Pedro Madureira

Palaeomagnetic study of a subaerial volcanic ridge (Sao Jorge Island, Azores) for the past 1.3 Myr: evidence for the Cobb Mountain Subchron, volcano flank instability and tectonomagmatic implications

We present a palaeomagnetic study on 38 lava flows and 20 dykes encompassing the past 1.3 Myr on S. Jorge Island (Azores ArchipelagoNorth Atlantic Ocean). The sections sampled in the southeastern and central/western parts of the island record reversed and normal polarities, respectively. They indicate a mean palaeomagnetic pole (81.3 degrees N, 160.7 degrees E, K= 33 and A95= 3.4 degrees) with a latitude shallower than that expected from Geocentric Axial Dipole assumption, suggesting an effect of non-dipolar components of the Earth magnetic field. Virtual Geomagnetic Poles of eight flows and two dykes closely follow the contemporaneous records of the Cobb Mountain Subchron (ODP/DSDP programs) and constrain the age transition from reversed to normal polarity at ca. 1.207 +/- 0.017 Ma. Volcano flank instabilities, probably related to dyke emplacement along an NNWSSE direction, led to southwestward tilting of the lava pile towards the sea. Two spatially and temporally distinct dyke systems have been recognized on the island. The eastern is dominated by NNWSSE trending dykes emplaced before the end of the Matuyama Chron, whereas in the central/western parts the eruptive fissures oriented WNWESE controlled the westward growth of the S. Jorge Island during the Brunhes Chron. Both directions are consistent with the present-day regional stress conditions deduced from plate kinematics and tectonomorphology and suggest the emplacement of dykes along pre-existing fractures. The distinct timing and location of each dyke system likely results from a slight shift of the magmatic source.

Evolution of submarine eruptive activity during the 2011–2012 El Hierro event as documented by hydroacoustic images and remotely operated vehicle observations

Submarine volcanic eruptions are frequent and important events, yet they are rarely observed. Here we relate bathymetric and hydroacoustic images from the 2011 to 2012 El Hierro eruption with surface observations and deposits imaged and sampled by ROV. As a result of the shallow submarine eruption, a new volcano named Tagoro grew from 375 to 89 m depth. The eruption consisted of two main phases of edifice construction intercalated with collapse events. Hydroacoustic images show that the eruptions ranged from explosive to effusive with variable plume types and resulting deposits, even over short time intervals. At the base of the edifice, ROV observations show large accumulations of lava balloons changing in size and type downslope, coinciding with the area where floating lava balloon fallout was observed. Peaks in eruption intensity during explosive phases generated vigorous bubbling at the surface, extensive ash, vesicular lapilli and formed high-density currents, which together with periods of edifice gravitational collapse, produced extensive deep volcaniclastic aprons. Secondary cones developed in the last stages and show evidence for effusive activity with lava ponds and lava flows that cover deposits of stacked lava balloons. Chaotic masses of heterometric boulders around the summit of the principal cone are related to progressive sealing of the vent with decreasing or variable magma supply. Hornitos represent the final eruptive activity with hydrothermal alteration and bacterial mats at the summit. Our study documents the distinct evolution of a submarine volcano and highlights the range of deposit types that may form and be rapidly destroyed in such eruptions.

Noble Gas Constraints on the Origin of the Azores Hotspot

Noble gas geochemistry is a valuable tool for assessing the nature of contributors to magma mantle sources. In this chapter, we analyse previously published data regarding helium and neon in the Azores to discuss the origin of the Azores archipelago. After the pioneering works of Kurz et al. (1982a, 1990) examining helium isotopic variations along the Mid-Atlantic Ridge (MAR) between 28°N and 53°N and on Sao Miguel Island, a systematic study was conducted by Moreira et al. (1999) on several Azores islands. These authors analysed He isotopic ratios from phenocrysts collected in five islands of the archipelago. Helium isotopic data from Terceira (minimum 4He/3He ratio of ≈63,700; R/Ra ≈11.3) were interpreted as the presence of a relatively primitive component in the mantle source, whereas the radiogenic 4He/3He ratios found at Sao Miguel (from 121,600 to 276,800; R/Ra from ≈5.9 to ≈2.6) have been interpreted as resulting from the melting of an enriched and ancient recycled material. Moreira and Allegre (2002) analysed noble gas isotopic data for MORB glasses dredged along a MAR segment (from 21.25°N to 39.9°N), which also includes the Azores triple junction area. The obtained 4He/3He ratios decrease from 90,000 at 37°N to 75,000 at 38.5°N and later increase to 100,000 at 40.5°N. The low 4He/3He ratio measured on the ridge at 38.5°N (76,000; R/Ra = 9.5) was interpreted as the result of present-day interaction between the ridge source and the Azores plume (see also Madureira et al. 2014), as sampled by lava erupted at Terceira and Sao Jorge islands (the latter with 4He/3He ratios down to 40,000; R/Ra ≈18). Madureira et al. (2005) focused their study on Terceira Island, for which He and Ne isotopic ratios were determined from olivine phenocrysts. Ne isotopic data corroborated the presence of a relatively primitive component in the Terceira mantle source (21Ne/22Necorr = 0.052), which is, however, dominated by a MORB-type component. Jean-Baptiste et al. (2009) presented helium isotope data for thermal waters and gas emissions sampled at Terceira, Graciosa and Sao Miguel islands, as well as Faial, Pico and Flores islands. The results were interpreted as the presence of relatively primitive He isotopic ratios at Terceira (4He/3He ≈53,500; R/Ra ≈13.5), which also extend to Graciosa Island (4He/3He ≈64,500; R/Ra ≈11.2). These values contrast with those obtained by these authors for Sao Miguel Island, which are significantly more radiogenic than those typical of MORB (4He/3He from ≈120,400 to ≈138,900; R/Ra from 5.2 to 6.0) and are in agreement with the helium measurements in lavas from the eastern part of Sao Miguel (Moreira et al. 2012). Noble gas geochemistry suggests, despite the strong dilution of its primitive signature by MORB-like material, that a lower mantle-derived mantle plume is located under the central group of islands, particularly under Sao Jorge, in agreement with seismic tomographic images. The peculiar helium isotopic ratios observed in Sao Miguel lavas can be explained by different scenarios invoked for the Sr–Pb isotopic and trace element systematics, which suggest that the Sao Miguel source contains recycled mafic material ~3 Ga in age (e.g., Beier et al. 2007; Elliott et al. 2007).

Palaeomagnetic study of a subaerial volcanic ridge (São Jorge Island, Azores) for the cobb mountain subhron, volcano flank instability and tectonomagmatic implications

We present a palaeomagnetic study on 38 lava flows and 20 dykes encompassing the past 1.3 Myr on S. Jorge Island (Azores ArchipelagoNorth Atlantic Ocean). The sections sampled in the southeastern and central/western parts of the island record reversed and normal polarities, respectively. They indicate a mean palaeomagnetic pole (81.3 degrees N, 160.7 degrees E, K= 33 and A95= 3.4 degrees) with a latitude shallower than that expected from Geocentric Axial Dipole assumption, suggesting an effect of non-dipolar components of the Earth magnetic field. Virtual Geomagnetic Poles of eight flows and two dykes closely follow the contemporaneous records of the Cobb Mountain Subchron (ODP/DSDP programs) and constrain the age transition from reversed to normal polarity at ca. 1.207 +/- 0.017 Ma. Volcano flank instabilities, probably related to dyke emplacement along an NNWSSE direction, led to southwestward tilting of the lava pile towards the sea. Two spatially and temporally distinct dyke systems have been recognized on the island. The eastern is dominated by NNWSSE trending dykes emplaced before the end of the Matuyama Chron, whereas in the central/western parts the eruptive fissures oriented WNWESE controlled the westward growth of the S. Jorge Island during the Brunhes Chron. Both directions are consistent with the present-day regional stress conditions deduced from plate kinematics and tectonomorphology and suggest the emplacement of dykes along pre-existing fractures. The distinct timing and location of each dyke system likely results from a slight shift of the magmatic source.

Palaeomagnetic study of a subaerial volcanic ridge (São Jorge Island, Azores) for the past 1.3 Myr: evidence for the Cobb Mountain Subchron, volcano flank instability and tectonomagmatic implications: Palaeomagnetic study of S. Jorge Island

We present a palaeomagnetic study on 38 lava flows and 20 dykes encompassing the past 1.3 Myr on S. Jorge Island (Azores ArchipelagoNorth Atlantic Ocean). The sections sampled in the southeastern and central/western parts of the island record reversed and normal polarities, respectively. They indicate a mean palaeomagnetic pole (81.3 degrees N, 160.7 degrees E, K= 33 and A95= 3.4 degrees) with a latitude shallower than that expected from Geocentric Axial Dipole assumption, suggesting an effect of non-dipolar components of the Earth magnetic field. Virtual Geomagnetic Poles of eight flows and two dykes closely follow the contemporaneous records of the Cobb Mountain Subchron (ODP/DSDP programs) and constrain the age transition from reversed to normal polarity at ca. 1.207 +/- 0.017 Ma. Volcano flank instabilities, probably related to dyke emplacement along an NNWSSE direction, led to southwestward tilting of the lava pile towards the sea. Two spatially and temporally distinct dyke systems have been recognized on the island. The eastern is dominated by NNWSSE trending dykes emplaced before the end of the Matuyama Chron, whereas in the central/western parts the eruptive fissures oriented WNWESE controlled the westward growth of the S. Jorge Island during the Brunhes Chron. Both directions are consistent with the present-day regional stress conditions deduced from plate kinematics and tectonomorphology and suggest the emplacement of dykes along pre-existing fractures. The distinct timing and location of each dyke system likely results from a slight shift of the magmatic source.