Microcrysts Record Transient Convection at Piton de la Fournaise Volcano (La Réunion Hotspot)

We studied the morphologies of olivine, clinopyroxene and plagioclase microcrysts in olivine-rich basalts from the December 2005 eruption at Piton de la Fournaise, to reconstruct the thermal history of the magma prior to reaching the surface. Olivine microcrysts and their melt inclusions show textural and compositional evidence for rapid growth caused by successive cooling and reheating cycles below the liquidus of olivine (1230°C). Clinopyroxene crystals are corroded, which indicates reheating above the liquidus of clinopyroxene (1140°C). Clinopyroxene crystallization–dissolution is also evidenced by the composition of melt inclusions in olivine microcrysts. We interpret the cooling–heating cycles experienced by olivine and clinopyroxene microcrysts as evidence for thermal convection in the shallow magma storage zone (2–2·5 km depth) of the volcano. Plagioclase microcrysts do not record temperature fluctuations, and hence probably crystallized in the conduit. Given the small size of the olivine and clinopyroxene microcrysts and their rapid growth, it is likely that convection preceded the December 2005 eruption by only a few days at most. Our results suggest that convection is transient and proceeds in small-scale cells at low Rayleigh number (< 10
⁴
), rather than vigorous and continuous convection at high Rayleigh number.