Description: Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 109 (2004): C06012, doi:10.1029/2003JC002028.

Description: The World Ocean Circulation Experiment Indian Ocean helium isotope data are mapped and features of intermediate and deep circulation are inferred and discussed. The 3He added to the deep Indian Ocean originates from (1) a strong source on the mid-ocean ridge at about 19°S/65°E, (2) a source located in the Gulf of Aden in the northwestern Indian Ocean, (3) sources located in the convergent margins in the northeastern Indian Ocean, and (4) water imported from the Indonesian Seas. The main circulation features inferred from the 3He distribution include (1) deep (2000–3000 m) eastward flow in the central Indian Ocean, which overflows into the West Australian Basin through saddles in the Ninetyeast Ridge, (2) a deep (2000–3000 m) southwestward flow in the western Indian Ocean, and (3) influx of Banda Sea Intermediate Waters associated with the deep core (1000–1500 m) of the through flow from the Pacific Ocean. The large-scale 3He distribution is consonant with the known pathways of deep and bottom water circulation in the Indian Ocean.

Description: National Science Foundation support is acknowledged for the UM part of the work through grants OCE-9820131 and OCE-998150. Support for the LDEO portion of the work was obtained from the National Science Foundation through awards OCE 94-13162 and OCE 98-20130.

Description: Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 9 (2008): Q06T01, doi:10.1029/2008GC002104.

Description: As part of a rapid response cruise in May 2006, we surveyed water column hydrothermal plumes and bottom conditions on the East Pacific Rise between 9°46.0′N and 9°57.6′N, where recent seafloor volcanic activity was suspected. Real-time measurements included temperature, light transmission, and salinity. Samples of the plume waters were analyzed for methane, manganese, helium concentrations, and the δ 13C of methane. These data allow us to examine the effects of the 2005–2006 volcanic eruption(s) on plume chemistry. Methane and manganese are sensitive tracers of hydrothermal plumes, and both were present in high concentrations. Methane reached 347 nM in upper plume samples (250 m above seafloor) and exceeded 1085 nM in a near-bottom sample. Mn reached 54 nM in the upper plume and 98 nM in near-bottom samples. The concentrations of methane and Mn were higher than measurements made after a volcanic eruption in the same area in 1991, but the ratio of CH4/Mn, at 6.7, is slightly lower, though still well above the ratios measured in chronic plumes. High concentrations of methane in near-bottom samples were associated with areas of microbial mats and diffuse venting documented in seafloor imagery. The isotopic composition of the methane carbon shows evidence of active microbial oxidation; however, neither the fractionation factor nor the source of the eruption-associated methane can be determined with any certainty. Considerable scatter in the isotopic data is due to diverse sources for the methane as well as fractionation as methane is consumed. One sample at +21‰ versus Peedee belemnite standard is among the most enriched methane carbon values reported in a hydrothermal plume to date.

Description: This field work was supported by NSF awards OCE0222069 (J.P.C., M.D.L.); OCE0525863 (D.J.F.); and OCE0327261 (T.M..S.); and the NASA Astrobiology Institute (JPC). The NOAA-VENTS program provided additional support through a grant to the Joint Institute for the Study of the Atmosphere and Ocean (JISAO) under NOAA Cooperative Agreement NA17RJ1232.

Description: Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 15 (2014): 4093–4115, doi:10.1002/2014GC005387.

Description: We present multiple lines of evidence for years to decade-long changes in the location and character of volcanic activity at West Mata seamount in the NE Lau basin over a 16 year period, and a hiatus in summit eruptions from early 2011 to at least September 2012. Boninite lava and pyroclasts were observed erupting from its summit in 2009, and hydroacoustic data from a succession of hydrophones moored nearby show near-continuous eruptive activity from January 2009 to early 2011. Successive differencing of seven multibeam bathymetric surveys of the volcano made in the 1996–2012 period reveals a pattern of extended constructional volcanism on the summit and northwest flank punctuated by eruptions along the volcano's WSW rift zone (WSWRZ). Away from the summit, the volumetrically largest eruption during the observational period occurred between May 2010 and November 2011 at ∼2920 m depth near the base of the WSWRZ. The (nearly) equally long ENE rift zone did not experience any volcanic activity during the 1996–2012 period. The cessation of summit volcanism recorded on the moored hydrophone was accompanied or followed by the formation of a small summit crater and a landslide on the eastern flank. Water column sensors, analysis of gas samples in the overlying hydrothermal plume and dives with a remotely operated vehicle in September 2012 confirmed that the summit eruption had ceased. Based on the historical eruption rates calculated using the bathymetric differencing technique, the volcano could be as young as several thousand years.

Description: Support for R.W.E. during this study was by internal NOAA funding to the NOAA Vents Program (now Earth-Ocean Interactions Program). The NSF Ridge 2000 and MARGINS programs played a major role in the planning and justification for the 2009 rapid response proposal that funded the May 2009 expedition. MBARI provided support and outstanding postprocessing of the multibeam bathymetry from the D. Allan B. AUV multibeam sonar used in this study. NSF also provided major funding for the 2009 expedition (OCE930025 and OCE-0934660 to JAR) and for the 210Po-210Pb radiometric dating (OCE-0929881 and for the 210Po-210Pb radiometric dating (OCE-0929881 to KHR)). The NOAA Office of Exploration and Research provided major funding for the 2009 and 2012 field programs.

Description: 2015-04-30

Description: 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 and are typically 2–3 m in height, with some reaching 6–7 m. Their ages (at time of sampling) fall broadly into three groups: 〈4, 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. Rarer are massive sulfide crusts 2–3 m thick. Two main types of chimney predominate: Cu-rich (up to 28.5 wt.% Cu) and, more commonly, Zn-rich (up to 43.8 wt.% Zn). Geochemical results show that Mo, Bi, Co, Se, Sn, and Au (up to 91 ppm) are correlated with the Cu mineralization, whereas Cd, Hg, Sb, Ag, and As are associated with the dominant Zn-rich mineralization. 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 contrast to venting at both the Upper Cone and NW Caldera sites. Individual vents are marked by low-relief (≤0.5 m) mounds comprising predominately native sulfur with bacterial mats. Vent fluids of the NW Caldera field are focused, hot (≤300°C), acidic (pH ≥ 2.8), metal-rich, and gas-poor. Calculated end-member fluids from NW Caldera vents indicate that phase separation has occurred, with Cl values ranging from 93% to 137% of seawater values. By contrast, vent fluids at the Cone site are diffuse, noticeably cooler (≤122°C), more acidic (pH 1.9), metal-poor, and 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. Iron oxide crusts 3 years in age cover the main cone summit and 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; negative δD and δ18OH2O for vent fluids; negative δ34S for sulfides (to −4.6‰), sulfur (to −10.2‰), and δ15N2 (to −3.5‰); vent fluid pH values to 1.9; and mineral assemblages common to high-sulfidation systems. Changing physicochemical conditions at the Brothers hydrothermal system, and especially the Cone site, occur over periods of months to hundreds of years, as shown by interlayered Cu + Au- and Zn-rich zones in chimneys, variable fluid and isotopic compositions, similar shifts in 3He/4He values for both Cone and NW Caldera sites, and overprinting of “magmatic” mineral assemblages by water/rock-dominated assemblages. 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 relatively shallow depths. The NW Caldera and Cone sites are considered to represent stages along a continuum between water/rock- and magmatic/hydrothermal-dominated end-members.

Description: Along the Galápagos Spreading Center (GSC), 3He/4He varies from 8.5–5.9 RA. High 3He/4He ratios, resembling those in the western and southern Galápagos islands, are absent. This lack of high 3He/4He contrasts markedly with other localities of plume–ridge interaction, such as Iceland, Easter, and Amsterdam/St. Paul. The most striking feature is a 3He/4He gradient, decreasing westward from 8.4–7.0 RA between 89 and 93°W, where the GSC is shallowest and shows “axial high” morphology. The intra-segment 3He/4He variability within this region indicates that magma crosses the mantle/crust boundary at multiple points beneath individual ridge segments, and lateral mixing within the crust and upper mantle is limited. Some of the 3He/4He variability may also reflect transfer of discrete heterogeneity from beneath the northern sector of the Galápagos plateau. One possible explanation for the absence of high 3He/4He along the GSC is that helium is a relatively ineffective downstream tracer of mantle material from the core of the Galápagos plume, due to its preferential extraction beneath the archipelago compared to other incompatible, lithophile tracers. A second explanation is that the heterogeneous Galápagos plume is sheared in the upper mantle by motion of the Nazca Plate relative to the migrating GSC. In this case, plume core material having high 3He/4He (from beneath Fernandina and Isabela) would be dispersed mostly away from the ridge, while plume edge material having low 3He/4He plus enriched Sr and Pb isotope signatures (from beneath the northern periphery of the archipelago) is smeared into the sub-ridge mantle.

Description: Subduction of oceanic crust and the formation of volcanic arcs above the subduction zone are important components in Earth’s geological and geochemical cycles. Subduction consumes and recycles material from the oceanic plates, releasing fluids and gases that enhance magmatic activity, feed hydrothermal systems, generate ore deposits and nurture chemosynthetic biological communities. Among the first lavas to erupt at the surface from a nascent subduction zone are a type classified as boninites. These lavas contain information about the early stages of subduction, yet because most subduction systems on Earth are old and well-established, boninite lavas have previously only been observed in the ancient geological record. Here we observe and sample an active boninite eruption occurring at 1,200 m depth at the West Mata submarine volcano in the northeast Lau Basin, southwest Pacific Ocean. We find that large volumes of H2O, CO2 and sulphur are emitted, which we suggest are derived from the subducting slab. These volatiles drive explosive eruptions that fragment rocks and generate abundant incandescent magma-skinned bubbles and pillow lavas. The eruption has been ongoing for at least 2.5 years and we conclude that this boninite eruption is a multi-year, low-mass-transfer-rate eruption. Thus the Lau Basin may provide an important site for the long-term study of submarine volcanic eruptions related to the early stages of subduction.

Description: Strombolian-style volcanic activity has persisted for six years at the NW Rota-1 submarine volcano in the southern Mariana Arc, allowing direct observation and sampling of gas-rich fluids produced by actively degassing lavas, and permitting study of the magma-hydrothermal transition zone. Fluids sampled centimeters above erupting lava and percolating through volcaniclastic sediments around an active vent have dissolved sulfite 〉100 mmol/kg, total dissolved sulfide 1 mmol/kg. If NW Rota is representative of submarine arc eruptions, then volcanic vent fluids from seawater-lava interaction on submarine arcs have a significant impact on the global hydrothermal flux of sulfur and Al to the oceans, but a minimal impact on Mg removal. Gas ratios (SO2, CO2, H2, and He) are variable on small spatial and temporal scales, indicative of solubility fractionation and gas scrubbing. Elemental sulfur (Se) is abundant in solid and molten form, produced primarily by disproportionation of magmatic SO2 injected into seawater. Se accumulates within the porous rock surrounding the lava conduit connecting the magma source to the seafloor. Accumulated Se can be heated, melted, and pushed upward by rising magma to produce molten Se flows and lavas saturated with Se. Molten Se near the top of the lava conduit may be ejected up into the water column by escaping gases or boiling water. This mechanism of Se accumulation and refluxing may underlie the relatively widespread occurrence of Se deposits of many sizes found on submarine arc volcanoes.