Magmareservoirs und Aufstiegsdynamik von historischen und prähistorischen Eruptionen von Fogo, Kapverdische Inseln: ein barometrischer, petrologischer und geochemischer Ansatz

Abstract

Fogo is one of the most active oceanic intra-plate volcanoes in the world and the only island of the Cape Verde Archipelago showing historic activity. This study has been conducted to shed light on the magma plumbing systems of the island. In particular it aims to gain information on the depths of magma stagnation and differentiation, and on the ascent dynamics. Fogo was affected by a giant prehistoric lateral collapse that led to the removal of the summit and the eastern flank of the former Monte Amarelo volcano. The scar was partly refilled by intensive subsequent volcanic activity that is represented by the present-day Cha das Caldeiras plain and the young Pico do Fogo stratovolcano. During the 20th century Fogo experienced two eruptions in 1951 and 1995 that were both fed from fissures on the flanks of Pico do Fogo. The 1995 eruption differed from earlier eruptions in a distinct chemical and mineralogical bimodality with phonotephrites (2.4-2.8 wt% MgO) being erupted in the first days and basanites (5.2-6.7 wt% MgO) in the later phase, the uncommon southwest orientation of the eruption fissure, and pre-eruptive seismicity between Fogo and the adjacent island of Brava. Geochemical modeling of major and trace elements shows that the phonotephrites formed out of the basanites by crystal fractionation. Clinopyroxene-melt barometry of phenocrysts yields overlapping pressure ranges for final crystal growth before eruption of 460-680 MPa for the basanites and 460-520 MPa for the phonotephrites, corresponding to 16-24 km depth in the lithospheric mantle. Microthermometry of CO2-dominated fluid inclusions in basanites yield pressures of 270-440 MPa for olivine-hosted and 240-270 MPa for clinopyroxene hosted ones. Inclusions in phenocrysts of the phonotephrites yield a pressure range of 320-470 MPa for olivine-hosted and 200-310 MPa for clinopyroxene-hosted fluid inclusions. Fluid inclusions in olivine, especially of phonotephrites show overlapping pressures with the data of clinopyroxene-melt barometry. The lower pressures derived for clinopyroxene-hosted inclusions are interpreted to reflect a level of syn-eruptive short-term magma stagnation in the lower crust at 8-11 km depth. Chemical zonations of olivine phenocrysts indicate a rapid final magma ascent during eruption in 100 ka, which includes the Monte Amarelo collapse, indicate that magma storage and differentiation occurred in the lithospheric mantle at pressures of 420-870 MPa (15-30 km depth) presumably throughout the subaerial evolution of Fogo. The fractionation depths decreased through time, though this trend was temporarily interrupted by the giant collapse because the oldest post-collapse eruption shows a deeper stagnation level than the youngest pre-collapse one. The petrologic data indicate that large flank collapses may significantly influence deep-seated magma plumbing systems beneath ocean islands. Historic eruptions show shallower and broader pressure ranges and more complexly zoned clinopyroxene phenocrysts, suggesting an increase in complexity of the magma storage system. The lack of shallow persistent magma chambers, however, may be a consequence of the cool Mesozoic crust and thick lithosphere beneath Fogo