Ponding Conditions and Degassing Dynamics of Mafic Magmas Beneath the Azores Islands

Abstract:

The Azores archipelago is located at the triple junction between the North American, African and Eurasian plates, in an area dominated by transtensive tectonic. The magmatism is concentrated along elongated volcanic ridges, generally orthogonal to the Mid Atlantic Ridge (MAR), where central volcanoes alternate with fissure zones. In order to better understand the relationships between the regional and local tectonics and the magmatism, we carried out a systematic study of basaltic pyroclasts from monogenic Strombolian cones built up on both fissure zones and central volcanoes, on five Azores Islands. We combined the major and trace element geochemistry of bulk rocks, melt inclusions and minerals with microthermometric data of coexisting CO₂-rich fluid inclusions. These latter, trapped in Fo_88-82 olivines, reveal pressure decrease from west to east for each ridge, but the highest pressures are recorded by fluids trapped in mineral assemblage forming ultramafic cumulates (dunites, harzburgites and wehrlites). All these results fully confirm variable depths of the Moho Transition Zone (MTZ), which marks the upper limit for underplating and correspond to the magma ponding zones where the main processes of magmatic evolution occur. The MTZ is located at 25 km beneath Flores Island, on the North American plate, and 29.5 km beneath the island of São Miguel, 380 km eastward. It reaches a minimum (18.5 km depth) under Faial at ~120 km east of the MAR. In these pressure conditions, the volatile composition is X_H2O =0.1 and X_CO2=0.9, at 1155-1175 °C, under QFM redox conditions. The maximum dissolved volatile content achieves 1.8-1.9 wt% of H₂O and 0.4-0.5 wt% of CO₂ in the central archipelago, and 2.3-2.6 wt% of H₂O and 0.8-1.0 wt% of CO₂ at São Miguel. However, the total pressures (P_CO2+P_H2O) and the dissolved H₂O content recorded by melt inclusions are commonly underestimated. The initial H₂O content of the basaltic magmas characteristic of each volcanic system was re-assessed from the maximum value of the H₂O/Ce ratio, and their degassing paths calculated during their transfer in the crust, where fluid inclusions indicated the presence of small ponding areas. We used these data to discuss the influence of the different tectonic settings, associated to the studied systems, on the magma degassing paths.