V54A-01 16:00h
Magmatic Contribution of Ore Metals to the Conical Seamount Hydrothermal System, Papua New Guinea: a High-Precision Pb Isotope Study.
During RV SONNE cruise SO-133 in 1998 polymetallic gold mineralization was discovered at Conical Seamount, located on the flank of Lihir island, Papua New Guinea. The seamount and the island are composed mainly of trachybasalts and basaltic trachyandesites, although some monzonites are found on Lihir. Sr isotopic analyses suggest that most of the Sr in the mineralized samples is derived from the local alkaline lavas. Elevated 87Sr/86Sr ratios in some of the samples suggest that during the waning stages of the hydrothermal system, some of the Sr was contributed either from seawater or the thick sequence of marine sediments underlying the island. High-precision Pb isotopic analyses conducted with MC-ICP-MS show that the ores and volcanic rocks share similar Pb isotopic compositions, suggesting that the Pb in the mineralized zones was ultimately derived from local magmatic sources. The Pb isotopic data, however, reveal small, but significant, differences between the mineralized zones and the associated host lavas. Mineralized samples from Lihir have slightly less radiogenic lead isotopic ratios than their host lavas. These lead isotopic compositions are similar, however, to some of the fresh lavas recovered from Conical seamount and to a monzonite intrusion underlying the Ladolam deposit. Lead isotopic ratios in mineralized samples from Conical seamount, however, are slightly more radiogenic than their host lavas and similar to those of fresh lavas recovered from nearby Tubaf and Edison seamounts. Petrographic data reveal a complex magmatic history for the magma chamber inferred beneath Conical seamount. Based on zoning patterns in the Conical clinopyroxenes, it appears that a sub-seamount magma chamber was recharged with a mafic magma similar to the most primitive, volatile-rich, and xenolith-bearing lavas recovered from Tubaf and Edison seamounts. Rapid cooling of this mafic magma accompanied by exsolution of metal-bearing fluids in the relatively shallow magma chamber is proposed as the mechanism that ultimately resulted in the observed gold mineralization. Similarity of the lead isotopic ratios between the ores and the mafic magmas in the area suggest that the ore metals were primarily derived from the recharging magma, not by hydrothermal leaching of the host lavas by seawater-derived hydrothermal fluids.
V54A-02 16:15h
Pb and Other Trace Elements in Melt Inclusions From Modern Seafloor Tectonic Settings
Evidence for the contribution of magmatic metal-bearing fluids to modern seafloor hydrothermal systems has been presented for key ore metals such as Cu, Zn, Au and Ag but not for Pb. The source of Pb in these systems can be attributed to leaching of host rocks by hydrothermal fluids and/or potentially by direct degassing of magma. We present data from 103 melt inclusions mainly in plagioclase crystals hosted in basaltic to rhyolitic lavas from 9 modern seafloor tectonic settings: oceanic back-arcs (Manus Basin including an ODP Leg 193 drill hole, Bransfield Straits), continental back-arc (Okinawa Trough), mid-ocean ridges (Explorer, MAR), seamounts (Foundation, Axial, Pito), and a transform (Garrett). Most sites host active hydrothermal systems that are producing polymetallic sulfides. Inclusions were analyzed for Pb and other trace elements by LA- ICP-MS at ETH Zurich. Although inclusion trace element compositions vary between tectonic settings, for a given setting variability between different volcanic sequences is low and follows a general mafic to felsic fractionation trend. N-MORB compositions dominate most inclusion populations with exceptions seen at Explorer (plume-ridge interaction), Garrett (near-ultramafic source) and in some Foundation samples (seamounts affected by ridge interaction). Results show high Pb concentrations in inclusions from oceanic back arc and continental back-arc settings coinciding with evolved volcanic chemistries and related in the latter to contamination from sediments or continental crust. Sulfide deposits in these back-arcs are also Pb-rich. Inclusions from primitive basalt and near-ultramafic hosted samples from MOR, intraplate seamounts and transforms have low Pb abundances. In the most Pb abundant settings, melt inclusions have Pb concentrations 1-3 times higher than that of the bulk rock or volcanic glass indicating possible Pb removal from the magma prior to eruption. This loss may be explained by direct degassing of the magma into the hydrothermal circulation. Exceptions to the behavior of Pb in the studied settings include: elevated Pb in inclusions hosted in primitive basalts TAG (MAR) and some samples from the Okinawa Trough showing low Pb with respect to bulk rock.
V54A-03 16:30h
Behavior of trace metals in the hydrothermal plume at two sites on the Izu-Bonin-Mariana Arc
Deep-sea hydrothermal systems play an important role in the oceanic geochemical cycles of trace metals. High concentration of trace metals of the basalt origin is discharged into the deep sea via the hydrothermal plume. The hydrothermal plume is widely diffused to the ocean by mixing with ambient seawater. The processes of input and removal in the diffusing hydrothermal plume differ by individual hydrothermal systems. In this presentation, the behavior of trace metals in the hydrothermal plume of two sites on the Izu-Bonin-Mariana Arc is compared. This study was funded by the O`Archaean ParkO_L project of MEXT. The hydrothermal plume samples were taken from the Suiyo Seamount and the southern Mariana Trough (Pika Site). The mini CTDT-RMS mounted twelve 1.2L Niskin bottles was installed onto the manned submersible. And the hydrothermal plume samples were collected with monitoring the anomaly of temperature and turbidity. The samples were immediately filtered in an onboard clean bench. Unfiltered sample for analysis of total (particulate + dissolved) trace metal and filtered sample for analysis of dissolved trace metal were acidified. Trace metals (Al, Mn, Fe, Cu and Zn) in the hydrothermal plume samples were analyzed by GFAAS. The ranges of concentration of Al, Mn, Fe, Cu and Zn in the hydrothermal plume samples collected from two sites are _`15uM, _`5uM, _`5uM, _` 0.2uM and _`0.6uM, respectively. The particulate phase is predominant form in Al, Fe, Cu and Zn, and Mn shows the superiority of dissolved form. At the Suiyo Seamount, the hydrothermal active site is located in the bottom of caldera. On the other hand, the hydrothermal active site exists on the top of off-ridge seamount at the southern Mariana Trough. The diffusion process of trace metals in the hydrothermal plume to the ocean differed by the topographic factor in two sites. It suggests that trace metals discharged from the vents are hardly diffused to the ocean surmounting the Suiyo Seamount caldera because the metals almost removed from the hydrothermal plume in the caldera.
V54A-04 16:45h
Hydrothermal Mineralization Along the Volcanically Active Mariana Arc
In March and April, 2004, ROPOS ROV dives took place from the R/V T.G. Thompson along the volcanically active Mariana arc to ground truth CTD data collected a year earlier that indicated hydrothermal activity. Dives took place on seven volcanoes, six of which showed hydrothermal activity. We present data on samples collected from NW Rota-1 ($14\deg$, 36'N, $144\deg$, 46'E), E. Diamante ($15\deg$, 56'N, $145\deg$, 41'E), and NW Eifuku ($21\deg$, 29'N, $144\deg$, 03'E), the three sites most studied. All the hydrothermal systems found are associated with volcano summits, or with resurgent domes inside a caldera. Brimstone vent at NW Rota-1 provided a dramatic display of thick, bellowing, yellow plumes that contained ash and molten sulfur. This site occurs at 500 m water depth and clearly shows closely associated magmatic-hydrothermal discharge. Sulfur was the dominant hydrothermal mineral deposited around the vent and occurs as spheres in the surrounding volcaniclastic sediment, fracture fill and veins, and massive deposits. The Black Forest vent field at E Diamante consists of a sulfide-sulfate chimney system developed at about 650 m water depth. This is the only mature system discovered and consists of numerous tall (up to 9 m) chimneys. The measured fluid temperature of $240\deg$ C produces boiling at the depth of the vents. The chimneys and mounds are composed of varying amounts of pyrite, sphalerite, chalcopyrite, barite, and anhydrite. Hydrothermal Mn oxides occur on the surface of inactive chimneys. This mineralogy contrasts with the other two systems, which deposit sulfur as the dominant hydrothermal product. The Cu-Zn-Fe-Ba mineralization is perhaps largely controlled by water/rock interaction. A unique hydrothermal field (Champagne field) was found at NW Eifuku where liquid CO$_{2}$ is discharging from focused- and diffuse-flow vents at 1600 m water depth. The focused-flow vents consist of small chimneys and mounds up to a meter high that are composed of sulfur and yet to be identified minerals. Sulfides were not recovered from this site. At Champagne, sulfur also occurs as crusts on the surface of sediment in areas of diffuse-flow venting, as fracture fill and veins, and as massive deposits. The dominant characteristics of the hydrothermal mineralization along the Mariana arc are shallow-water systems, highly permeable rocks, gas (SO$_{2}$, H$_{2}$S, CO$_{2}$)-rich discharge, and magmatic-hydrothermal systems controlled by the depth-to-boiling-point curve. These characteristics result predominantly in the subsurface deposition of sulfide mineralization, with E Diamante being the exception.
V54A-05 17:00h
Microbeam Analyses of Rare-Earth Element (REE) and Sr Isotopes of Anhydrite Veins From the PACMANUS Hydrothermal System
Ocean Drilling Program Leg 193 drilled into areas of focused and diffuse venting from the PACMANUS hydrothermal field along Pual Ridge, eastern Manus Basin. Pual Ridge is primarily composed of fresh dacite that is pervasively altered to argillaceous (illite-chlorite) and overprinting acid-sulfate (pyrophyllite-anhydrite) and siliceous secondary mineral assemblages. Anhydrite is an abundant precipitate in all cores recovered beneath the active vent field. Isotopic (Sr, S), elemental (Mg, Sr, REE) and fluid inclusion analyses on anhydrite mineral separates demonstrate that pathways of fluid evolution (magmatic input, variable fluid composition, seawater entrainment) differ on the meter scale within this system. In addition, large variations in REE content, S ($\delta^{34}$S $>$ 5$\permil$) and Sr ($^{87}$Sr/$^{86}$Sr $>$ 1000 ppm) isotope composition are observed within individual anhydrite veins. Using microbeam techniques, we have analyzed this small scale chemical and isotopic heterogeneity in order to unravel the record of hydrothermal processes occurring at PACMANUS. Cathodoluminescence microscopy is used to distinguish between different crystal growth textures (e.g., oscillatory versus sector zoning). A Finnigan Neptune laser-ablation MC-ICP-MS is used to measure $^{87}$Sr/$^{86}$Sr, and a Cameca 3f ion microprobe and a Finnigan Element2 laser-ablation MC-ICP-MS to measure REE and minor element concentrations in these zones. Initial microbeam measurements of sector-zoned crystals reveal REE patterns trending from mid-REE enriched to MREE depleted with distance from crystal center and with a uniform $^{87}$Sr/$^{86}$Sr (0.7087). Analyses for oscillatory zoned crystals reveal variable and increasing $^{87}$Sr/$^{86}$Sr (0.7043 to 0.7089) and REE patterns trending from light-REE enriched to LREE depleted with both negative and positive Eu anomalies during crystal growth. $^{87}$Sr/$^{86}$Sr reproducibility for an isotopically homogenous anhydrite is better than 40 ppm. The Sr isotopic composition range for heterogenous crystals ($>$ 4000 ppm) is hence greater than 100 times the analytical uncertainty of our LA-MC-ICP-MS method. The general trend of these data can be modeled by Rayleigh-type fractional crystallization of anhydrite, with sector zoned anhydrite precipitating from a seawater dominated fluid and oscillatory zoned anhydrite forming from a hydrothermally dominated fluid following introduction of seawater into the basement. Future microbeam REE, Sr and S-isotope analyses are planned to provide a more comprehensive view of fluid evolution and mineral formation within this backarc hydrothermal system.
V54A-06 17:15h
Geomicrobiological exploration and characterization of novel deep-sea hydrothermal activities accompanying with extremely acidic white smokers and elemental sulfur chimneys at the TOTO caldera in the Mariana Volcanic Arc
Novel hydrothermal activities accompanying effluent white smokers and elemental sulfur chimney structures at the northeast lava dome of the TOTO caldera depression in the Mariana Volcanic Arc were explored by the manned submersible Shinkai 6500 and characterized by geochemical and microbiological surveys. The white smoker hydrothermal fluids were observed in the potential hydrothermal activity center of the field and represented a maximal temperature of 172 degree C and a lowest pH of 1.59, that was the lowest pH of the hydrothermal fluid ever recorded. The chimney structures consisting all of elemental sulfur (sulfur chimney) were also peculiar to the TOTO caldera hydrothermal field in the world. The geochemical characterization strongly suggested that the TOTO caldera hydrothermal field was a novel system driven by subseafloor mixing between the oxygenated seawater and the superheated volcanic gasses. Microbial community structures in a sulfur chimney structure and its formation hydrothermal fluid with a high concentration of hydrogen sulfide (15 mM) were investigated by culture-dependent and _|independent analyses. Ribosomal rRNA gene clone analysis and fluorescence in situ hybridization (FISH) analysis revealed that epsilon-Proteobacteria, specifically classified into Group G and Group B, dominated the microbial communities in the sulfur chimney structure and formed a dense microbial mat covering the sulfur chimney surface. Archaeal phylotypes were consistently minor components in the communities and related to the genera Thermococcus, Pyrodictium, Aeropyrum, and the uncultivated archaeal group of Deep-sea Hydrothermal Vent Euryarchaeotal Group. Cultivation analysis suggested that the microbial components inhabiting in the sulfur chimney structure might be entrained by hydrothermal fluids from the potential subsurface habitats
V54A-07 17:30h
Hydrothermal Plume Distributions Along the Valu Fa Ridge and East Lau Spreading Center, Lau Backarc Basin
Extensive studies along midocean ridges (MORs) from ultraslow- to superfast-spreading find a robust correlation, at the multi-segment scale, between the incidence of hydrothermal plumes (p$_{h}$) and the spreading rate, a proxy for the long-term melt supply. On the segment and sub-segment scales, these studies likewise find a good correlation between p$_{h}$ and cross-axial inflation, a proxy for higher frequency variability in the melt supply. To test the validity of these correlations in a backarc setting, where complex tectonics may weaken the connection between spreading rate and melt supply, we conducted multiple surveys of the hydrothermal plume distribution along three geophysically and morphologically distinct ridge sections in the Lau backarc basin: Valu Fa Ridge (VFR, 22.7-21.43$\deg$S), southern East Lau Spreading Center (S-ELSC, 21.43-20.53$\deg$S), and N-ELSC (20.53-19.3$\deg$S). On the TELVE cruise in March 2003 we used an optical sensor on CTDO tow-yos to map plumes on the southern 88 km of the VFR. In April 2004, on the initial cruise of the RIDGE ISS Lau project, we used both CTDO tow-yos and a vertical array of optical/temperature sensors (MAPRs) on the DSL-120 sidescan vehicle to conduct multiple surveys of the entire VFR/ELSC. The VFR/ELSC complex is distinctly different than typical MORs in several ways: (1) spreading rate more than doubles from 39 to 96 mm/yr over just 400 km between the southern end of the VFR and the northern end of the ELSC; (2) cross-axial inflation and axial depth are inversely correlated with spreading rate, decreasing northward from +5 km$^{2}$ and $\sim$1800 m, respectively, to -5 km$^{2}$ and $\sim$3000 m; and (3) geophysical studies suggest that melt production is also inversely correlated with spreading rate, owing to increasing distance from the melt-rich arc front from south to north along the ridge complex. Our surveys found $>$20 hydrothermal plume areas, ranging from $<$1 km to $>$20 km in axial extent, and rising as high as 500 m above the seafloor. The p$_{h}$ (fraction of axis overlain by a plume) increases northward as spreading rate increases, from $<$0.3 along the VFR (39-61 mm/yr) to $>$0.4 along the N-ELSC (76-96 mm/yr). This trend is reasonably consistent with the global MOR trend of spreading rate vs. p$_{h}$, though the relatively short length of these segments (100-150 km) makes comparisons tentative. On the subsegment scale, the lack of any consistent correlation between axial inflation and p$_{h}$ is strikingly different than most MORs. We speculate that despite a northward diminishment of melt supply, resulting in a thinner crust and deeper seafloor, hydrothermal discharge is enhanced by heat from mantle rocks accessed by the pervasive faulting and fracturing that increases northward with spreading rate. Forthcoming data from the chemical analyses of the plumes and the fine-scale sidescan imagery will provide further evidence to support or reject this hypothesis.
V54A-08 17:45h
A Systematic Reconnaissance of Submarine Hydrothermal Venting Along the South Tonga (Tofua) Intra-oceanic arc
The Tonga-Eastern Lau Vents Expedition (TELVE) of March-April 2003 provided the first survey of hydrothermal plumes along 425 km of the south Tonga (Tofua) arc, between $\sim$$21\deg$S and $25\deg$S. The volcanic front in this arc section lies $\sim$180 km west of the deepest point in the southern hemisphere (Horizon Deep in the Tonga Trench), $\sim$55 km west of the Tonga ridge, and $\sim$40 km east at closest approach to the Valu Fa and Eastern Lau back-arc spreading complex. Twenty-four of the 25 volcanic centers that comprise this arc section are submarine. We used conventional hydrothermal plume reconnaissance protocols (towed CTDO-Eh sensors + discrete water sampling for helium abundance and isotopic ratios, methane, pH, hydrogen sulphide, and total dissolvable Mn and Fe) to systematically survey 19 submarine volcanoes of the south Tonga arc for magmatic-hydrothermal fluid emissions. We found about one-third of the volcanoes (seven) to be hydrothermally active, with four venting at dual, spatially discrete sites and depths, for a total of eleven venting sites. While the deepest plume was sampled at $\sim$1400 m depth, plumes at nine sites were $<$1000 m deep and at six sites $<$500 m deep. This site frequency for venting is intermediate to that found during recent surveys along other Western Pacific intra-oceanic arcs (Kermadec higher, Mariana lower) and the relatively shallow depth regime for discharge, compared to most MOR venting sites, is similar. The compositions of the south Tonga arc plumes were chemically diverse: Fe/Mn varied from $<$1 to $>$50, total dissolved Fe concentrations ranged from $<$10 nM to $>$300 nM, only one site provided a large negative anomaly in pH (an indicator for ultra-high volatile content), and the presence of hydrogen sulphide was never detected, contrary to our experience at other submarine arcs. While high $\delta$$^{3}$He values were observed (74% maximum), sources located deeper than the arc sites appear to be volumetrically more important to the regional depth distribution of $^{3}$He. We place these results within the contexts of regional and global submarine hydrothermal emissions to assess their significance.