ALBERT

Description: Volcanism in the Eastern Azores Plateau occurs at large central volcanoes and along subaerial and submarine fissure zones, resulting from a mantle melting anomaly combined with transtensional stresses. Volcanic structures are aligned WNW‐ESE and NW‐SE, reflecting two tectonic stress fields that control the direction of lateral melt transport. Terceira Island is influenced by both stress fields, dividing the island into an eastern and western part. Several submarine volcanic ridges with variable orientations are located west of Santa Bárbara, the youngest central volcano on Terceira. Major, trace element and Sr‐Nd‐Pb‐Hf isotope compositions from submarine lavas and glasses, in part associated with the 1998-2001 Serreta Ridge eruption, vary between different lava suites, suggesting a formation from different mantle sources. Submarine lavas are more primitive than those from Santa Bárbara volcano, indicating that they are not laterally connected with the shallow magma reservoir located in 2‐ to 5‐km depth beneath the central volcano. Mineral thermobarometric data suggest that the older Serreta magmas were laterally transported at depths 〉5 km from Santa Bárbara predominantly in WNW direction. We propose that lithospheric extension controls magma transport from the central volcano to Serreta Ridge. The youngest Serreta lavas differ from Santa Bárbara and other submarine ridges in having less radiogenic Pb and higher Hf isotope ratios representing a new magma pulse ascending from the mantle. We conclude that lateral magma transport and the morphology of volcanic ridges are controlled by tectonic stresses in the lithosphere, whereas vertical melt transport is initiated by processes in the mantle.

Description: The D. João de Castro seamount is located on the ultraslow diverging Terceira Rift, Azores. D. João de Castro is a young central volcano (〈0.5 Ma) with recent volcanic and hydrothermal activity. The central volcano is located on a volcanically active fissure zone where two volcanic ridges extent rift-parallel from the edifice. Submarine lava samples and volcanic glasses were taken from D. João de Castro volcano and the adjacent submarine volcanic ridges (N and NW Castro Ridges). Samples range from the uppermost flanks at ~300 m water depth to ~1350 m water depth at the NW and N Castro Ridges. Samples were taken by TV-guided grab (GEOMAR Helmholtz-Zentrum für Ozeanforschung) and with a Remotely Operated Vehicle (ROV MARUM Quest 4000) during cruises M113 and M128 with the German research vessel R/V Meteor in 2015 and 2016, respectively. Major element concentrations of all lavas were analyzed in order to estimate the compositional variability of the edifice and the volcanic ridges. Trace element and Sr-Nd-Pb isotopes analyses where then carried out on selected lavas to reconstruct the mantle source signatures and the conditions of melting under which lavas from D. João de Castro formed. The sampling of these volcanic formations was designed to improve our understanding of how central volcanoes are formed in oceanic rift systems.