Magma storage and ascent of historic and prehistoric eruptions of Fogo, Cape Verde Islands: A barometric, petrologic and geochemical approach

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

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

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

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 (~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

Barometry of lavas from the 1951 eruption of Fogo, Cape Verde Islands: Implications for historic and prehistoric magma plumbing systems

Fogo is one of the most active oceanic volcanoes in the world. The island was affected by a prehistoric giant lateral collapse that decapitated the summit of the former Monte Amarelo volcano. Subsequent volcanism has partly filled the collapse scar and built up the present-day Cha das Caldeiras plain and the Pico do Fogo stratovolcano. We have conducted a thermobarometric study of historic and prehistoric, basanitic to tephritic rocks in order to gain insight into Fogo's magma plumbing system and the impact of the collapse event on fractionation depths. A main focus was the penultimate 1951 eruption, which produced basanites to tephrites (5.0–8.2 wt.% MgO) at two sites south and northwest of Pico do Fogo. Clinopyroxene–melt barometry of phenocrysts yields a well-confined pressure range of 480–650 MPa for the final crystallization level. Microthermometric data of CO2-dominated fluid inclusions in olivine and clinopyroxene phenocrysts yield systematically lower pressures of 250–430 MPa. Inclusions in cumulate xenoliths yield pressures of 100–290 MPa. The combined data indicate pre-eruptive magma storage in the uppermost mantle between 17 and 22 km depth and syn-eruptive short-term magma stalling within the lower crust at 8–13 km depth. The lower pressures revealed by fluid inclusions in xenoliths may indicate that they originate from pre- 1951 magma pulses that stalled and crystallized at variable levels in the crust. There is, however, no petrologic evidence for persistent crustal magma chambers. Clinopyroxene–melt barometric data of other historic and prehistoric eruptions indicate that magma storage and differentiation occurred in the uppermost mantle at pressures between 420 and 870 MPa (15–30 km depth) throughout the subaerial evolution of Fogo. Our data indicate that fractionation depths decreased significantly during a period of about 100 ka representing a strong growth phase of Fogo edifice leading up to the giant Monte Amarelo flank collapse at 123–62 ka. This trend seems to have been interrupted by the flank collapse, as early post-collapse magmas stagnated at deeper levels than late pre-collapse ones. Historic eruptions show shallower and broader pressure ranges, indicating an increase in complexity of the magma storage systems. Thus petrologic data indicate that flank collapse events may significantly influence deep-seated magma plumbing systems beneath ocean islands