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.

Description: Supra-subduction zone (SSZ) ophiolites such as the Troodos Ophiolite of Cyprus are thought to have formed at spreading centres close to subduction zones. Similarities in the geochemistry between lavas from SSZ ophiolites and fore-arc lavas from active subduction zones, and the presence of boninites in both, have led to the suggestion that SSZ ophiolites represent fragments of fore-arc crust and mantle, formed during subduction initiation. Here we present major and trace element and Sr, Nd and Pb isotope data for fresh volcanic glasses from a section through the lava pile on the southern margin of the Troodos Ophiolite, and compare these to lavas from the Izu–Bonin–Mariana fore-arc. In Troodos, boninites and tholeiitic basalts are interbedded and were derived from a highly depleted mantle source that was later enriched by both fluids and melts derived largely from subducted sediment, before melting beneath the spreading axis. Troodos boninites differ from Izu–Bonin–Mariana boninites by their greater source depletion, enrichment in Nb by small degree melts, and lack of Zr enrichment relative to Sm. Together with the lack of fore-arc basalts in Troodos, our data imply that the Troodos Ophiolite was formed in a fore-arc location at a back-arc spreading centre that propagated into arc crust. The Troodos Ophiolite was thus not formed during subduction initiation and thus may not be used as analogue for the formation of fore-arc basalts.

Description: The Mahoney Seamount is a recently discovered volcanic edifice located 4 km north of the ultra-slow spreading Southwest Indian Ridge (SWIR). The SWIR is one of the slowest spreading ridges worldwide with a full spreading rate of ∼14 mm/year and low magmatic productivity. We report that highly vesicular basalts from the Mahoney Seamount have unradiogenic Nd-Hf together with radiogenic Sr isotopic compositions. Their distinct low 206Pb/204Pb isotope signature combined with high 207Pb/204Pb and 208Pb/204Pb is best explained by melting of a mantle that has been strongly influenced by stranded lower continental crust. The geographic distribution of the isotopic variability favors the idea of shallow recycling of lower continental crust isolated for a longer period contributing to melts forming Mahoney Seamount through off-axis fault systems. The isotopic composition of Mahoney Seamount lavas shares many characteristics with EM-1 sources and the DUPAL signature. Previous isotopic studies of the SWIR basalts proposed recycling of ancient subcontinental lithospheric mantle (SCLM) or pelagic sediments with oceanic crust to be responsible for this enriched isotopic signature. Lu/Hf and Sm/Nd ratios of pelagic sediments would result in decoupled 143Nd/144Nd and 176Hf/177Hf ratios. This decoupling is also observed in Ejeda-Bekily dikes from Madagascar, but those are believed to sample the SCLM dispersed in the Indian Ocean. However, Mahoney Seamount shows no decoupling in those isotopic systems and the restricted occurrence of the extreme lower continental crustal signature at Mahoney Seamount implies that the enriched isotopic signature has a different origin.