Amphibole megacrysts with cavities: rapid crystal growth at mantle depth during the 1951 eruption of Fogo (Cape Verdes)

The late basanitic pyroclasts of the 1951 eruption at Fogo Island (Cape Verdes) contain ultramafic cumulate xenoliths and euhedral kaersutite megacrysts up to 12 cm in size. The megacrysts are characterized by abundant cavities that locally contain vesicular basaltic glass with small clinopyroxene phenocrysts. Some of these cavities are crystallographically oriented and remind of hopper textures. Micro-CT analyses reveal that the cavities are only in part interconnected and locally form funnel-like openings to the crystal surface. The kaersutite megacrysts also contain numerous pyrrhotite rods aligned perpendicular to the crystal surface, some pyrrhotite blebs, and locally stellate spinel aggregates. We interpret these structures to reflect rapid growth of the megacrysts in a volatile-rich silicate melt under the presence of a CO₂-H₂O-dominated fluid phase and abundant droplets of exsolved sulfide melt. Glass compositions within the cavities suggest that this melt was slightly more primitive than the erupted host melt. Barometric data based on clinopyroxene-melt equilibria within cavity glass, as well as kaersutite composition, indicate that megacrysts grew at pressures of 630–700 MPa, overlapping with a range of 620–640 MPa obtained for coevally erupted cumulate xenoliths, which is within the uppermost mantle. Possible cause for the rapid growth at such depths could be sudden liquidus increase due to H₂O loss of the host melt, following magma mixing or CO₂ flushing. We suggest that this occurred within a crystal mush where mobility of coexisting fluid and sulfide phase was limited.