evolution of an alkaline intra-plate volcano of non-plume and non-rifting origin

Vesteris Seamount is a solitary, alkaline intraplate volcano of non-plume and non-rifting origin located in the Norwegian-Greenland Basin. During RV Maria S. Merian Cruise MSM86 highly detailed sampling of the Seamount recovered over 70 rock samples from the whole volcanic edifice during TV-Grab and ROV Marum SQUID operations. Major- and trace element geochemical analyses of matrix compositions and phenocrysts were conducted by electron microprobe and laser ablation ICP-MS. To investigate the petrogenesis of the Vesteris rock suite and the magma plumbing dynamics the analytical results were applied to least-squares mass balance and clinopyroxene-liquid thermobarometry models.

Primitive basanites and alkali-basalts reflect individual melt batches from a deep mantle source. A tight liquid line of descent can be observed from basanitic to trachytic compositions. The main control on the magmatic evolution is exerted by fractional crystallization. Textural and chemical evidence indicate a more pronounced influence of magma mixing during the genesis of the evolved tephriphonolitic and trachytic compositions.

Results from our clinopyroxene-melt thermobarometry calculations show a bimodal pressure distribution that indicates upper mantle storage between 500 and 850 MPa (15-27 km depth) and shallower crustal storage between 250 and 460 MPa (7-14 km). The magmatic evolution of basanites and alkali-basalts by crystal fractionation began in the upper mantle and subsequently migrated into crustal levels where phonotephritic to trachytic magma compositions developed. A multi-stage model for the evolution of the magma plumbing system below Vesteris Seamount is proposed based on the geochemical and thermobarometric data.

Initially, dykes transported deep mantle melts to the surface. Subsequent dyke and sill emplacement and high magma fluxes led to the formation of a reservoir in the upper mantle and magma accumulation zone at the uppermost mantle to Moho levels. Progressive weakening of the crust and continuing high magma fluxes led to and development of a crustal storage zone. At a later stage volcanic activity became more episodic and magmatic evolution was controlled mainly by fractional crystallization. The basanites and phonotephrites were stored at upper mantle levels, evolved tephriphonolitic and trachytic magmas were stored at crustal levels prior to eruption. During episodes of increased volcanic activity, the crustal reservoir was recharged by batches of poorly differentiated melts and initiated magma mixing.