Changing depths of magma fractionation and stagnation during the evolution of an oceanic island volcano:: La Palma (Canary Islands)

La Palma (Canary Islands) represents an oceanic island volcano with an active rift zone, inferred to have formed during the last 800 ka following southward growth of the former radial-symmetrical stratovolcano Taburiente. We carried out clinopyroxene-melt thermobarometry and microthermometry of fluid inclusions to reconstruct the evolution of the magma plumbing systems over time and to understand the genetic relationship between Taburiente and the presently active Cumbre Vieja rift zone. Clinopyroxene-melt equilibria of phenocryst rims and glassy groundmass indicate pressures of 0.60-1.04 GPa (similar to 19-34 km depth) for Taburiente, 0.47-1.17 GPa (16-40 km) for the former Cumbre Nueva rift arm of Taburiente, and 0.50-0.78 GPa (1626 km) for Bejenado volcano that formed after collapse of the Cumbre Nueva rift. These pressures are interpreted to reflect depths of magma storage and major crystal fractionation. CO2-dominated fluid inclusions hosted by clinopyroxenes and olivines indicate pressures of formation or re-equilibration within an overall range of 0.25-0.61 GPa (similar to 8-19 km depth). Respective frequency maxima are at 0.41-0.50 GPa for Taburiente dunite xenoliths, 0.26-0.43 GPa for Cumbre Nueva ankaramites, and 0.26-0.32 GPa for Bejenado cumulate xenoliths. These pressures are interpreted to reflect levels of temporary magma stagnation during ascent. Our data show that the magma pathways during all volcanic phases including the presently active Cumbre Vieja rift [Kliigel, A., Hansteen, T.H., Galipp, K., 2005. Magma storage and underplating beneath Cumbre Vieja volcano, La Palma (Canary Islands). Earth and Planetary Science Letters 236, 211-226] are characterized by two distinct storage levels: a system of prolonged storage within the upper mantle, and a system of short-term stagnation within the lower crust or near to the Moho. Both the mantle and crustal storage systems show a migration to shallower levels from 1.0 Ma to present, probably as a result of changing thermomechanical properties of the mantle and crust and possibly stoping. Our combined barometric data and field observations suggest that the extinct Taburiente/Cumbre Nueva and the active Cumbre Vieja represent two distinct volcanoes with separate magma plumbing systems. In this case, the present rift configuration does not reflect continuous growth of the Taburiente shield volcano during the last 800 ka. None of the La Palma volcanoes shows any indicators of along-lived shallow magma reservoir where magmas fractionate and from which rift zones emanate, which is an important difference to Hawaiian shield volcanoes characterized by shallow subcaldera magma chambers. (c) 2006 Elsevier B.V. All rights reserved.