Manus Basin Research Articles

Characterization of 18 polymorphic microsatellite loci from Bathymodiolus manusensis (Bivalvia, Mytilidae) from deep-sea hydrothermal vents

Species in the genus Bathymodiolus are mytilid mussels found at deep-sea hydrothermal vents and cold seeps. Next-generation sequencing techniques were employed to identify eighteen unlinked polymorphic microsatellite loci for Bathymodiolus manusensis from Manus Basin in the western Pacific. Allele frequencies for eight loci conform to Hardy–Weinberg expectations and observed heterozygosities ranged from 0.04 to 0.90 (mean HO = 0.45, SD = 0.25). Ten of eighteen loci cross-amplified in Bathymodiolus heckerae from Atlantic seeps. Microsatellites developed for Bathymodiolus manusensis are being deployed to study connectivity among populations of this species colonizing geographically discrete back-arc basin vent systems.

Rare earth element abundances in hydrothermal fluids from the Manus Basin, Papua New Guinea: Indicators of sub-seafloor hydrothermal processes in back-arc basins

Rare earth element (REE) concentrations are reported for a large suite of seafloor vent fluids from four hydrothermal systems in the Manus back-arc basin (Vienna Woods, PACMANUS, DESMOS and SuSu Knolls vent areas). Sampled vent fluids show a wide range of absolute REE concentrations and chondrite-normalized (REE N) distribution patterns (La N /Sm N ∼ 0.6–11; La N /Yb N ∼ 0.6 – 71; Eu N / Eu N ∗ ∼ 1 – 55 ). REE N distribution patterns in different vent fluids range from light-REE enriched, to mid- and heavy-REE enriched, to flat, and have a range of positive Eu-anomalies. This heterogeneity contrasts markedly with relatively uniform REE N distribution patterns of mid-ocean ridge hydrothermal fluids. In Manus Basin fluids, aqueous REE compositions do not inherit directly or show a clear relationship with the REE compositions of primary crustal rocks with which hydrothermal fluids interact. These results suggest that the REEs are less sensitive indicators of primary crustal rock composition despite crustal rocks being the dominant source of REEs in submarine hydrothermal fluids. In contrast, differences in aqueous REE compositions are consistently correlated with differences in fluid pH and ligand (chloride, fluoride and sulfate) concentrations. Our results suggest that the REEs can be used as an indicator of the type of magmatic acid volatile (i.e., presence of HF, SO 2) degassing in submarine hydrothermal systems. Additional fluid data suggest that near-seafloor mixing between high-temperature hydrothermal fluid and locally entrained seawater at many vent areas in the Manus Basin causes anhydrite precipitation. Anhydrite effectively incorporates REE and likely affects measured fluid REE concentrations, but does not affect their relative distributions.

Insights to magmatic–hydrothermal processes in the Manus back-arc basin as recorded by anhydrite

Microchemical analyses of rare earth element (REE) concentrations and Sr and S isotope ratios of anhydrite are used to identify sub-seafloor processes governing the formation of hydrothermal fluids in the convergent margin Manus Basin, Papua New Guinea. Samples comprise drill-core vein anhydrite and seafloor massive anhydrite from the PACMANUS (Roman Ruins, Snowcap and Fenway) and SuSu Knolls (North Su) active hydrothermal fields. Chondrite-normalized REE patterns in anhydrite show remarkable heterogeneity on the scale of individual grains, different from the near uniform REE N patterns measured in anhydrite from mid-ocean ridge deposits. The REE N patterns in anhydrite are correlated with REE distributions measured in hydrothermal fluids venting at the seafloor at these vent fields and are interpreted to record episodes of hydrothermal fluid formation affected by magmatic volatile degassing. 87Sr/ 86Sr ratios vary dramatically within individual grains between that of contemporary seawater and that of endmember hydrothermal fluid. Anhydrite was precipitated from a highly variable mixture of the two. The intra-grain heterogeneity implies that anhydrite preserves periods of contrasting hydrothermal versus seawater dominant near-seafloor fluid circulation. Most sulfate δ 34S values of anhydrite cluster around that of contemporary seawater, consistent with anhydrite precipitating from hydrothermal fluid mixed with locally entrained seawater. Sulfate δ 34S isotope ratios in some anhydrites are, however, lighter than that of seawater, which are interpreted as recording a source of sulfate derived from magmatic SO 2 degassed from underlying felsic magmas in the Manus Basin. The range of elemental and isotopic signatures observed in anhydrite records a range of sub-seafloor processes including high-temperature hydrothermal fluid circulation, varying extents of magmatic volatile degassing, seawater entrainment and fluid mixing. The chemical and isotopic heterogeneity recorded in anhydrite at the inter- and intra-grain scale captures the dynamics of hydrothermal fluid formation and sub-seafloor circulation that is highly variable both spatially and temporally on timescales over which hydrothermal deposits are formed. Microchemical analysis of hydrothermal minerals can provide information about the temporal history of submarine hydrothermal systems that are variable over time and cannot necessarily be inferred only from the study of vent fluids.

Influence of subducted components on back‐arc melting dynamics in the Manus Basin

Lavas erupted in back‐arc basins afford the opportunity to explore the extent to which decompression and subduction‐related components influence partial melting in this setting. We present U‐Th‐Ra disequilibria data from 24 well‐characterized lavas from the Manus Basin behind the New Britain volcanic arc, supplemented by some additional trace element and Sr‐Nd‐Pb isotope data. The lavas range in composition from 49.6 to 57.7 wt % SiO2 and can be subdivided into those that are broadly like mid‐ocean ridge basalts (MORB) with Ba/Nb < 16 and back‐arc basin basalts (BABB) that are variably influenced by subduction components and have Ba/Nb > 16. Rifts closest to the arc are dominated by BABB, whereas both lavas types erupt further away at the Manus Spreading Center. The MORB have small 230Th excesses (up to 5%) and are displaced below the global correlation of (230Th/238U) with ridge depth. In most respects the BABB closely resemble lavas erupted along the New Britain arc front, including 238U excesses that reach 26%. The Pb isotope data can be explained by mixing of a subduction component into an Indian MORB mantle source. The Pb in the subduction component is derived from both the subducted sediment (5%) and fluids from the subducting altered Solomon Sea oceanic crust (95%), and these were mixed prior to addition to the mantle wedge. U/Th ratios, Fe3+/ΣFe, and H2O contents all increase with increasing 206Pb/204Pb. A model in which addition of the subduction component to the mantle wedge is followed by 230Th in‐growth during decompression and dynamic melting all less than 140 kyr prior to eruption can simulate the data. However, our preferred model is one of dynamic decompression melting in which subduction‐modified, more oxidized mantle had DU ≪ DTh leading to 238U excesses in contrast to unmodified mantle that yields 230Th excess. Large 226Ra excesses in some southern rift samples require addition of a fluid <8 kyr ago but elsewhere reflect melting under low‐porosity conditions.

Characterization of nine polymorphic microsatellite loci in Chorocaris sp. (Crustacea, Caridea, Alvinocarididae) from deep-sea hydrothermal vents

Species in the genus Chorocaris are caridean shrimp found at deep-sea hydrothermal vents at mid-ocean spreading centers. Nine unlinked polymorphic microsatellite loci were developed for this Chorocaris sp. 2 from the Manus back-arc basin, southwest Pacific. Three loci deviated significantly from Hardy–Weinberg expectations. Average observed heterozygosity ranged from 0.17 to 0.74 (mean HO = 0.45, SD = 0.21). Eight out of nine loci cross-amplified in one or two additional species of alvinocaridid shrimp that co-occur with Chorocaris sp. 2 at western Pacific vent habitats. The efficacy of our markers to detect genetic population structure is demonstrated using 362 individuals from 3 sites in Manus Basin. Microsatellites developed for Chorocaris sp. 2 are being deployed to study connectivity among populations of this species colonizing geographically discrete back-arc basin vent systems.

Mapping elemental distributions in submarine hydrothermal sulfide smokers using proton induced X-ray emission

PIXE analysis using a 3 MeV proton beam on the CSIRO Nuclear Microprobe was carried out on samples of four typical undersea sulfide chimneys from the Rogers Ruins and Fenway hydrothermal sites, PACMANUS field, Eastern Manus Basin, Papua New Guinea. The ability of PIXE to map the spatial association of trace elements within the sulfides across multiple mineralogical zones provides important insights into the mode of formation of structures and the nature of interaction between ∼250 and 350 °C hydrothermal fluids and 3–4 °C ambient seawater within the chimney walls.

Tracing the origin of subduction components beneath the South East rift in the Manus Basin, Papua New Guinea

The Manus Basin to the northeast of Papua New Guinea is an actively spreading/rifting back-arc basin in the Bismarck Sea located between the inactive Manus-Kilinailau trench on the Pacific-plate side and the active New Britain trench on the Solomon-plate side. Spreading/rifting in the Manus Basin takes place in the last 0.78 Myr or so. We present major and trace elements, and Sr–Nd–Pb isotope compositions of rock samples taken from the South East Rift (SER) at the eastern end of the Manus Basin. The strong enrichment of Pb and LILE (large ion lithophile elements) relative to HFSE (high field strength elements) and REE (rare earth elements) in the SER lava is also quite similar to other island arc lavas, suggesting that substantial amount of subduction components were present in its source mantle. To investigate the origin of the subduction components in SER lavas, we compare the geochemical data of SER lavas to published data from New Britain Arc (NBA) and Tabar–Lihir–Tanga–Feni (TLTF) island chain. The volcanism in NBA is related to presently active subduction of the Solomon slab, whereas the TLTF volcanism is located in the forearc area of New Ireland arc which was formed during a former subduction of the Pacific slab. In other words, the NBA and TLTF lavas were influenced by subduction components from the present and former subduction, respectively. We argue that the subduction components in SER lava were incorporated in the mantle lithosphere during the active arc volcanism on New Ireland because the amount of the subduction component in SER decreases with increasing in distance from New Ireland. On the other hand, no relationships are found with respect to New Britain. The Sr–Nd–Pb isotopes indicate that SER lavas contain little sediment component and less amount of fluid component derived from altered oceanic crust compared to the TLTF lavas. This is probably due to the fact that SER is located in backarc settings in contrast to TLTF which is located in forearc setting with respect to the Pacific slab. Thus it is likely that the sediment was removed from the slab in the forearc and/or arc areas, and therefore little or none was introduced in the backarc mantle, which is the source region for SER magmas at present. Fluid derived from altered oceanic crust also may have made its way into the sub-forearc region more effectively than backare region by shallow dehydration process.

Pb IN THE PACMANUS SEA-FLOOR HYDROTHERMAL SYSTEM, EASTERN MANUS BASIN: NUMERICAL MODELING OF A MAGMATIC VERSUS LEACHED ORIGIN

Mass balance calculations of metal leaching and deposition in the PACMANUS submarine hydrothermal vent field in the eastern Manus basin in the Bismarck Sea off the east coast of Papua New Guinea indicate that simple leaching of the underlying volcanic pile by hydrothermal fluids cannot account for the Pb found in associated sulfide deposits and Pb-enriched altered volcanic rocks. With 100 percent leaching, transport, and precipitation efficiencies, 1.69 km3 of volcanic rock would need to be leached to account for Pb. Applying realistic leaching efficiencies of 50 to 60 percent and transport-precipitation efficiencies of 10 to 20 percent, 14 to 34 km3 of volcanic rock would need to be leached, assuming a static reaction zone. The estimated available volume of leachable rock is 0.144 km3, much less than required. Applying a dynamic reaction zone model, 98 to 235 volumes of fresh, leachable rock (1 volume = 0.144 km3) would need to be exposed to hydrothermal fluids. This translates into a 588 to 1,410 m migration of the reaction zone. A model incorporating such values would not be tenable. Previously published compositions of melt inclusions in volcanic rocks from PACMANUS indicate an aver-age of 199 ppm Cu, 68 ppm Zn and 5.2 ppm Pb in the melt and 1.3 to 1.7 wt percent of degassed H2O. These data suggest that 37.7 million tonnes (Mt) of magmatic fluid containing 7.2 wt percent Cu and 2.3 wt percent Zn could be released from a 1 km3 body of magma. Using the same metal transfer efficiency of 80 percent, the degassed fluid would have a maximum concentration of 0.17 wt percent Pb, which translates to 58,000 t of Pb from the melt. For the highest known fluid discharge rate of 92,000 g/s and over a 100,000-year period, a concentration of only 12 ppm Pb in the original magmatic component could account for all the Pb in the PACMANUS deposits, assuming that 1 percent of the mixed end-member fluid is of magmatic origin. A wide range of durations of hydrothermal activity and discharge rates, assuming a maximum 0.17 wt percent Pb concentration in the degassed magmatic fluid, can account for the total Pb in the deposit.

Macrobenthos community structure and trophic relationships within active and inactive Pacific hydrothermal sediments

Hydrothermal fluids passing through sediments create a habitat hypothesized to influence the community structure of infaunal macrobenthos. Here we characterize the density, biomass, species composition, diversity, distributions, lifestyle, and nutritional sources of macroinfauna in hydrothermal sediments in NE and SW Pacific settings, and draw comparisons in search of faunal attributes characteristic of this habitat. There is increasing likelihood that seafloor massive sulfide deposits, associated with active and inactive hydrothermal venting, will be mined commercially. This creates a growing imperative for a more thorough understanding of the structure, dynamics, and resilience of the associated sediment faunas, and has stimulated the research presented here. Macrobenthic assemblages were studied at Manus Basin (1430–1634 m, Papua New Guinea [PNG]) as a function of location (South Su vs. Solwara 1), and hydrothermal activity (active vs. inactive), and at Middle Valley (2406–2411 m, near Juan de Fuca Ridge) as a function of habitat (active clam bed, microbial mat, hot mud, inactive background sediment). The studies conducted in PNG formed part of the environmental impact assessment work for the Solwara 1 Project of Nautilus Minerals Niugini Limited. We hypothesized that hydrothermally active sites should support (a) higher densities and biomass, (b) greater dominance and lower diversity, (c) a higher fraction of deposit feeders, and (d) greater isotopic evidence for chemosynthetic food sources than inactive sites. Manus Basin macrofauna generally had low density (o1000 ind. m

Evidence for a chemoautotrophically based food web at inactive hydrothermal vents (Manus Basin)

Hydrothermal vents are ephemeral systems. When venting shuts down, sulfide-dependent taxa die off, and non-vent taxa can colonize the hard substrata. In Manus Basin (Papua New Guinea), where hydrothermally active and inactive sites are interspersed, hydroids, cladorhizid sponges, barnacles, bamboo corals, and other invertebrate types may occupy inactive sites. Carbon and nitrogen isotopic compositions of animals occupying inactive sites are consistent with nutritional dependence on either chemoautotrophically or photosynthetically produced organic material, but sulfur isotopic compositions of these animals point to a chemoautotrophic source of sulfur from dissolved sulfide in vent fluids rather than sulfur derived from seawater sulfate through photosynthesis. Given that suspension-feeding and micro-carnivorous invertebrates are the biomass dominants at inactive sites, the primary source of chemoautotrophic nutrition is likely suspended particulates and organisms delivered from nearby active vents.

Molecular phylogenetic analysis of a known and a new hydrothermal vent octopod: their relationships with the genus Benthoctopus (Cephalopoda: Octopodidae)

The resolution of evolutionary relationships among deep-sea incirrate octopuses has been hindered by the paucity of individuals available for morphological studies and by the lack of tissue samples preserved using fixatives compatible with simple DNA extraction techniques. Evolutionary relationships from 11 species of deep-sea incirrate octopuses were investigated using 2392 base pairs (bp) of DNA from four mitochondrial genes (12S rDNA, 16S rDNA, cytochrome c oxidase subunit III, and cytochrome b) and the nuclear gene, rhodopsin. Morphological examination of these species was also undertaken. Molecular analyses distinguish a species of octopus from hydrothermal vents at Manus Basin from the vent octopodid Vulcanoctopus hydrothermalis known from vents on the East Pacific Rise. Both are herein considered members of the clade currently assigned the name Benthoctopus, although taxonomic implications preclude formally naming Vulcanoctopus as a junior synonym. Morphological investigations led to the conclusion that Benthoctopus macrophallus is a junior synonym of Benthoctopus yaquinae. An amended diagnosis of Benthoctopus is provided with additional information on male reproductive characteristics.

Two species of caridean shrimps (Decapoda: Hippolytidae and Nematocarcinidae) newly recorded from hydrothermal vents on the Manus Basin, southwestern Pacific

Two species of caridean shrimps (Decapoda: Hippolytidae and Nematocarcinidae) newly recorded from hydrothermal vents on the Manus Basin, southwestern Pacific

Structural features of ZnS from modern submarine sulfide ores of the Manus Basin (Pacific Ocean): Results of electron microscope study

Structural features of ZnS from modern submarine sulfide ores of the Manus Basin (Pacific Ocean): Results of electron microscope study

Effects of hydrothermal alteration on Pb in the active PACMANUS hydrothermal field, ODP Leg 193, Manus Basin, Papua New Guinea: A LA-ICP-MS study

The conventional model of leaching volcanic rocks as a source of metals in a seafloor hydrothermal systems has been tested by examining the behavior of Pb and other trace elements during hydrothermal alteration. ODP Leg 193 drill sites 1188 (Snowcap) and 1189 (Roman Ruins) on Pual Ridge in the eastern Manus Basin offshore eastern Papua New Guinea provide a unique three-dimensional window into an active back-arc hydrothermal system. We investigate by means of a LA-ICP-MS microbeam technique the capacity of Pb to be leached from a host volcanic rock exposed to various types and intensities of alteration. Our results are in general agreement with previous studies that utilized bulk analytical techniques but provide a more detailed explanation of the processes. Fresh representative dacitic lavas from the Pual Ridge have an average whole rock Pb content of 5.2 ppm, an average interstitial glass Pb content of 5.6 ppm and an average plagioclase Pb content of 1.0 ppm. Altered matrix samples have highly variable Pb values ranging from 0 to 52.4 ppm. High Pb values in altered samples are associated with a low temperature chlorite and clay mineral assemblage, in some cases overprinted by a high temperature (up to 350 °C) silica-rich “bleaching” alteration. Only the most highly altered matrix samples have REE patterns that differ from the fresh Pual Ridge dacite. This may represent either different lava histories or alteration characteristics that have affected normally immobile REEs. Altered samples with the highest Pb values have similar REE patterns to those of the local unaltered lavas. They are compositionally similar to typical Pual Ridge dacites indicating a genetic relationship between the main regional volcanic suite and the subseafloor hydrothermally altered, Pb-enriched material. Relative loss/gain for Pb between the analyzed altered samples and a calculated precursor show a maximum relative gain of 901%. Samples with relative Pb gain from both drill sites are associated with lower temperature alteration mineral assemblages characterized by pervasive chloritization. The related lower temperature (220–250 °C) neutral to slightly acidic fluids have been ascribed by others to return circulation of hydrothermal fluids that did not interact with seawater. Because altered samples have a higher Pb content than the fresh precursor, leaching of fresh volcanic rocks cannot be the source of Pb in the hydrothermal systems.

Pb and other ore metals in modern seafloor tectonic environments: Evidence from melt inclusions

Many modern seafloor tectonic environments are host to hydrothermal systems and associated polymetallic sulfide deposits. Metal transport and precipitation are controlled by magmatic processes such as pre-eruptive degassing and the hydrothermal cycle. The original availability of Pb and other ore metals in a given setting is dependent on concentrations in the original magmatic source or additional enrichment processes. We have examined the Pb budget of melt inclusions from nine modern seafloor settings representing back-arcs, mid-ocean ridges and seamounts. Melt inclusions provide information on the characteristics of parental magmas, including insights into metal budgets. Trace element data in melt inclusions hosted in plagioclase, olivine and pyroxene were obtained by laser-ablation inductively-coupled mass-spectrometry. Results from back-arcs emphasize the impact of slab-subduction and dehydration processes on the chemical characteristics of generated magmas. Volatile- and fluid-mobile element-rich melt inclusions at Manus basin and Okinawa trough reflect a robust contribution of elements from the subducting slab as evidenced by relatively low Ce/Pb ratios. At Bransfield strait, on the other hand, melt inclusions are volatile poor, and fluid-mobile element ratios are similar to mid-ocean ridge values indicating little or no contribution from the slab. High Cu concentrations at Manus basin and Okinawa trough can be explained by fluxing of ferric iron from the subducting slab benefiting the production of sulfate over sulfide. Metal budgets for seamounts located on and nearby the axis of mid-ocean ridge segments appear to be independent of any input of mantle plume material. Results from the southern Explorer ridge (strong lower mantle influence, transitional- and enriched-MORBs), Pito and Axial seamounts (moderate lower mantle influence, transitional-MORBs) and a Foundation near-ridge seamount (little to no mantle influence, normal-MORB) show that, despite similar tectonic environments and varying contributions of mantle plume material, Cu, Zn and Pb values do not vary significantly between the enriched and non-enriched magma components of a given setting.

Metalliferous Sediments Associated with Presently Forming Volcanogenic Massive Sulfides: The SuSu Knolls Hydrothermal Field, Eastern Manus Basin, Papua New Guinea

Short sediment cores were examined from the active SuSu Knolls hydrothermal field in the eastern Manus back-arc basin, Papua New Guinea, in order to explain the origin of the currently accumulating metalliferous sediments. The mineralogy and geochemistry of the sediments were studied by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray fluorescence, and instrumental neutron activation analysis. Sediments of the Suzette site originated from the deposition of volcaniclastic material with the addition of a hydrothermal component from the mass wasting of old oxidized chimneys. A strong mass wasting event was recognized at the base of the studied sedimentary succession from the occurrence of chalcopyrite, pyrite, and barite aggregates, similar to those found in chimneys, as well as the presence of atacamite. The metalliferous component is characterized by high concentrations of Cu (up to 2.3%) and Au (up to 3.5 ppm), elevated concentrations of As, Ba, Zn, and Fe, as well as a positive Eu anomaly. The material derived from eroded chimneys was deposited together with abundant glass fragments eroded from the volcanic edifice. The mass wasting was succeeded by deposition of volcaniclastic sediment containing dacite fragments, Ca plagioclase, pyroxene, glass shards, cristobalite, Si-rich amorphous material, alunite, pyrite, barite, and magnetite. The volcaniclastic sediment may have originated from hydrothermal eruptions at North Su and South Su. The present-day plume at Suzette is a likely source of rare barite aggregates, amorphous Cu-containing aggregates, and Cu-Fe sulfides creating geochemical anomalies of Ba and Cu in the uppermost sediments at Suzette. Interbedded volcaniclastic and hemipelagic sediments were retrieved east and west of SuSu Knolls. The enrichment of Mn in these sediments is considered to be a result of settling of Mn oxides from a hydrothermal plume.

Chlorine in submarine volcanic glasses from the eastern manus basin

Submarine volcanic glasses from the eastern Manus Basin of Papua New Guinea, ranging from basalt to rhyodacite, clarify the geochemical behavior of Cl in arc-type magmas. For the Manus samples, Cl is well correlated with non-volatile highly incompatible trace elements, suggesting it was not highly volatile and discounting significant seawater contamination. The Cl partition coefficient is close to but slightly lower than that of Nb and K 2O, a behavior similar to that in mid-ocean ridge basalts (MORB) and ocean island basalts (OIB). The similar incompatibilities of Cl and Nb imply that the Cl/Nb values of the eastern Manus Basin glasses reflect their magma source. For glasses from other west Pacific back-arc basins, Cl/Nb, Ba/Nb, and U/Nb increase towards the subduction trench, indicating increased contribution of a component enriched in Cl, Ba, and U, likely from subduction-released slab fluids. It is estimate that ∼80% of the Cl in the Manus arc-type glasses was added directly from subducted slab-derived fluids. We have also modeled Cl behavior during magma evolution in general. Our results show that the behavior of Cl in magma is strongly influenced by pressure, initial H 2O content, and the degree of magmatic fractionation. At early stages of magmatic evolution, for magmas with initial H 2O content of <4.0 wt%, Cl is highly incompatible under all pressures. By contrast, for more evolved magmas at moderately high pressure and high H 2O contents, considerable amounts of Cl can be extracted from the magma once H 2O saturation is reached. Accordingly, Cl is usually highly incompatible in MORB and OIB because of their low H 2O contents and relatively low degrees of fractional crystallization. The behavior of Cl in arc magmas is more complicated, ranging from highly incompatible to compatible depending on H 2O content and depth of magma chambers. The behavior of Cl in the eastern Manus Basin magmas is consistent with low H 2O contents (1.1–1.7 wt%) and evolution at low pressures (<0.1 GPa). Modeling results also indicate that Cl will behave differently in intrusive rocks compared to volcanic rocks because of the different pressures involved. This may have a strong influence on the mechanisms of ore genesis in these two tectonic settings.

Phylogenetic relationships of a tube worm (Lamellibrachia juni) from three hydrothermal vent fields in the South Pacific

The phylogenetic relationships were analysed based on nucleotide sequences of a mitochondrial gene for cytochrome c oxidase subunit I (COI) for the vestimentiferan tube worm Lamellibrachia juni, which inhabits three extremely acidic hydrothermal vent fields in the South Pacific: the TOTO Caldera in the South Mariana Volcanic Arc, the DESMOS site in the Manus Basin, and the Brothers Caldera in the Kermadec Arc. Six haplotypes were obtained from 34 lamellibrachiids from the three sites. Each haplotype was obtained only from a single site, which suggests that the three populations have been isolated from each other. Haplotypes from the DESMOS site formed a monophyletic cluster. All ten individuals from the TOTO Caldera were classified into the same haplotype, which was shown to form a monophyletic cluster with one of three haplotypes from the Brothers Caldera. The population in the Brothers Caldera was shown to consist of two genetically distinct groups.

Textural and mineralogical changes associated with the incipient hydrothermal alteration of glassy dacite at the submarine PACMANUS hydrothermal system, eastern Manus Basin

Variably altered dacite from the PACMANUS vent field in the eastern Manus back-arc basin, Papua New Guinea, was studied to determine the textural and mineralogical characteristics of hydrothermal alteration taking place in the immediate subsurface of this modern seafloor hydrothermal system. Detailed textural investigations show that fluid flow through the glassy dacite has been strongly controlled by the primary volcanic textures. Quench fractures and networks of interconnected perlitic cracks linking vesicles provided pathways for hydrothermal fluids flowing up to the seafloor. Hydrothermal alteration along these pathways resulted in the formation of pseudoclastic textures. Textural evidence suggests that alteration of the glassy dacite has not been sustained. The samples have been affected by incipient hydrothermal alteration that is typically not preserved in ancient volcanic-rock-hosted massive sulfide deposits. Interaction of the glassy dacite with hydrothermal fluids primarily resulted in the conversion of volcanic glass to dioctahedral smectite. Only minor amounts of trioctahedral smectite were formed. Destruction of the volcanic glass and the formation of smectite caused pronounced changes in the chemistry of the dacite samples, in particular a decrease in the SiO 2 whole-rock content and the Na 2O/K 2O ratio. The two alkali elements behaved differently during hydrothermal alteration due to preferential incorporation of K into the interlayer position of the newly formed dioctahedral smectite. Smectite formation occurred under rock-dominated conditions although the addition of Mg was required to form trioctahedral smectite from the silicic volcanic glass. Primary plagioclase was resistant to hydrothermal alteration highlighting the fact that the destruction of volcanic glass and feldspar are not necessarily contemporaneous in massive sulfide forming hydrothermal systems. Incipient alteration of the glassy dacite close to the seafloor occurred at temperatures below 150 °C in an environment that allowed the development of steep temperature gradients. Comparison of the new data to the findings of deep drilling during ODP Leg 193 suggests that the smectite-rich alteration in the immediate subsurface of the PACMANUS hydrothermal vent field represents the low-temperature equivalent of illite- and chlorite-rich alteration associations forming in the upflow zones of the hydrothermal fluids in the deeper portion of the volcanic sequence.

Origin of subduction components beneath the south east rift in the Manus Basin, Papua New Guinea and its tectonic implications

Origin of subduction components beneath the south east rift in the Manus Basin, Papua New Guinea and its tectonic implications