Magma storage and ascent during the 1995 eruption of Fogo, Cape Verde Archipelago

The 1995 eruption of Fogo (Cape Verde Islands) differed from previous eruptions by the occurrence of evolved lavas, the SW-orientation of vents, and pre-eruptive seismicity between Fogo and the adjacent (similar to 20 km) island of Brava. We have conducted a thermobarometric and chemical study of this eruption in order to reconstruct its magma plumbing system and to test for possible connections to Brava. The bimodal eruption produced basanites (5.2-6.7 wt% MgO) and phonotephrites (2.4-2.8 wt% MgO) that are related by fractional crystallization. Clinopyroxene-melt-barometry of phenocrysts yields pressure ranges of 460-680 MPa for the basanites and 460-520 MPa for the phonotephrites. Microthermometry of CO2-dominated fluid inclusions in olivine and clinopyroxene phenocrysts yields systematically lower pressure ranges of 200-310 MPa for basanites and 270-470 MPa for phonotephrites. The combined data indicate pre-eruptive storage of the 1995 magmas within the lithospheric mantle between 16 and 24 km depth. During eruption, the ascending magmas stalled temporarily at 8-11 km depth, within the lower crust, before they ascended to the surface in a few hours as indicated by zonations of olivine phenocrysts. Our data provide no evidence for magma storage at shallow levels (< 200 MPa) or lateral magma movements beneath the Fogo-Brava platform. Sr-Nd-Pb isotope ratios of samples from Brava differ significantly from those of the 1995 and older Fogo lavas, which rules out contamination of the 1995 magmas by Brava material and indicates different mantle sources and magma plumbing systems for both islands.