V44A-01 16:00h
Overview of Vent Fluid Chemistry From the Marianas Volcanic Arc
In March and April 2004, a research expedition on the {\it R.V.T.G. Thompason} with the ROV {\it ROPOS} investigated and sampled hydrothermal systems on six submarine volcanoes of the Marianas volcanic arc between 14.3 and 21.5 degrees N. In this two-year project sponsored by NOAA Ocean Exploration, dive targets were selected based on bathymetric and hydrothermal plume mapping conducted in 2003. Hydrothermal plume intesity and chemistry was used with success to target and sample a remarkable variety of volcanic and hydrothermal features. At NW Rota-1 submarine volcano, there is clear evidence of ongoing eruptive activity producing clouds of particulate and molten sulfur as well as mm to cm-size glassy volcanic ejecta. Fluids (34 deg C) sampled directly from an eruptive pit crater has pH of 2.0, with a high content of particulated sulfur, excess sulfate relative to seawater, and very low H2$^{s}$ content. Fluids percolating through volcaniclastic sand adjacent to the pit reached 100 deg C and had higher silica, slightly higher pH, and millimolar levels of H2$^{s}$. The chemistry of both types of fluids is indicative of input of volcanic SO$^{2}$ and disproportionation into sulfate and H2$^{s}$ (in volcaniclastic sands) and elemental sulfur (in the pit crater). Molten sulfur droplets indicate a high-temperature source at the base of the pit crater that rapidly mixes with seawater, while fluids venting through the sand remain hotter and react with volcanic glass. DNA was collected by in-situ filtration and both archaea and bacteria were amplified by PCR. Bacterial clone libraries were dominated by epsilon Proteobacteria with a high degree of relatedness to microaerophilic sulfur-, sulfide-, and hydrogen oxidizers belonging to the genera Sulfurimonas and Calderomonas. At East Diamante submarine volcano, several hydrothermal areas were found on resurgent domes within the large caldera. One of these sites hosted the only high-temperature deposits. The high volatile content of the volcanic arc environment is evident in the CO$^{2}$ and SO$^{2}$ dominated fluids sampled on the expedition. The submarine volcanoes explored were doninated by diffuse, low-temperature venting, significatly below the boiling temperature even at shallow depths.
V44A-02 16:15h
Shrimp Populations on Northwest Rota, an Active Volcano of the Mariana Volcanic Arc
NW Rota-1 is a submarine volcano that manifested active volcanic and hydrothermal activity during submersible surveys in March 2004 (see Embley et al.). Substratum on the volcano summit (520 m depth) was entirely basalt outcrop or variously-sized ejecta lying near the angle of repose. While no fauna inhabited the rim of the volcanic pit, patches of shrimp were located within 25 m and on the nearby summit. Two species are present. {\it Opaepele cf. loihi} shows few morphological differences from either a nearby population on Eifuku Volcano (see Chadwick et al.) at 1700 m depth or from the type locality in Hawaii. A molecular comparison of COI sequences of 13 specimens found little difference from two Hawaiian sequences. Video observations detail frequent feeding activity using spatulate chelipeds to trim microbial filaments as the cephalothorax sways across the substratum. The second species is an undescribed {\it Alvinocaris}. Juveniles of this species appear to form clusters distinct from {\it Opaepele} where they also graze on filaments. Sparse adults of {\it Alvinocaris} range up to 5.5 cm long and display aggressive behaviour moving through patches of smaller shrimp. Densities of {\it Opaepele} were highest on sloping rock walls (over 500 per sq.m.) whereas adult {\it Alvinocaris} were more abundant on rubble. This division may reflect food preference: microbial filaments versus polychaetes and meiofauna. Characterization of particulates from these substrata was conducted using visual sorting and stable isotope composition. As {\it Alvinocaris} matures, the chelipeds enlarge, enabling a greater predatory capacity. Measurements of {\it Opaepele} from digital in situ images reveal a population structure suggesting a recent recruitment. Average size is significantly smaller than the Eifuku population and no egg-bearing females were collected. The disjunct range of this species where it occurs on active volcanoes 6000 km apart is puzzling. Further work on intermediate sites and into the reproductive strategy of the species is required.
V44A-03 16:30h
Discovery and drilling of on- and off-axis hydrothermal sites in backarc spreading center of southern Mariana Trough, Western Pacific
The Mariana Trough is an actively spreading backarc basin that is located along the eastern margin of Philippine Sea Plate. GPS monitoring indicates that the rate of spreading is about 45 mm/yr in the southern section (Kato et al., 2003). No transform fault offsets exist despite significant changes in the trend of the spreading center. Fryer et al. (1998) pointed out the close proximity of submarine arc volcanoes to the spreading center and tectonic fabric that is at a high angle to the trend of the spreading center on the eastern flank. Three hydrothermal sites were discovered along such tectonic lineament in southern Mariana Trough (12$^{o}$55-57'N, 143$^{o}$37-39'E). On-axis site (so-called Fryer site, depth: 2,850 m) consists of a hydrothermal mound about 20 m in diameter that develops on pillow lava of a segment center of the spreading axis. The segment is characterized by highly variable rock composition (up to 68% SiO2). Repeated temperature measurements revealed rapid cooling of the hydrothermal system from 240$^{o}$C in April 2003, through 112$^{o}$C in October 2003 to 69$^{o}$C in March 2004. On the other hand, two off-axis sites seem to have longevity of life: The Archaean site which locates about 2 km off-axis on the eastern (arc side) skirt is characterized by its huge sulfide spire; 50 m in height and 20 m in diameter. It is composed of pyrite, chalcopyrite and sphalerite, and is emanating hydrothermal fluids up to 220$^{o}$C. In the third site (Pika site), active black smokers (max. temp. = 330$^{o}$C), numerous dead chimneys and sulfide mounds were found on a basaltic seamount about 5 km off-axis. These lines of evidence support the idea of Fryer et al. (1998) that the backarc magma is replenished by arc/off-axis magma along the tectonic lineation. The first and third sites been drilled and cased using a tethered, submarine rock-drill system BMS (Benthic Multi-coring System) on-board the {\it R/V Hakurei-Maru # 2} as a part of {\bf Archaean Park Project}*. Rocks from two holes (7.5 m and 4.1 m) in the Fryer site are porous basaltic lava with frequent fractures. Fluid 76$^{o}$C was discharged from one of the holes. The Pika site has two holes; one hole (5.61 m) is entirely composed of massive sulfides (pyrite-sphalerite) beneath thin manganese oxide cover. The other (4.61 m) shares common features with those of Fryer site. No venting was witnessed during the drilling but the latter hole was found venting fluid (10.1$^{o}$C) month later during the {\it ROPOS/ Thompson} cruise when extensive sampling and monitoring were done on these holes. * Funded by MEXT through the Special Coordination Fund.
V44A-04 16:45h
Contrasting sulfur isotope compositions of sulfide minerals between on-ridge and off-ridge hydrothermal fields in the southern Mariana back-arc region
New submarine hydrothermal fields were discovered in the southern Mariana back-arc spreading region during the Yokosuka-Shinkai 6500 cruise (October, 2003). One is located on the ridge of spreading center and the other is located on the off-rige site: A low-temperature hydrothermal activity and 10 m-high sulfide chimneys were found on the ridge site and the black-smoker activity with various sulfide chimneys was found on the top of the off-ridge seamount. Both hydrothermal fields were directly drilled by the benthic-multiple coring system during the Hakurei 2 cruise (February, 2004), in order to examine the subsurface hydrothermal processes. Elemental maps of drilled core samples and surface chimneys were constructed using X-ray scanning microscope, and an alteration pattern and types of sulfide minerals were examined. Forming steps of Fe-rich clays near the seafloor were traced and vesicle-filling process by clays and sulfides were found in the examined samples. Sulfur isotope analyses were performed using EA-IRMS on the separated sulfide and sulfate minerals. Sulfur isotope compositions of sulfate minerals are the identical to the seawater sulfate value. Sulfur isotope compositions of sulfides range from +2.9 to +8.7?n at the on-ridge site and _|3.2 to +3.6 ?n at the off-ridge site, respectively. Such regional difference in sulfur isotope compositions of sulfides is probably reflecting the difference in crustal processes: either (1) involvement of sulfate reduction near the discharge zone or (2) isotope exchange among several sulfide and sulfate phases in the deep reaction zone. Chronological change of sulfur isotope compositions was also found in each region: lighter sulfur isotope compositions were found in the younger generation of sulfides. This suggests the style of sulfur cycle in the basaltic crusts (e.g., depth and temperature change for sulfate reduction, potential biological process, etc.) are varying through the development and/or decay of each hydrothermal field.
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V44A-05 17:00h
Geochemistry of Hydrothermal Fluids in South Mariana Backarc Spreading Center
Fluid samples were collected from active hydrothermal sites along the south Mariana backarc spreading center, during dive programs with SHINKAI6500 (JAMSTEC) / {\it Yokosuka} (YK03-09 cruise) in Oct. 2003, and with ROPOS (CSSF) / {\it Thomas G. Thompson} (TN167A cruise) in March 2004. Fluid geochemistry shows diversity among three sites, probably reflecting their geological settings. High temperature fluid (T=$315\deg$C) from black smokers at the top of an off-axis seamount (Pika site: 12$\deg$55.1'N, 143$\deg$38.9'E, depth=2830m) shows metal-rich signature and high Cl concentration (=600mM). Modest high temperature fluid (T=$213\deg$C) from giant sulfide structure of a few ten meters high (Archaean Site: 12$\deg$56.3'N, 143$\deg$38.0'E, depth=2990m) shows slight K-rich from ridge-type fluids and low Cl concentration (=470mM). On the other hand, along the spreading axis, only shimmering venting directly from basaltic seafloor was observed (Fryer site: 12$\deg$57.2'N, 143$\deg$37.2'E, depth=2880m). Although temperature of the venting fluid were reported as $240\deg$C when this site was discovered at May 2003, it has decreased to $110\deg$C at Oct. 2003, and to $70\deg$C at March 2004. Based on fluid chemistry composition, the shimmering fluid is considered as formed by mixing between hydrothermal fluid endmember (Mg=0) with seawater, and hydrothermal contribution has diminished for this one year. We also successfully collected fluid samples venting from the casing pipes which had been drilled by BMS at January 2004. The fluid (T=$30\deg$C) from APM01 located in the vicinity of Fryer site showed chemistry well explained by mixing of the same hydrothermal fluid endmember with seawater. Under the framework of Achaean Park Project, samples for microbiological studies were collected simultaneously, and these geochemical data provides basic information for them.
V44A-06 17:15h
Diversity of Active Seafloor Hydrothermal Mineralization in the Manus Back-Arc Basin, Papua New Guinea
The Manus back-arc basin in the Western Pacific hosts three contrasting style of mineralization called Vienna Wood, Pacmanus and Onsen site. The three deposits have distinct geology and geochemical characteristics. The Vienna Wood site in the Central Manus Basin (CMB) is hosted by mid-oceanic ridge type basalt. The mineralization is dominated by wurtzite, marcasite, pyrite, chalcopyrite, Fe-oxide and gangue minerals of anhydrite, gypsum and quartz. The filling temperatures and salinities of the inclusion range from 200-260_E#8249;C and 4.8-6.6 NaCl equiv. wt% respectively. Bulk chemical analyses of the ores indicate that the mineralization in the Vienna Wood site can be classified into the Zn-Cu type. The simple mineral assemblage indicates simple basalt-seawater interaction and is a modern analogue of the ophiolite hosted VVMS deposits. The mineralization in the Pacmanus site is hosted by dacitic lavas is characterised by a complex mineral assemblage of marcasite, pyrite, chalcopyrite, sphalerite, bornite, enargite, covellite, chalcocite, digenite, galena, tennantite, native gold, PbAs-sulfosalts and gangue minerals of barite, rare silica, anhydrite and native sulphur. The temperature of mineralization ranges from 230-280_E#8249;C. Bulk chemical composition of the ore samples indicates that the mineralization in the Pacmanus site can be classified into the Zn-Cu-Pb-Au type. The mineral assemblage, character of the ores and tectonic setting resembles those of ancient Kuroko and currently forming hydrothermal deposits in Lau Basin and in Okinawa Trough. The Onsen hydrothermal site in the Desmos caldera is hosted by basaltic andesite. The advanced argillic alteration is characterised by both high and low temperature acid stable minerals of pyrophyllite, natroalunite, quartz, cristobalite, amorphous silica, anhydrite, gypsum, pyrite, marcasite, enargite, covellite and native sulphur. The mineralization occurs at a temperature range of 240 to 340_E#8249;C. The high sulfidation mineralization is considered to be a product of magmatic degassing and is a typical example of an acid-sulphate type of hydrothermal activity developing on the seafloor.
V44A-07 INVITED 17:30h
Characteristics of Hydrothermal Activity in the Lau Back Arc Basin
In April and May 2004, we conducted extensive surveys of hydrothermal plumes along the East Lau Spreading Center (ELSC), including the Valu Fa Ridge, in an initial effort to deliniate the extent and nature of hydrothermal venting along these ridges. This was the first step in identifying a focus site for the Ridge 2000 Lau Back Arc Basin ISS. We sampled these plumes from the RV Kila Moana using CTD tows and casts and by mounting a variety of plume-sensitive sensors on the DSL-20 side scan sonar system. Our survey documented extensive venting with complex chemical signatures at more than 20 locations along 400 km of ridge axis. Here, we examine the chemistry of the plumes based on samples that were collected for Helium isotopes, total carbon dioxide, pH, particulate chemistry, and total and dissolved Fe and Mn. There were several predominant chemical characteristics of the plumes: 1. The particulate matter that produced the optical backscatter plumes in the VFR region (22.7-21.4S) was dominated by Fe-oxyhydroxides and metal sulfides while the majority of the plumes on the ELSC (21.4-19.3S) were dominated by elemental sulfur and, to a lesser extent, by Fe-oxyhydroxides. 2. The ratio of Fe to S at Valu Fa was $\sim$2:1, but was a factor of 10 lower ($\sim$0.2:1) in the plumes further north. 3. In the areas where particulate sulfur was rich, there was a wide range of dissolved Mn concentrations. 4. Elevated levels of particulate Mn are concentrated at 21°S suggesting a buildup of hydrothermal effluent in the basin. The differences between plumes over Valu Fa and the rest of the ELSC have significant implications for the establishment of the Lau ISS. The high Fe:S ratio at Valu Fa suggests that organisms that require hydrogen sulfide for their metabolism might be scarce in this area, while areas farther north may better support vent organisms. The abundance of particulate elemental S in the north further suggests (based on MOR models)that the vents forming the plumes are located in areas of recent volcanism (0-5+ years). These are areas where evolutionary characteristics of back arc hydrothermal systems might be studied. However, the proximity of the Valu Fa Ridge to a volcanic arc and the silicic lavas erupted there distinguish it from well studied MOR spreading centers. These results indicate that the RIDGE community will need to consider along-ridge differences in vent chemistry ecological richness, and geology in the decision on where to establish the focus site of the Lau ISS.
V44A-08 INVITED 17:45h
Systematic Survey of the Kermadec-Tonga Intra-oceanic arc Between 1999 and 2004: a Significant Source of Diverse Submarine Hydrothermal Emissions
The 1999 NZAPLUME cruise was the first of several expeditions to systematically locate, and chemically characterise, submarine hydrothermal vents associated with arc volcanoes of the southern part the Kermadec-Tonga intra-oceanic arc system. This was followed by the 2002 NZAPLUME II and 2004 NZAPLUME III cruises to the mid- and northern-sections of the Kermadec arc, respectively, and the 2001 TELVE cruise to the southern part of the Tonga (Tofua) arc. Combined, $\sim$1,800 km of this arc system have been mapped, including $\sim$60 major volcanoes and numerous subordinate volcanic edifices, making this the longest continuous stretch of intra-oceanic arc to be surveyed for hydrothermal emissions. Most of the volcanoes are simple cones although $\sim$25% are caldera volcanoes with their compositions ranging from basalt through rhyo-dacite. Results from the NZAPLUME I and II cruises show 16 of the 26 major volcanoes surveyed are hydrothermally active, while 7 of 19 are active along the Tongan section of the arc, a $\sim$50% frequency of venting. Depths to venting range from $\sim$120 m to $\sim$1,650 m. Chemical analysis of the hydrothermal plumes shows a large range in composition, including different compositions for plumes from vent sites at the same volcano, with indications locally of a magmatic fluid component. An expedition in Oct./Nov. 2004 by the deep-sea submersible Shinkai 6500 will enable vent sites at Brothers and Healy volcanoes to be mapped and mineralization, animals to be sampled.