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International audience; The seismicity of the North Atlantic was monitored from May 2002 to September 2003 by the 'SIRENA array' of autonomous hydrophones. The hydroacoustic signals provide a unique data set documenting numerous low-magnitude earthquakes along the section of the Mid-Atlantic Ridge (MAR) located in a ridge-hot spot interaction context. During the experiment, 1696 events were detected along the MAR axis between 40°N and 51°N, with a magnitude of completeness level of mb ≈ 2.4. Inside the array, location errors are in the order of 2 km, and errors in the origin time are less than 1 s. From this catalog, 15 clusters were detected. The distribution of source level (SL) versus time within each cluster is used to discriminate clusters occurring in a tectonic context from those attributed to non-tectonic (i.e. volcanic or hydrothermal) processes. The location of tectonic and non-tectonic sequences correlates well with regions with positive and negative Mantle Bouguer Anomalies (MBAs), indicating the presence of thinner/colder and thicker/warmer crust respectively. At the scale of the entire array, both the complete and declustered catalogs derived from the hydroacoustic signals show an increase of the seismicity rate from the Azores up to 43°30′N suggesting a diminishing influence of the Azores hot spot on the ridge-axis temperature, and well correlated with a similar increase in the along-axis MBAs. The comparison of the MAR seismicity with the Residual MBA (RMBA) at different scales leads us to think that the low-magnitude seismicity rates are directly related to along-axis variations in lithosphere rheology and temperatures.

The seismicity of the North Atlantic was monitored from May 2002 to September 2003 by the 'SIRENA array' of autonomous hydrophones. The hydroacoustic signals provide a unique data set documenting numerous low-magnitude earthquakes along the section of the Mid-Atlantic Ridge (MAR) located in a ridge-hot spot interaction context. During the experiment, 1696 events were detected along the MAR axis between 40 degrees N and 51 degrees N, with a magnitude of completeness level of m(b) approximate to 2.4. Inside the array, location errors are in the order of 2 km, and errors in the origin time are less than 1 s. From this catalog, 15 clusters were detected. The distribution of source level (SL) versus time within each cluster is used to discriminate clusters occurring in a tectonic context from those attributed to non-tectonic (i.e. volcanic or hydrothermal) processes. The location of tectonic and non-tectonic sequences correlates well with regions with positive and negative Mantle Bouguer Anomalies (MBAs), indicating the presence of thinner/colder and thicker/warmer crust respectively. At the scale of the entire array, both the complete and declustered catalogs derived from the hydroacoustic signals show an increase of the seismicity rate from the Azores up to 43 degrees 30'N suggesting a diminishing influence of the Azores hot spot on the ridge-axis temperature, and well correlated with a similar increase in the along-axis MBAs. The comparison of the MAR seismicity with the Residual MBA (RMBA) at different scales leads us to think that the low-magnitude seismicity rates are directly related to along-axis variations in lithosphere rheology and temperatures. info:eu-repo/semantics/publishedVersion

Brothers volcano, of the Kermadec intraoceanic arc, is host to a hydrothermal system unique among seafloor hydrothermal systems known anywhere in the world. It has two distinct vent fields, known as the NW caldera and cone sites, whose geology, permeability, vent fluid compositions, mineralogy and ore forming conditions are in stark contrast to each other. The NW caldera site strikes for ~600 m in a SW-NE direction with chimneys occurring over a ~145 m depth interval, between ~1,690 and 1,545 m. At least 100 dead and active sulfide chimney spires occur in this field, as seen during manned submersible dives, and are typically 2-3 m in height, although locally reach 6-7 m. Their ages fall into three groups: 0-4 years (at time of sampling), 23 and 35 years old. The chimneys typically occur near the base of individual fault-controlled benches on the caldera wall, striking in lines orthogonal to the slopes. More rare are massive sulfide crusts up to 2-3 m thick. Two main types of chimney predominate: Cu-rich chimneys (with parts up to 28.5 wt.% Cu) and more commonly, Zn-rich chimneys (up to 43.8 wt.% Zn). Geochemical results indicate a "magmatic" suite of elements associated with the Cu mineralization, including up to 91 ppm Au, and an ‘epithermal’ suite of elements with the dominant Zn-rich chimneys. The cone site comprises the Upper cone site atop the summit of the recent (main) dacite cone, and the Lower cone site that straddles the summit of an older, smaller, more degraded dacite cone on the NE flank of the main cone. Huge volumes of diffuse venting are seen at the Lower cone site, in complete contrast to venting at both the Upper cone and NW caldera sites. Individual vents are marked by low relief (≤0.5 m) mounds comprised predominately of native sulfur with bacterial mats. Vent fluids of the NW caldera field are focused, hot (≤300°C), acidic (pH ≥ 2.8), metal-rich and relatively gas-poor. Calculated end-member fluids from various NW caldera vents indicate phase-separation has occurred with Cl values ranging from 93% to 137% seawater values. By contrast, vent fluids for the cone site are diffuse, noticeably cooler (≤122°C), very acidic (to pH = 1.86), relatively metal-poor and very gas-rich. Higher-than-seawater values of SO4 and Mg in the cone vent fluids show that these ions are being added to the hydrothermal fluid and are not being depleted via normal water/rock interactions. Three year old Fe-oxide crusts covering the main cone summit appear to have formed from Fe-rich brines. Evidence for magmatic contributions to the hydrothermal system at Brothers includes: high concentrations of dissolved CO2 (e.g., 206 mM/kg at the cone site), high CO2/3He values, negative δD and δ18OH2O values for vent fluids, negative δ34S values for sulfides (to -4.6‰), sulfur (to -10.2‰) and δ15N2 (to -3.5‰), vent fluid pH values to -1.9 and mineral assemblages of chalcopyrite, bornite, chalcocite, covelllite and euhedral grains of hematite. Changing physio-chemical conditions at the Brothers hydrothermal system, and especially the cone site, occur over months to hundreds of years, as shown by interlayed Cu/Au- and Zn-rich zones in chimneys, variable fluid and isotopic compositions between vents, similar shifts in 3He/4He values for both cone and NW caldera sites, and overprinting of "magmatic" mineral assemblages by water/rock dominated ones. Metals, especially Cu and possibly Au, may be entering the hydrothermal system via the dissolution of metal-rich glasses. They are then transported rapidly up into the system via magmatic volatiles utilizing vertical (~2.5 km long), narrow (~300 m diameter) "pipes", consistent with evidence of vent fluids forming at higher than hydrostatic pressures, at relatively shallow depths. The NW caldera and cone sites are considered to represent stages along a continuum between magmatic-hydrothermal and water/rock-dominated end-members.

The East Blanco Depression (EBD), a pull‐apart basin within the western Blanco Transform Fault Zone (BTFZ), was the site of an intense earthquake T‐wave swarm that began at 1317Z on January 9, 1994. Although technically generated earthquakes occur frequently along the BTFZ, this swarm was unusual in that it was preceded and accompanied by periodic, low‐frequency, long‐duration acoustic signals, that originated from near the swarm epicenters. These tremor‐like signals were very similar in character to acoustic energy produced by a shallow‐submarine eruption near Socorro Island, a seamount several hundred km west of Baja, California. The ∼69 earthquakes and ∼400 tremor‐like events at the EBD occurred sporadically, with two periods of peak activity occurring between January 5–16 and 27–31. The swarm‐like character of the earthquakes and the similarity of the tremor activity to the Socorro eruption indicated that the EBD was undergoing an intrusion or eruption episode. On January 27, six CTD/rosette casts were conducted at the site. Water samples from two of the stations yielded anomalous ³He concentrations, with maxima at ∼2800 m depth over the main basin. In June 1994 two camera tows within the basin yielded evidence of pillow‐lava volcanism and hydrothermal deposits, but no conclusive evidence of a recent seafloor eruption. In September 1994, deployments of the U.S. Navy's Advanced Tethered Vehicle resulted in the discovery of an active hydrothermal mound on the flanks of a pillow‐lava volcano. The hydrothermal mound consists of Fe‐rich hydrothermal precipitate and bacterial mats. Temperatures to 60°C were measured 30 cm below the surface. This is the first discovery of active hydrothermal vents along an oceanic fracture zone. Although no conclusive evidence of volcanic activity associated with the T‐wave event swarm was found during these response efforts, the EBD has been the site of recent seafloor eruptions.