Thermal Regime and Crystal-accumulation - Insights on Oceanic Islands Differentiation Processes at Fogo Island, Cape Verde

Abstract : Magma differentiation at oceanic islands is generally considered to happen in small magmatic reservoirs connected by vertical conduits allowing magma transportation. Here, we show that these vertical conduits are not only responsible for magma transportation, but may also act as magmatic reservoirs hosting differentiation processes. Fogo Island is the most active volcano of the Cape Verde Archipelago with the most recent eruption occurring in 2014. The volcano summit exposes ca. 150 Ka of volcanic activity along the caldera walls composed of series of aphanitic to ankaramitic lava flows and magmatic breccia cross-cut by numerous dykes. Bulk rock geochemistry reveals a wide range of compositions from foidites to tephriphonolites measured in the dykes and lavas flows (2-15 wt% MgO). A large part of this variability is linked to mineral accumulation, which can reach up to 70%. The combination of whole rock and mineral compositions allows to correct this cumulative effect and to propose a chemical differentiation model involving the crystallization of a phase assemblage of dominantly clinopyroxene with minor olivine and Fe-Ti oxides. Interestingly, clinopyroxene analyses reveal a constant Mg# (74-78) cluster in all lavas and dykes. This highlights the role of the thermal structure of the edifice to control the conditions of crystallization, as previously shown for the pyroxenites complex of Fuerteventura Island1, Canary Islands. To support this hypothesis, we present a 1-D thermal numerical model of multiple dyke injections based on the eruption frequency estimated for Fogo Island. We show that the temperature in the volcano root zone stays above 950-1000 °C for prolonged periods of time. The persistent high-temperature regime controls the magma differentiation trend and explains the presence of intermediate magma compositions indicated by the consistent clinopyroxene compositions. We suggest that vertical conduits act as ephemeral magma reservoirs, where minerals crystallize and segregate during magma transport and/or between eruptions. New pulses of magma later remobilize magma stored in these vertical conduits. This process explains the mineral accumulation in ankaramitic rocks and the presence of various cumulates observed in dykes and lavas from Fogo Island.