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Thermal Dynamics and Crystal Accumulation: Unraveling Magma Differentiation at Fogo Island, Cape Verde

Sebastien Pilet, Selena Galdini, Annelore Bessat, Cindy Luisier, Evelyne Tornare

Abstract

Magma differentiation at oceanic islands typically occurs in small reservoirs connected by vertical conduits that transport magma. Here, we explore whether these conduits can also serve as temporary magma reservoirs. Fogo Island is the most active volcano in the Cape Verde Archipelago, last erupted in 2014. The volcano summit exposes ca. 150 Ka of volcanic activity along the caldera walls with aphanitic to ankaramitic lava flows and magmatic breccias intersected by numerous dykes. Geochemical analyses of dykes and lava flows show a wide compositional range from foidites to tephriphonolites (2--15 wt% MgO), largely influenced by mineral accumulation of up to 70%. By combining whole-rock and mineral chemistry, this accumulation effect can be corrected, revealing a differentiation model dominated by clinopyroxene crystallization, with minor olivine and Fe-Ti oxides. Clinopyroxene consistently displays a Mg# of 74--78 across all samples, suggesting that the volcano's thermal structure governs crystallization conditions. A 1-D thermal model simulating repeated dyke injections---based on eruption frequency---shows that temperatures in the volcano's root zone remain above 950--1000 °C for extended periods. This sustained heat controls magma differentiation and explains the intermediate compositions observed. We propose that vertical conduits temporarily store magma, allowing minerals to crystallize and segregate during transport or between eruptions. Subsequent magma injections remobilize this stored magma, accounting for mineral accumulation in ankaramitic rocks and the diverse cumulates in Fogo's dykes and lava flows. This model offers valuable insights for forecasting the magma composition of upcoming volcanic activity on oceanic islands.