V34B-01
Drilling of Submarine Shallow-water Hydrothermal Systems in Volcanic Arcs of the Tyrrhenian Sea, Italy
Seafloor hydrothermal systems related to volcanic arcs are known from several localities in the Tyrrhenian Sea in water depths ranging from 650 m (Palinuro Seamount) to less than 50 m (Panarea). At Palinuro Seamount 13 holes (<5m) were drilled using Rockdrill 1 of the British Geological Survey 1 into the heavily sediment-covered deposit recovering 11 m of semi-massive to massive sulfides. Maximum recovery within a single core was 4.8 m of massive sulfides/sulfates with abundant late native sulfur overprint. The deposit is open to all sides and to depth since all drill holes ended in mineralization. Metal enrichment at the top of the deposit is evident in some cores with polymetallic (Zn, Pb, Ag) sulfides overlying more massive and dense pyritic ore. The massive sulfide mineralization at Palinuro Seamount contains a number of unusual minerals, including enargite, tennantite, luzonite, and Ag-sulfosalts, that are not commonly encountered in mid-ocean ridge massive sulfides. In analogy to epithermal deposits forming on land, the occurrence of these minerals suggests a high sulfidation state of the hydrothermal fluids during deposition implying that the mineralizing fluids were acidic and oxidizing rather than near-neutral and reducing as those forming typical base metal rich massive sulfides along mid-ocean ridges. Oxidizing conditions during sulfide deposition can probably be related to the presence of magmatic volatiles in the mineralizing fluids that may be derived from a degassing magma chamber. Elevated temperatures within sediment cores and TV-grab stations (up to 60°C) indicate present day hydrothermal fluid flow. This is also indicated by the presence of small tube-worm bushes present on top the sediment. A number of drill holes were placed around the known phreatic gas-rich vents of Panarea and recovered intense clay-alteration in some holes as well as abundant massive anhydrite/gypsum with only trace sulfides along a structural depression suggesting the presence of an anhydrite seal to a larger hydrothermal system at depth. The aim of this study is to understand the role that magmatic volatiles and phase separation play in the formation of these precious and trace element-rich shallow water (<750m) hydrothermal systems in the volcanic arcs of the Tyrrhenian Sea.
V34B-02
Venting of a Separate CO2-Rich Gas Phase from Submarine Arc Volcanoes
Although CO2 is typically the most abundant gas dissolved in submarine hydrothermal fluids, it rarely appears as a completely separate phase. Among mid-ocean ridge hydrothermal systems, a significant discharge of pure CO2 has been found at only one site: the Magic Mountain vent field on the Explorer Ridge, northeast Pacific. In contrast to MOR systems, recent studies of submarine volcanoes on the Mariana and Kermadec Arcs have found several sites that, in addition to discharging hot vent fluid, are also venting a separate CO2-rich phase either in the form of gas bubbles or liquid CO2 droplets. One of the most impressive is the Champagne vent site on NW Eifuku in the northern Mariana Arc. This relatively small vent field is discharging cold droplets of liquid CO2 at an estimated rate of 23 moles CO2/sec, about 0.1% or the global MOR carbon flux (see Lupton et al., 2006). Three other Mariana Arc submarine volcanoes, NW Rota-1, Nikko, and Daikoku, all have vent fields discharging CO2 in the form of gas bubbles. At Nikko and Daikoku the CO2 gas is bubbling up through pools of liquid sulfur. In addition, Pisces dives on the Kermadec Arc in 2005 found venting of CO-rich gas bubbles at Giggenbach volcano and Volcano-1. Based on this limited data set, it appears that a separate CO2-rich gas phase is a relatively common occurrence on volcanic arcs and almost non-existent on mid-ocean ridges. This difference is probably due to the supply of subducted marine carbonates and organic matter incorporated into the melting process along volcanic arcs, although the shallower depth of submarine arc volcanoes also favors formation of a separate gas phase. At each of the 6 volcanoes considered here, the separate gas phase is also accompanied by venting of hot hydrothermal fluid. Our preliminary analysis indicates that the vent fluid is not in equilibrium with the gas phase, suggesting that the two phases separated at depth in the system. One possibility is that the gas phase results from direct CO2 degassing from a magma chamber, while the hot vent fluid originates from seawater circulating through the volcanic edifice. These findings indicate that carbon fluxes from submarine arcs may be higher than previously estimated. Detailed experiments to estimate carbon fluxes at submarine arc volcanoes would help to resolve this question.
V34B-03
Sulfur Lakes and Sulfur-rich Volcanic Hydrothermal Systems on the Mariana Arc
During the Submarine Ring of Fire expeditions in 2004 and 2006, and the Natsushima NT-05-18 expedition in 2005 we investigated and sampled hydrothermal systems on ten submarine volcanoes in the Mariana arc between 13.5 and 23.1 degrees N. The high volatile content of the volcanic arc environment is evident in the CO2 and SO2 dominated fluids sampled and differentiates volcanic arc hydrothermal chemistry from more rock- buffered mid-ocean ridge systems. Sulfur-dominated sites appear to be common on submarine arc volcanoes at water depths shallower than 700 meters. At NW Rota-1 volcano, there is clear evidence of ongoing eruptive activity producing clouds of particulate and molten sulfur as well as mm to m-size glassy volcanic ejecta. The hydrothermal system at NW Rota-1 represents a direct connection to a sub-seafloor magma body, and is one of the only known sites in the world where we can directly sample the solid, liquid and gaseous products of a submarine magmatic hydrothermal system. Fluids (30 to 260 deg C) sampled directly from an eruptive vent have pH as low as 1.0, with a high content of particulate sulfur, excess sulfurous and sulfuric acid, and very low H2S content. Fluids percolating through volcaniclastic sand adjacent to the vent reached 100 deg C and had higher silica, slightly higher pH, and millimolar levels of H2S. The chemistry of both types of fluids is indicative of input of volcanic SO2 and incomplete disproportionation into sulfuric acid and either H2S (in volcaniclastic sands) or elemental sulfur (in the eruptive vent). Highly acidic aqueous fluids attack the basaltic substrate, and carry high levels of iron and aluminum. Daikoku (21.3°N) and Nikko (23.1°N) submarine volcanoes both host active molten sulfur ponds and a wide variety of sulfur flows and deposits. The remarkable molten sulfur pools (~180-200°C) occur without widespread focused venting of hot water and may be maintained by active magmatic degassing of hot CO2/SO2-rich gases that bubble up through the pools. Although diffuse low-temperature hydrothermal vents are widespread on Daikoku and Nikko, they are less vigorous, less acidic and less enriched in sulfur and metals than the vents on NW Rota, possibly reflecting a lower SO2 content in the magmatic gas and a longer pathway between the magmatic heat source and the seafloor.
V34B-04
Volatile Chemistry at Lau Basin Hydrothermal Sites: Basin-Wide Trends of Slab Carbonate Influence and Suggestions of Abiotic Methane Oxidation at the Mariner Vent Site
The Lau Basin is actively spreading along three major spreading centers: the Central Lau Spreading Center (CLSC) in the north, the Eastern Lau Spreading Center (ELSC), and the Valu Fa Ridge (VFR) to the south. A southward progression along these spreading centers reveals decreasing spreading rates, decreasing distance to the arc, and a corresponding increase in the influence of the subducted Pacific plate slab (1, 2). Analysis of the d13CO2 and He data from volatile samples collected during a 2005 expedition to six hydrothermal sites along the ELSC and VFR reveals a systematic and continuous relationship between latitude (distance from the arc) and the arc-derived component of the signal. This smooth and linear transition from arc-like hydrothermal volatiles in the south to more MORB-like hydrothermal volatiles in the north contrasts the interpretation from the dredged geologic record that suggests an extremely sharp andesite-basalt transition occurring along the southern portion of the ESLC (3). The Mariner vent site, the southernmost high-temperature site visited in 2005, exhibits an arc-influenced volatile chemistry that is different from the other Lau Basin sites. The ESLC and northern VFR vents are characterized by CO2 concentrations ranging from 1-12 mmol/kg while the Mariner site has higher CO2 values, up to 60 mmol/kg. The opposite trend is seen in the methane concentrations as the northern vents range from 25-55 μmol/kg while Mariner methane values are much lower, at 5-7 μmol/kg. Isotopic results also illustrate the unique chemistry of the Mariner site: d13CO2 values are in the typical MOR-hydrothermal range of -7 to -4 per mil at northern vents and near 0 per mil at Mariner, and d13CH4 values are in the typical MOR-hydrothermal range of - 25 to -20 per mil at northern vents, and highly enriched, -7 to +0.7 per mil at Mariner. The CO2/3He ratio and the d13CO2 values at Mariner suggest that 80 percent of the CO2 at Mariner is sourced from carbonates, presumably from authigenically formed carbonates in the downgoing slab. The low CH4 concentrations and uniquely enriched d13CH4 signatures at Mariner are hypothesized to be the result of abiotic methane oxidation at hydrothermal temperatures. 1. Jacobs, A.M., A. J. Harding, G. M. Kent, Earth and Planetary Science Letters 259, 239 (2007). 2. Martinez, F., B. Taylor, E. T. Baker, J. A. Resing, S. L. Walker, Earth and Planetary Science Letters 245, 655 (2006). 3. Bezos, A., et al., Eos, Transactions, American Geophysical Union 86, Abstract V41C (2005).
V34B-05
The Role of Magmatic Volatile Input, Near-surface Seawater Entrainment and Sulfide Deposition in Regulating Metal Concentrations Within Manus Basin Hydrothermal Systems
Analyses of Fe, Mn, Cu, Zn, Pb, Ag, Cd, Co and Sb in vent fluid samples from four hydrothermal systems in the Manus back-arc basin, Papua New Guinea, were carried out by ICP-MS. Vienna Woods is located on the well- defined, basalt-dominated Manus Spreading Center, while the other systems are hosted in felsic volcanics on the Pual Ridge (PACMANUS), within a caldera (DESMOS), and on volcanic cones (SuSu Knolls). Metal concentrations were coupled with other fluid data (pH, SO4, Ca, H2S) to discriminate effects of deep- seated water-rock reaction and magmatic volatile input from near surface seawater entrainment, mixing, and consequent mineral precipitation and metal remobilization. Both magmatic volatile input (e.g. SO2, HCl, HF) and sulfide precipitation can increase fluid acidity and thus affect the aqueous mobility of metals. At Vienna Woods, 280°C end-member (Mg = 0) fluids have high pH (>4.2) and low metal contents (Fe <160 uM, Cu <10 uM, Zn <40 uM) relative to most mid-ocean ridge (MOR) vent fluids. The high pH and lack of evidence for magmatic volatile input are consistent with fluid compositions regulated by subsurface seawater- basalt/andesite reactions. Despite low aqueous Zn concentrations, Zn-rich (wurtzite-lined) chimneys are common at Vienna Woods active vents, reflecting deposition from fluids characterized by low Fe and Cu and high pH. At PACMANUS, black smoker fluids (T >300°C, pH ~ 2.7) are enriched in sulfide-forming metals by an order of magnitude relative to Vienna Woods fluids. Enrichments at PACMANUS reflect efficient leaching of metals at low pH, with the lower pH likely a result of input of magmatic volatiles. In addition, some vents fluids show clear evidence for seawater entrainment, subsurface precipitation of Cu-Fe-sulfides and preferential remobilization of Zn-sulfides (lower T, non-zero Mg, lower Fe, Cu, H2S and pH (2.3–2.4), but higher Zn, Pb, Cd and Ag, compared to black smokers). The higher metal concentrations and lower pH of fluids from PACMANUS versus Vienna Woods are reflected in chimney deposit compositions with Zn-poor sulfide linings composed of Cu-Fe-sulfides and As-Sb-sulfosalts in high T and lower T vents, respectively. At DESMOS caldera, fluid data suggest extensive magmatic volatile input (e.g. pH <1.5, elevated F and SO4) but lesser reaction with the basement felsic rocks (low Li, Rb, Mn). Sampled "acid-sulfate" fluids are low temperature (T ~180°C) with Mg >46 mM, and very high concentrations of some metals for these Mg concentrations (Fe >5 mM, Zn >50 – 400 uM). At SuSu Knolls, vent fluid compositions similar to those at both PACMANUS and DESMOS are observed. Smoker fluids have high but variable metal concentrations of similar magnitude to PACMANUS. Acid-sulfate fluids from North Su have low pH (<2), non-zero Mg (>40 mM), and high Fe and Zn concentrations, similar to DESMOS fluids. At SuSu Knolls, fluid compositions reflect either high temperature water-rock reaction (smoker fluids) or magmatic volatile input (acid-sulfate fluids). As at PACMANUS, chimney deposits that correspond to venting fluids are Cu-Fe-As-Sb-rich and Zn-poor, likely reflecting deposition from low pH, high Cu and Fe fluids.
V34B-06
Vapor-rich Hydrothermal Fluid Migration Within Pumiceous Sediment in the Iheya North Knoll, Okinawa Trough
The newly developed ROV NSS (Navigable Sampling System) enabled pin-point piston core sampling from the active hydrothermal field. In the Iheya North hydrothermal field in the mid-Okinawa Trough (27°47.5'N, 126°53.8'E, depth = 1000m), animal colonies are observed not only around the central mound structure (named as NBC) which discharges vigrously high temperature (T=311°C) clear fluid, but also as Calyptogena colony at 200m east from the NBC mound and as tube-worm colony at 250m southeast from the NBC mound. During Leg 3 of KY05-14 cruise (R/V Kaiyo of JAMSTEC) in Jan. 2005, fourn piston cores were successfully recovered with length from 65cm to 250cm. Surface sediments from the Calyptogena colony and the tube-worm colony were revealed as mainly composed of pumiceous sediment. Pore fluids from the Calyptogena Field showed unusual chemistry characterized as very low salinity (Cl=420mM), low Mg concentration, significantly lower Na/Cl ratio than seawater and high methane concentration, which suggests contribution of a vapor-rich hydrothermal component migrating within the pumice layer from the activity center. Moreover, decrease of SO4 accompanied by increase of alkalinity is notable even less than one meter depth below the seafloor. In situ sulfate reduction in the surface sediment caused by entrainment of the hydrothermal component would be a source of hydrogen sulfide that supports Calyptogena colony.
V34B-07
Preliminary Results of a Near-Bottom Integrated Seafloor and Water Column survey of Brothers volcano, Kermadec arc, Using the Autonomous Vehicle ABE
Brothers volcano, located about 310 km NE of New Zealand along the magmatic front of the Kermadec arc, is one of the best studied intraoceanic arc submarine volcanoes. Its 3.0 x 3.5 km caldera is slightly elliptical, with the long axis oriented about N320°E and has more than 300 m relief from a rim at ~1500 m to a maximum depth of 1880 m in its NW corner. Two major hydrothermal systems were discovered on it in the late 1990s, a high temperature field (up to 302°C) on the NW wall and a lower temperature gas-rich system on the summits of a pair of dacitic cones that occupy the SE half of the caldera. Although the caldera and cones were partly explored by submersibles in 2004 and 2005, the base map, made with a surface ship multibeam, was not detailed enough to understand the context of the seafloor observations. We used the autonomous vehicle ABE launched and recovered from the R/V SONNE in July-August 2007 to conduct high resolution near-bottom surveys of the caldera and its hydrothermal systems using a multibeam sonar, magnetometer, and CTD. The caldera wall, the dacite cones and part of the flat caldera rim were mapped in 96 hours of survey time over 8 dives. In addition, very detailed water column surveys at lower altitude and closer line spacing were conducted over the two most intense hydrothermal sites (i.e., the NW caldera wall and the smaller dacite cone). Although the results are preliminary, there are obvious correlations between hydrothermal activity, wall geomorphology, structural lineations, and the magnetic signature. New hydrothermal sites were discovered on the uppermost NW rim of the caldera and on the SW wall. This new map, along with the previously collected suites of fluid, mineral and seafloor observations, provides a baseline for future monitoring of Brothers' hydrothermal and volcanic activity. It will also provide a better understanding of how the long-term interplay of hydrothermal and volcanic activity affects the geomorphic evolution of submarine arc volcanoes. http://www.oceanexplorer.noaa.gov/explorations/07fire/welcome.html
V34B-08
Lithium Isotopic Composition of Intra-Oceanic Arc Hydrothermal Fluids - Initial Results From the Tonga Kermadec Arc
Active hydrothermal venting has recently been discovered on submarine volcanoes along the Tonga-Kermadec Arc. Li isotopic compositions of high temperature fluids from mid-ocean ridge systems have been well studied as an indicator of seawater-basalt exchange. we present here initial Li isotope data for hydrothermal fluids collected from the volcanoes along the arc: Volcano 1 and Volcano 19 on the Tonga arc and Monowai, Giggenbach, Macauley, Brothers, and Healy along the Kermadec arc. The system features shallow venting depths, phase separation, magmatic degassing, and pervasive diffuse flows. Black smoker venting occurs on Volcano 19 (maximum fluid temperature 245°C) and Brothers NW vents (290°C). δ7Li (relative to L-SVEC) of high temperature end-member fluids at these two locations are 8.2 and 7.2‰ respectively. These isotopic values are in the typical range for mid-ocean ridge crest hot springs, which average 7± 1‰ globally (Bray, 2001). The similarity in fluid composition may reflect comparable Li isotopic compositions of arc volcanic substrates and mid-ocean ridge basalts. Most low temperature fluids (<70°C, e.g. at Brothers cones, Healy, and Giggenbach) are variably enriched in Li and have δ7Li between 15‰ and the seawater value (32‰). Only Monowai shows slight Li depletion and higher δ7Li (33.5‰) than seawater. The trends of Li versus Mg vary widely between sites, reflecting very different water/rock ratios. However, high and low temperature fluids converge to a general linear relationship between δ7Li and Mg/Li and Cl/Li suggesting that the diffuse flows are essentially the product of subsurface mixing of ambient seawater and high temperature vent fluids. The compositions of diffuse flows differ from that of Baby Bare, a 60°C warm spring water from a ridge flank hydrothermal system in that the latter has lost both Mg and Li as a result of low temperature reaction (Wheat and Mottl, 2000). High δ7Li (33‰) relative to Mg/Li and Cl/Li has been observed at Macauley and, together with high Mg and Si, is consistent with acid leaching of chemically weathered rocks. Several sites (Volcanoes 1 and 19, Monowai, Giggenbach and Brothers) provide evidence of phase separation, but Li isotopic fractionation associated with this process appears to be insignificant. Available data also do not indicate anomalous composition in samples containing magmatic volatiles. Our initial results thus suggest that high temperature vent fluids from the Tonga-Kermadec main arc have similar Li isotope compositions as those from mid-ocean ridge systems. Further work is in progress to investigate the nature of diffuse flows. References: Bray, A.M., 2001, PhD thesis, University of New Hampshire. Wheat, C.G., Mottl, M.J., 2000. Geochim. Cosmochim. Acta 64, 629-642.