A CO2-rich basanitic magma source for Fogo Volcano (Cape Verde Archipelago) inferred from volatile contents in silicate melt inclusions.

Understanding the pre-eruptive volatile contents in magmas is critical to charactering the magmatic plumbying systems that feed acative volcanoes, and is key to volcano monitoring and volcanic hazard assessment. Silicate melt inclusions (MIs) hosted in primitive minerals are a powerful tool to definite parental melt volatile contents, and to track the volatile degassing path upon magma ascent and decompression.

Here, we apply different analyses (Raman Spectroscopy, Nano SIMS, Electron microprobe, Laser Ablation ICPMS) for the characterisation of major and trace elements and volatiles in silicate melt inclusions entrapped in minerals from recently erupted tephra by Fogo Volcano in Cape Verde archipelago, one of the most active intraplate volcanic systems on Earth.Our aims are to (i) characterise the pressure-dependent magma compositional changes taking place during magma storage and ascent, (ii) model magmatic degassing and (iii) constrain the magmatic source, and the rates/modes of magma ascent prior and during eruption.

Seventeen MIs hosted in twelve olivine phenocrysts (Fo_79-85) were examined from tephra samples of two distinct periods of the last 10 ky of activity of the volcano. In detail, we studied basanitic (SiO₂ ~42 wt.%, MgO ~4.8 wt. %) and alkali-rich (Na₂O + K₂O = 6.9 wt.%) tephra samples of São Jorge (early Holocene activity, ~10 ka) and of the most recent eruptions (1951 and 2014/15).

Results reveal high concentrations of incompatible trace elements (e.g.,˜70 ppm Nb) and dissolved volatiles ( ˜2.1 wt.% H₂O and ≥1 wt.% CO₂) in the parental (un-degassed) magma. We use different H₂O-CO₂ solubility models to estimate MI entrapment pressures along the magma plumbing system. The deepest entrapment pressures of  ˜1000-1400 MPa (corresponding to  ˜ 30-46 km) are recorded in Holocene products, while the inclusions from the recent eruptions indicate shallower entrapment pressures of  ˜ 350-1100 MPa (˜ 11-35km). These entrapment pressure data, combined with previous independent barometric results, demonstrate a relatively deep (30-40 km) magma source for Fogo eruptions. Our results are the first to unambiguously demonstrate the CO₂-rich nature of alkali-rich mafic melts feeding intraplate volcanism at Cape Verde.