Bach Wolfgang

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Bach

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Wolfgang

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0000-0002-3099-7142

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Geochemistry of hydrothermal fluids from the PACMANUS, Northeast Pual and Vienna Woods hydrothermal fields, Manus Basin, Papua New Guinea

( 2010-10-29) Reeves, Eoghan P. ; Seewald, Jeffrey S. ; Saccocia, Peter J. ; Bach, Wolfgang ; Craddock, Paul R. ; Shanks, Wayne C. ; Sylva, Sean P. ; Walsh, Emily ; Pichler, Thomas ; Rosner, Martin

Processes controlling the composition of seafloor hydrothermal fluids in silicic back-arc or neararc crustal settings remain poorly constrained despite growing evidence for extensive magmatichydrothermal activity in such environments. We conducted a survey of vent fluid compositions from two contrasting sites in the Manus back-arc basin, Papua New Guinea, to examine the influence of variations in host rock composition and magmatic inputs (both a function of arc proximity) on hydrothermal fluid chemistry. Fluid samples were collected from felsic-hosted hydrothermal vent fields located on Pual Ridge (PACMANUS and Northeast (NE) Pual) near the active New Britain Arc and a basalt-hosted vent field (Vienna Woods) located farther from the arc on the Manus Spreading Center. Vienna Woods fluids were characterized by relatively uniform endmember temperatures (273–285°C) and major element compositions, low dissolved CO2 concentrations (4.4mmol/kg) and high measured pH (4.2–4.9 at 25°C). Temperatures and compositions were highly variable at PACMANUS/NE Pual and a large, newly discovered vent area (Fenway) was observed to be vigorously venting boiling (358°C) fluid. All PACMANUS fluids are characterized by negative δDH2O values, in contrast to positive values at Vienna Woods, suggesting substantial magmatic water input to circulating fluids at Pual Ridge. Low measured pH (25°C) values (~2.6 to 2.7), high endmember CO2 (up to 274 mmol/kg) and negative δ34SH2S values (down to -2.7‰) in some vent fluids are also consistent with degassing of acid-volatile species from evolved magma. Dissolved CO2 at PACMANUS is more enriched in 13C (-4.1‰ to -2.3‰) than Vienna Woods (-5.2‰ to -5.7‰), suggesting a contribution of slab-derived carbon. The mobile elements (e.g. Li, K, Rb, Cs and B) are also greatly enriched in PACMANUS fluids reflecting increased abundances in the crust there relative to the Manus Spreading Center. Variations in alkali and dissolved gas abundances with Cl at PACMANUS and NE Pual suggest that phase separation has affected fluid chemistry despite the low temperatures of many vents. In further contrast to Vienna Woods, substantial modification of PACMANUS/NE Pual fluids has taken place as a result of seawater of seawater ingress into the upflow zone. Consistently high measured Mg concentrations, trends of increasingly non-conservative SO4 behavior, decreasing endmember Ca/Cl and Sr/Cl ratios with increased Mg indicate extensive subsurface anhydrite deposition is occurring as a result of subsurface seawater entrainment. Decreased pH and endmember Fe/Mn ratios in higher Mg fluids indicate that the associated mixing/cooling gives rise to sulfide deposition and secondary acidity production. Several low temperature (≤80°C) fluids at PACMANUS/NE Pual also show evidence for anhydrite dissolution and water-rock interaction (fixation of B) subsequent to seawater entrainment. Hence, the evolution of fluid compositions at Pual Ridge reflects the cumulative effects of water/rock interaction, admixing and reaction of fluids exsolved from silicic magma, phase separation/segregation and seawater ingress into upflow zones.
Unraveling the sequence of serpentinization reactions : petrography, mineral chemistry, and petrophysics of serpentinites from MAR 15°N (ODP Leg 209, Site 1274)

(American Geophysical Union, 2006-07-06) Bach, Wolfgang ; Paulick, Holger ; Garrido, Carlos J. ; Ildefonse, Benoit ; Meurer, William P. ; Humphris, Susan E.

The results of detailed textural, mineral chemical, and petrophysical studies shed new light on the poorly constrained fluid-rock reaction pathways during retrograde serpentinization at mid-ocean ridges. Uniformly depleted harzburgites and dunites from the Mid-Atlantic Ridge at 15°N show variable extents of static serpentinization. They reveal a simple sequence of reactions: serpentinization of olivine and development of a typical mesh texture with serpentine-brucite mesh rims, followed by replacement of olivine mesh centers by serpentine and brucite. The serpentine mesh rims on relic olivine are devoid of magnetite. Conversely, domains in the rock that are completely serpentinized show abundant magnetite. We propose that low-fluid-flux serpentinization of olivine to serpentine and ferroan brucite is followed by later stages of serpentinization under more open-system conditions and formation of magnetite by the breakdown of ferroan brucite. Modeling of this sequence of reactions can account for covariations in magnetic susceptibility and grain density of the rocks.
Rare earth element abundances in hydrothermal fluids from the Manus Basin, Papua New Guinea : indicators of sub-seafloor hydrothermal processes in back-arc basins

( 2010-05-02) Craddock, Paul R. ; Bach, Wolfgang ; Seewald, Jeffrey S. ; Rouxel, Olivier J. ; Reeves, Eoghan P. ; Tivey, Margaret K.

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 (REEN) distribution patterns (LaN/SmN ~ 0.6 – 11; LaN/YbN ~ 0.6 – 71; EuN/Eu*N ~ 1 – 55). REEN 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 REEN 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, SO2) 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.
Brucite formation and dissolution in oceanic serpentinite

(European Association of Geochemistry, 2020-10-27) Klein, Frieder ; Humphris, Susan E. ; Bach, Wolfgang

Brucite is an important, albeit elusive, hydrous mineral formed during serpentinisation, a vector of Mg from the mantle to seawater, and possibly a significant host of water in oceanic serpentinite. However, the abundance of brucite has not been quantified in oceanic serpentinite and its fate and related chemical fluxes remain uncertain. We used thermal analysis and confocal Raman spectroscopy to determine the abundance and distribution of brucite in serpentinite recovered by seafloor drilling (n = 48) and dredging (n = 22). Almost all (90 %) of the drilled serpentinite samples contained brucite. The brucite contents increased with increasing extent of serpentinisation and constituted up to 15.6 wt. % of the altered rock. In contrast, dredged serpentinites were devoid of brucite and lost 4.0 wt. % MgO on average, which translates to an estimated average annual flux of 1.3 × 1010 mole Mg and about 2 × 1010 mole alkalinity during seafloor weathering of serpentinite globally. Our data suggest that, on average, brucite stores ∼20 % of the water in unweathered serpentinite, making brucite one of the largest water carriers in slow and ultra-slow spreading oceanic lithosphere.
Hydrothermal venting in magma deserts : the ultraslow-spreading Gakkel and Southwest Indian Ridges

(American Geophysical Union, 2004-08-18) Baker, Edward T. ; Edmonds, Henrietta N. ; Michael, Peter J. ; Bach, Wolfgang ; Dick, Henry J. B. ; Snow, Jonathan E. ; Walker, Sharon L. ; Banerjee, Neil R. ; Langmuir, Charles H.

Detailed hydrothermal surveys over ridges with spreading rates of 50–150 mm/yr have found a linear relation between spreading rate and the spatial frequency of hydrothermal venting, but the validity of this relation at slow and ultraslow ridges is unproved. Here we compare hydrothermal plume surveys along three sections of the Gakkel Ridge (Arctic Ocean) and the Southwest Indian Ridge (SWIR) to determine if hydrothermal activity is similarly distributed among these ultraslow ridge sections and if these distributions follow the hypothesized linear trend derived from surveys along fast ridges. Along the Gakkel Ridge, most apparent vent sites occur on volcanic highs, and the extraordinarily weak vertical density gradient of the deep Arctic permits plumes to rise above the axial bathymetry. Individual plumes can thus be extensively dispersed along axis, to distances >200 km, and ∼75% of the total axial length surveyed is overlain by plumes. Detailed mapping of these plumes points to only 9–10 active sites in 850 km, however, yielding a site frequency F s , sites/100 km of ridge length, of 1.1–1.2. Plumes detected along the SWIR are considerably less extensive for two reasons: an apparent paucity of active vent fields on volcanic highs and a normal deep-ocean density gradient that prevents extended plume rise. Along a western SWIR section (10°–23°E) we identify 3–8 sites, so F s = 0.3–0.8; along a previously surveyed 440 km section of the eastern SWIR (58°–66°E), 6 sites yield F s = 1.3. Plotting spreading rate (us) versus F s, the ultraslow ridges and eight other ridge sections, spanning the global range of spreading rate, establish a robust linear trend (F s = 0.98 + 0.015us), implying that the long-term heat supply is the first-order control on the global distribution of hydrothermal activity. Normalizing F s to the delivery rate of basaltic magma suggests that ultraslow ridges are several times more efficient than faster-spreading ridges in supporting active vent fields. This increased efficiency could derive from some combination of three-dimensional magma focusing at volcanic centers, deep mining of heat from gabbroic intrusions and direct cooling of the upper mantle, and nonmagmatic heat supplied by exothermic serpentinization.
Application of B, mg, li, and sr isotopes in acid-sulfate vent fluids and volcanic rocks as tracers for fluid-rock interaction in back-arc hydrothermal systems

(American Geophysical Union, 2019-11-15) Wilckens, Frederike K. ; Reeves, Eoghan P. ; Bach, Wolfgang ; Seewald, Jeffrey S. ; Kasemann, Simone A.

The Manus Basin hosts a broad range of vent fluid compositions typical for arc and back‐arc settings, ranging from black smoker to acid‐sulfate styles of fluid venting, as well as novel intermediate temperature and composition “hybrid” smokers. We investigated B, Li, Mg, and Sr concentrations and isotopic compositions of these different fluid types as well as of fresh and altered rocks from the same study area to understand what controls their compositional variability. In particular, the formation of acid‐sulfate and hybrid smoker fluids is still poorly understood, and their high Mg concentrations are explained either by dissolution of Mg‐bearing minerals in the basement or by mixing between unmodified seawater and magmatic fluids. Mg isotope ratios of the acid‐sulfate fluids from the Manus Basin are seawater‐like, which supports the idea that acid‐sulfate fluids in this study area predominantly form by mixing between unmodified seawater and a Mg‐free magmatic fluid. Changes in the B, Li, and Sr isotope ratios relative to seawater indicate water‐rock interaction in all acid‐sulfate fluids. Further, the combination of δ7Li with B concentrations of the same fluids links changes in δ7Li to changes in (1) basement alteration, (2) water‐to‐rock ratios during water‐rock interaction, and/or (3) the reaction temperature. These isotope systems, thus, allow tracing of basement composition and acid‐sulfate‐driven alteration of the back‐arc crust and help increase our understanding of hydrothermal fluid‐rock interactions and the behavior of fluid‐mobile elements.
Fluid inclusion evidence for subsurface phase separation and variable fluid mixing regimes beneath the deep-sea PACMANUS hydrothermal field, Manus Basin back arc rift, Papua New Guinea

(American Geophysical Union, 2004-03-05) Vanko, David A. ; Bach, Wolfgang ; Roberts, Stephen ; Yeats, Christopher J. ; Scott, Steven D.

Altered volcanic rocks were cored from over 350 m below the seafloor at the Papua New Guinea-Australia-Canada Manus Basin Hydrothermal Field (PACMANUS) deep-sea hydrothermal field, in the eastern Manus back arc basin. Fluid inclusions in anhydrite veins reveal phase separation and fluid mixing beneath the seafloor. The anhydrite precipitated from high-temperature fluids (150–385°C). At Roman Ruins, a site of active high-temperature venting (220–276°C, measured by submersible), the fluid inclusion thermal depth profile is uniform and high temperature (242–368°C). At Snowcap, a site of warm water effusion (6–65°C), the fluid inclusions indicate high temperatures at depth (270–385°C) but both low and high temperatures in the shallower section. This indicates a flow regime dominated by vertical advection and shallow entrainment and mixing with cool seawater. Inclusions at Snowcap exhibit extreme salinity variations due to phase separation at temperatures above 350°C. Fluids contain Na, Cl, Fe, Zn, Mg, and Ba and a minor gas component such as CO2 or CH4. Most inclusions at Roman Ruins exhibit salinities that fall within the range of those observed at modern active vent sites along the mid-ocean ridge system. Fluid inclusion temperatures support a hypothesis, developed previously from Sr-isotopic analysis, that the subseafloor at Snowcap is characterized by mixing between deep-sourced hot hydrothermal fluids and cold seawater-like fluid. Both heating of seawater and cooling of upwelling hydrothermal fluids can be recognized by combining isotopic and fluid inclusion data. In contrast to Snowcap, the regime at Roman Ruins is less varied, with uniformly high-temperature upwelling fluids that have hydrothermally dominated Sr-isotopic ratios.
Manus 2006 : hydrothermal systems in the Eastern Manus Basin: fluid chemistry and magnetic structure as guides to subseafloor processes

(Woods Hole Oceanographic Institution, 2006) Tivey, Maurice A. ; Bach, Wolfgang ; Seewald, Jeffrey S. ; Tivey, Margaret K. ; Vanko, David A.

The hydrothermal systems in the Manus Basin of Papua New Guinea (PNG) were comprehensively investigated through a combination of sampling and mapping using the Remotely-Operated Vehicle (ROV) Jason, the autonomous underwater vehicle (AUV) ABE (Autonomous Benthic Explorer) and ship-based CTD work and multi-beam bathymetric mapping using the RV Melville. The objectives of the cruise (July 21st to Sept. 1st, 2006) were to identify the tectonic/geologic settings of the vent systems, examine the interactions of seawater with felsic rocks that constitute the high silica end-member range of seafloor basement compositions, determine the extent of volatile magmatic inputs into these systems and to examine the evolution of hydrothermal activity through time. The first 10-day portion of the cruise was funded by Nautilus Minerals in a collaborative research effort to examine the Manus Spreading Center and the Vienna Woods basalt-hosted hydrothermal vent systems. The second 32-day portion of the cruise, funded by the National Science Foundation (NSF), focused on the felsic-hosted hydrothermal systems of the PACMANUS (Papua New Guinea – Australia – Canada Manus) vents drilled by the Ocean Drilling Program (ODP) in 2000 and the nearby seafloor volcano vent systems of Desmos and SuSu Knolls. Nautilus Minerals generously funded the add-on use of ABE throughout the NSF program allowing for high resolution mapping to be completed on all the major vent sites within the eastern Manus Basin. A total of 30 ROV dives (497 operational hours) were completed collecting 198 vent sulfides, 83 altered substrate and 43 fresh lava samples along with 104 black, gray and clear fluid samples using gastight and major samplers. ABE successfully completed 14 high resolution bathymetric, CTD and magnetic field mapping dives covering a total of 364 line km of seafloor. We located and mapped in detail the Vienna Woods and nearby Tufar-2 and -3 vent areas on Manus Spreading Center documenting the strong tectonic control on the distribution of the vent systems and the presence of reduced magnetization i.e. “magnetic burnholes”, that help define the lateral extent of the vent fields. The Vienna Woods vent systems (273°-285°C) form treetrunk- like chimneys 5-15 m tall, that emit black to gray fluids with pH and compositions similar to other documented midocean ridge (MOR) systems like the East Pacific Rise. At PACMANUS, high-resolution mapping by ABE reveals a distinctive seafloor morphology associated with dacitic lava flows along with discrete magnetic burnholes associated with the active venting systems of Roman Ruins, Satanic Mills, Snowcap, Tsukushi and a new vigorous vent system discovered southeast of the Satanic Mills area named Fenway. Another vent field in its waning stages was also discovered ~8 km northeast of PACMANUS on the Northeast Pual Ridge. At PACMANUS, the 40 m diameter Fenway mound hosts outcrops of massive anhydrite on the seafloor beneath the sulfide chimneys, a rare occurrence as anhydrite is unstable at ambient seafloor conditions. Fenway is also boiling (356°C, 172 bar) with two-phase fluid producing a ”flashing” phenomenon when the Jason lights illuminated the vent orifices. The five PACMANUS vents (271° – 356°C) have ubiquitous low pH (2.3 to 2.8) relative to Vienna Woods and typical MOR fluids, presumably reflecting water-rock reaction with the felsic hosted lava, input of magmatic volatiles and the subsurface deposition of metal sulfides. We investigated two strongly magmatically influenced vent systems associated with seafloor volcanoes. Desmos is a breached caldera with white smokers (70°-115°C) that are highly acidic (pH 1 – 1.5) and sulfur lava flows. SuSu Knolls and the adjacent Suzette mound (Solwara-1 of Nautilus Minerals) were mapped in detail and sampled intensively. Hydrothermal activity at SuSu Knolls showed a remarkable range from boiling black smokers to white sulfur-rich fluids, native sulfur flows and massive anhydrite outcrops. Vent fluids from North Su (48° – 325°C) are 2 characterized by a measured pH of 0.87, more than an order of magnitude more acidic than any deep-sea vent fluid sampled to date. Many of the low pH fluids sampled at North Su and Desmos were actively precipitating native sulfur creating thick plumes of dense white smoke. In general, sampled fluids show a considerable range in pH and gas contents, sometimes within individual hydrothermal fields. The pronounced variability of fluid chemistry within 10’s to 100’s of m at North Su is probably unparalleled in systems studied to date. The most plausible explanation for the observed variability is that different fluid-rock reaction pathways are expressed in regimes of variable magmatic volatile input and extent of subsurface cooling. This hypothesis is supported by the distribution of alteration types at the seafloor, where the occurrence of advanced argillic alteration - that relates to interactions with acid-sulfate waters such as sampled at Desmos and North Su – is patchy and spatially confined to patches of active (Desmos, North Su) and past (Snowcap) venting of such fluids. In relationship to the ODP drilling results at PACMANUS we identified and sampled examples of advanced argillic rock alteration similar to that seen in the drill core. Good examples came from Snowcap and from the North Su pillar. We sampled highly clay-altered basement from just underneath extinct chimney complexes at two locations in the Satanic Mills hydrothermal field. Both samples have dense networks of sulfide veins and may represent the stockwork or feeder zone through which hydrothermal fluids rise up to the seafloor. These samples, in addition to the other altered rock types recovered, will provide useful stepping stones in bridging the knowledge gap between the extensive surface sampling now accomplished and the basement rocks recovered by ODP, where coring was almost nil shallower than 40 m subseafloor depth. Overall, the quality and quantity of solid and fluid samples that can be put in a direct geochemical context is remarkably high. This unique dataset encompasses a broad range of geological environments that includes hydrothermal activity in basalt-hosted oceanic style spreading centers to hydrothermal systems associated with arc-style volcanism. For the first time, alteration assemblages that are commonly observed in drillcore and outcrop on land have been observed in the aqueous environment responsible for their formation.
Geologic setting of PACManus hydrothermal vent fields – High-resolution mapping and in situ observations

( 2014-05) Thal, Janis ; Tivey, Maurice A. ; Yoerger, Dana R. ; Jons, Niels ; Bach, Wolfgang

This study presents a systematic analysis and interpretation of autonomous underwater vehicle-based microbathymetry combined with remotely operated vehicle (ROV) video recordings, rock analyses and temperature measurements within the PACManus hydrothermal area located on Pual Ridge in the Bismarck Sea of eastern Manus Basin. The data obtained during research cruise Magellan-06 and So-216 provides a framework for understanding the relationship between the volcanism, tectonism and hydrothermal activity. PACManus is a submarine felsic vocanically-hosted hydrothermal area that hosts multiple vent fields located within several hundred meters of one another but with different fluid chemistries, vent temperatures and morphologies. The total area of hydrothermal activity is estimated to be 20,279 m2. The microbathymetry maps combined with the ROV video observations allow for precise high-resolution mapping estimates of the areal extents of hydrothermal activity. We find the distribution of hydrothermal fields in the PACManus area is primarily controlled by volcanic features that include lava domes, thick and massive blocky lava flows, breccias and feeder dykes. Spatial variation in the permeability of local volcanic facies appears to control the distribution of venting within a field. We define a three-stage chronological sequence for the volcanic evolution of the PACManus based on lava flow morphology, sediment cover and lava SiO2 concentration. In Stage-1, sparsely to moderately porphyritic dacite lavas (68 - 69.8 wt. % SiO2) erupted to form domes or cryptodomes. In Stage-2, aphyric lava with slightly lower SiO2 concentrations (67.2 – 67.9 wt. % SiO2) formed jumbled and pillowed lava flows. In the most recent phase Stage-3, massive blocky lavas with 69 to 72.5 wt. % SiO2 were erupted through multiple vents constructing a volcanic ridge identified as the PACManus neovolcanic zone. The transition between these stages may be gradual and related to progressive heating of a silicic magma following a recharge event of hot, mantle-derived melts.
Carbonate veins trace seawater circulation during exhumation and uplift of mantle rock : results from ODP Leg 209

( 2011-09-02) Bach, Wolfgang ; Rosner, Martin ; Jons, Niels ; Rausch, Svenja ; Robinson, Laura F. ; Paulick, Holger ; Erzinger, Jorg

Carbonate veins hosted in ultramafic basement drilled at two sites in the Mid Atlantic Ridge 15°N area record two different stages of fluid-basement interaction. A first generation of carbonate veins consists of calcite and dolomite that formed syn- to postkinematically in tremolite–chlorite schists and serpentine schists that represent gently dipping large-offset faults. These veins formed at temperatures between 90 and 170 °C (oxygen isotope thermometry) and from fluids that show intense exchange of Sr and Li with the basement (87Sr/86Sr = 0.70387 to 0.70641, δ7LiL-SVEC = + 3.3 to + 8.6‰). Carbon isotopic compositions range to high δ13CPDB values (+ 8.7‰), indicating that methanogenesis took place at depth. The Sr–Li–C isotopic composition suggests temperatures of fluid-rock interaction that are much higher (T > 350–400 °C) than the temperatures of vein mineral precipitation inferred from oxygen isotopes. A possible explanation for this discrepancy is that fluids cooled conductively during upflow within the presumed detachment fault. Aragonite veins were formed during the last 130 kyrs at low-temperatures within the uplifted serpentinized peridotites. Chemical and isotopic data suggest that the aragonites precipitated from cold seawater, which underwent overall little exchange with the basement. Oxygen isotope compositions indicate an increase in formation temperature of the veins by 8–12 °C within the uppermost ~ 80 m of the subseafloor. This increase corresponds to a high regional geothermal gradient of 100–150 °C/km, characteristic of young lithosphere undergoing rapid uplift.
Subaqueous cryptodome eruption, hydrothermal activity and related seafloor morphologies on the andesitic North Su volcano

( 2016-04-28) Thal, Janis ; Tivey, Maurice A. ; Yoerger, Dana R. ; Bach, Wolfgang

North Su is a double-peaked active andesite submarine volcano located in the eastern Manus Basin of the Bismarck Sea that reaches a depth of 1154 m. It hosts a vigorous and varied hydrothermal system with black and white smoker vents along with several areas of diffuse venting and deposits of native sulfur. Geologic mapping based on ROV observations from 2006 and 2011 combined with morphologic features identified from repeated bathymetric surveys in 2002 and 2011 document the emplacement of a volcanic cryptodome between 2006 and 2011. We use our observations and rock analyses to interpret an eruption scenario where highly viscous, crystal-rich andesitic magma erupted slowly into the water-saturated, gravel-dominated slope of North Su. An intense fragmentation process produced abundant blocky clasts of a heterogeneous magma (olivine crystals within a rhyolitic groundmass) that only rarely breached through the clastic cover onto the seafloor. Phreatic and phreatomagmatic explosions beneath the seafloor cause mixing of juvenile and pre-existing lithic clasts and produce a volcaniclastic deposit. This volcaniclastic deposit consists of blocky, non-altered clasts next, variably (1-100 %) altered clasts, hydrothermal precipitates and crystal fragments. The usually applied parameters to identify juvenile subaqueous lava fragments, i.e. fluidal shape or chilled margin, were not applicable to distinguish between pre-existing non-altered clasts and juvenile clasts. This deposit is updomed during further injection of magma and mechanical disruption. Gas-propelled turbulent clast-recycling causes clasts to develop variably rounded shapes. An abundance of blocky clasts and the lack of clasts typical for the contact of liquid lava with water is interpreted to be the result of a cooled, high-viscosity, crystal-rich magma that failed as a brittle solid upon stress. The high viscosity allows the lava to form blocky and short lobes. The pervasive volcaniclastic cover on North Su is partly cemented by hydrothermal precipitates. These hydrothermally-cemented breccias, crusts and single pillars show that hydrothermal circulation through a thick layer of volcaniclastic deposits can temporarily increase slope stability through precipitation and cementation.
Hydrogen generation and iron partitioning during experimental serpentinization of an olivine-pyroxene mixture

(Elsevier, 2020-05-26) McCollom, Thomas M. ; Klein, Frieder ; Moskowitz, Bruce ; Berquo, Thelma S. ; Bach, Wolfgang ; Templeton, Alexis S.

A series of laboratory experiments was conducted to investigate serpentinization of olivine–pyroxene mixtures at 230 °C, with the objective of evaluating the effect of mixed compositions on Fe partitioning among product minerals, H2 generation, and reaction rates. An initial experiment reacted a mixture of 86 wt.% olivine and 14 wt.% orthopyroxene (Opx) with the same initial grain size for 387 days. The experiment resulted in extensive reaction (∼53% conversion), and solids recovered at termination of the experiment were dominated by Fe-bearing chrysotile and relict olivine along with minor brucite and magnetite. Only limited amounts of H2 were generated during the first ∼100 days of the experiment, but the rate of H2 generation then increased sharply coincident with an increase in pH from mildly alkaline to strongly alkaline conditions. Two shorter term experiments with the same reactants (26 and 113 days) produced a mixture of lizardite and talc that formed a thin coating on relict olivine and Opx grains, with virtually no generation of H2. Comparison of the results with reaction path models indicates that the Opx reacted about two times faster than olivine, which contrasts with some previous studies that suggested olivine should react more rapidly than Opx at the experimental conditions. The models also indicate that the long-term experiment transitioned from producing serpentine ± talc early in the early stages to precipitation of serpentine plus magnetite, with brucite beginning to precipitate only late in the experiment as Opx was depleted. The results indicate that overall reaction of olivine and Opx was initially relatively slow, but reaction rates accelerated substantially when the pH transitioned to strongly alkaline conditions. Serpentine and brucite precipitated from the olivine-Opx mixture had higher Fe contents than observed in olivine-only experiments at mildly alkaline pH, but had comparable Fe contents to reaction of olivine at strongly alkaline pH implying that higher pH may favor greater partitioning of Fe into serpentine and brucite and less into magnetite. Despite the presence of brucite, dissolved silica activities during the long-term olivine-Opx experiment maintained levels well above serpentine-brucite equilibrium. Instead, silica activities converged on levels close to metastable equilibrium between brucite and olivine. It is proposed that silica levels during the experiment may have been regulated by exchange of SiO2 between the fluid and a silica-depleted, brucite-like surface layer on dissolving olivine.
Seawater-peridotite interactions : first insights from ODP Leg 209, MAR 15°N

(American Geophysical Union, 2004-09-10) Bach, Wolfgang ; Garrido, Carlos J. ; Paulick, Holger ; Harvey, Jason ; Rosner, Martin

We present first results of a petrographic study of hydrothermally altered peridotites drilled during Ocean Drilling Program (ODP) Leg 209 in the 15°20′N fracture Zone area on the Mid-Atlantic Ridge (MAR). We find that serpentinization is extensive at all drill sites. Where serpentinization is incomplete, phase relations indicate two major reaction pathways. One is reaction of pyroxene to talc and tremolite, and the other is reaction of olivine to serpentine, magnetite, and brucite. We interpret these reactions in the light of recent peridotite-seawater reaction experiments and compositions of fluids venting from peridotite massifs at a range of temperatures. We suggest that the replacement of pyroxene by talc and tremolite takes place at temperatures >350°–400°C, where olivine is stable. The breakdown of olivine to serpentine, magnetite, and brucite is favored at temperatures below 250°C, where olivine reacts faster then pyroxene. High-temperature hydrothermal fluids venting at the Logatchev and Rainbow sites are consistent with rapid reaction of pyroxene and little or no reaction of olivine. Moderate-temperature fluids venting at the Lost City site are consistent with ongoing reaction of olivine to serpentine and brucite. Many completely serpentinized peridotites lack brucite and talc because once the more rapidly reacting phase is exhausted, interaction with the residual phase will change fluid pH and silica activity such that brucite or talc react to serpentine. At two sites we see strong evidence for continued fluid flow and fluid-rock interaction after serpentinization was complete. At Site 1268, serpentinites underwent massive replacement by talc under static conditions. This reaction requires either removal of Mg from or addition of Si to the system. We propose that the talc-altered rocks are Si-metasomatized and that the source of Si is likely gabbro-seawater reaction or breakdown of pyroxene deeper in the basement. The basement at Site 1268 is heavily veined, with talc and talc-oxide-sulfide veins being the most common vein types. It appears that the systems evolved from reducing (oxygen fugacity buffered by magnetite-pyrrhotite-pyrite and lower) to oxidizing (dominantly hematite). We propose that this transition is indicative of high fluid flux under retrograde conditions and that the abundance of hematite may relate to the Ca-depleted nature of the basement that prevents near-quantitative removal of seawater sulfate by anhydrite precipitation. At site 1272 we find abundant iowaite partly replacing brucite. While this is the first report of iowaite from a mid-ocean ridge setting, its presence indicates, again, fairly oxidizing conditions. Our preliminary results indicate that peridotite-seawater and serpentinite-seawater interactions can take place under a wider range of temperature and redox conditions than previously appreciated.
Sulfur isotope measurement of sulfate and sulfide by high-resolution MC-ICP-MS

( 2008-04) Craddock, Paul R. ; Rouxel, Olivier J. ; Ball, Lary A. ; Bach, Wolfgang

We have developed a technique for the accurate and precise determination of 34S/32S isotope ratios (δ34S) in sulfur-bearing minerals using solution and laser ablation multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). We have examined and determined rigorous corrections for analytical difficulties such as instrumental mass bias, unresolved isobaric interferences, blanks, and laser ablation- and matrix-induced isotopic fractionation. Use of high resolution sector-field mass spectrometry removes major isobaric interferences from O2+. Standard–sample bracketing is used to correct for the instrumental mass bias of unknown samples. Blanks on sulfur masses arising from memory effects and residual oxygen-tailing are typically minor (< 0.2‰, within analytical error), and are mathematically removed by on-peak zero subtraction and by bracketing of samples with standards determined at the same signal intensity (within 20%). Matrix effects are significant (up to 0.7‰) for matrix compositions relevant to many natural sulfur-bearing minerals. For solution analysis, sulfur isotope compositions are best determined using purified (matrix-clean) sulfur standards and sample solutions using the chemical purification protocol we present. For in situ analysis, where the complex matrix cannot be removed prior to analysis, appropriately matrix-matching standards and samples removes matrix artifacts and yields sulfur isotope ratios consistent with conventional techniques using matrix-clean analytes. Our method enables solid samples to be calibrated against aqueous standards; a consideration that is important when certified, isotopically-homogeneous and appropriately matrix-matched solid standards do not exist. Further, bulk and in situ analyses can be performed interchangeably in a single analytical session because the instrumental setup is identical for both. We validated the robustness of our analytical method through multiple isotope analyses of a range of reference materials and have compared these with isotope ratios determined using independent techniques. Long-term reproducibility of S isotope compositions is typically 0.20‰ and 0.45‰ (2σ) for solution and laser analysis, respectively. Our method affords the opportunity to make accurate and relatively precise S isotope measurement for a wide range of sulfur-bearing materials, and is particularly appropriate for geologic samples with complex matrix and for which high-resolution in situ analysis is critical.
Ultramafic clasts from the South Chamorro serpentine mud volcano reveal a polyphase serpentinization history of the Mariana forearc mantle

( 2014-11) Kahl, Wolf-Achim ; Jons, Niels ; Bach, Wolfgang ; Klein, Frieder ; Alt, Jeffrey C.

Serpentine seamounts located on the outer half of the pervasively fractured Mariana forearc provide an excellent window into the forearc devolatilization processes, which can strongly influence the cycling of volatiles and trace elements in subduction zones. Serpentinized ultramafic clasts recovered from an active mud volcano in the Mariana forearc reveal microstructures, mineral assemblages and compositions that are indicative of a complex polyphase alteration history. Petrologic phase relations and oxygen isotopes suggest that ultramafic clasts were serpentinized at temperatures below 200 °C. Several successive serpe ntinization events represented by different vein generations with distinct trace element contents can be recognized. Measured Rb/Cs ratios are fairly uniform ranging between 1 and 10, which is consistent with Cs mobilization from sediments at lower temperatures and lends further credence to the low-temperature conditions proposed in models of the thermal structure in forearc settings. Late veins show lower fluid mobile element (FME) concentrations than early veins, suggesting a deacreasing influence of fluid discharge from sediments on the composition of the serpentinizing fluids. The continuous microfabric and mineral chemical evolution observed in the ultramafic clasts may have implications as to the origin and nature of the serpentinizing fluids. We hypothesize that opal and smectite dehydration produce quartz-saturated fluids with high FME contents and Rb/Cs between 1 and 4 that cause the early pervasive serpentinization. The partially serpentinized material may then be eroded from the basal plane of the suprasubduction mantle wedge. Serpentinization continued but the interacting fluids did not carry the slab-flux signature, either because FME were no longer released from the slab, or due to an en route loss of FMEs. Late chrysotile veins that document the increased access of fluids in a now fluid-dominated regime are characterized by reduced trace element contents with a slightly increased Rb/Cs ratio near 10. This lack of geochemical slab signatures consistently displayed in all late serpentinization stages may indicate that the slab-derived fluids have been completely reset (i.e. the FME excesses were removed) by continued water-rock reaction within the subduction channel. The final stage of diapiric rise of matrix and clasts in the conduits is characterized by brucite-dominated alteration of the clasts from the clast rim inward (independent of the intra-clast fabric relations), which corresponds to re-equilibration with alkaline, low-silica activity fluids in the rising mud.
Insights to magmatic–hydrothermal processes in the Manus back-arc basin as recorded by anhydrite

( 2010-06-16) Craddock, Paul R. ; Bach, Wolfgang

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 REEN patterns measured in anhydrite from mid–ocean ridge deposits. The REEN 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 δ34S values of anhydrite cluster around that of contemporary seawater, consistent with anhydrite precipitating from hydrothermal fluid mixed with locally entrained seawater. Sulfate δ34S isotope ratios in some anhydrites are, however, lighter than that of seawater interpreted as recording a source of sulfate derived from magmatic SO2 degassed from underlying felsic magmas in the Manus. 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.
Geochemistry of abyssal peridotites (Mid-Atlantic Ridge, 15°20′N, ODP Leg 209) : implications for fluid/rock interaction in slow spreading environments

( 2006-04-21) Paulick, Holger ; Bach, Wolfgang ; Godard, M. ; de Hoog, Jan C. M. ; Suhr, G. ; Harvey, Jason

Abyssal peridotite from the 15°20’N area of the Mid-Atlantic Ridge show complex geochemical variations among the different sites drilled during ODP Leg 209. Major element compositions indicate variable degrees of melt depletion and refertilization as well as local hydrothermal metasomatism. Strongest evidence for melt-rock interactions are correlated Light Rare Earth Element (LREE) and High Field Strength Element (HFSE) additions at sites 1270 and 1271. In contrast, hydrothermal alteration at Sites 1274, 1272, and 1268 causes LREE mobility associated with minor HFSE variability, reflecting the low solubility of HFSE in aqueous solutions. Site 1274 contains the least-altered, highly refractory, peridotite with strong depletion in LREE and shows a gradual increase in the intensity of isochemical serpentinization; except for the addition of H2O which causes a mass gain of up to 20 g/100 g. The formation of magnetite is reflected in decreasing Fe2+/Fe3+ ratios. This style of alteration is referred to as rock-dominated serpentinization. In contrast, fluid-dominated serpentinization at Site 1268 is characterized by gains in sulfur and development of U-shaped REE pattern with strong positive Eu anomalies which are also characteristic for hot (350 to 400°C) vent-type fluids discharging from black smoker fields. Serpentinites at Site 1268 were overprinted by talc alteration under static conditions due to interaction with high aSiO2 fluids causing the development of smooth, LREE-enriched patterns with pronounced negative Eu anomalies. These results show that hydrothermal fluid-peridotite and fluid-serpentinite interaction processes are an important factor regarding the budget of exchange processes between the lithosphere and the hydrosphere in slow spreading environments.
Fluid-rock interactions in the shallow Mariana forearc: Carbon cycling and redox conditions

(European Geosciences Union, 2019-06-24) Albers, Elmar ; Bach, Wolfgang ; Klein, Frieder ; Menzies, Catriona D. ; Lucassen, Friedrich ; Teagle, Damon A.H.

Few data exist that provide insight into processes affecting the long-term carbon cycle at shallow forearc depths. To better understand the mobilization of C in sediments and crust of the subducting slab, we investigated carbonate materials that originate from the subduction channel at the Mariana forearc (< 20 km) and were recovered during International Ocean Discovery Program Expedition 366. Calcium carbonates occur as vein precipitates within metavolcanic and metasedimentary clasts. The clasts represent portions of the subducting lithosphere, including ocean island basalt, that were altered at lower blueschist facies conditions and were subsequently transported to the forearc seafloor by serpentinite mud volcanism. Euhedral aragonite and calcite and the lack of deformation within the veins suggest carbonate formation in a stress-free environment after peak metamorphism affected their hosts. Intergrowth with barite and marked negative Ce anomalies in carbonate attest the precipitation within a generally oxic environment, that is an environment not controlled by serpentinization. Strontium and O isotopic compositions in carbonate (87Sr∕86Sr = 0.7052 to 0.7054, δ18OVSMOW = 20 to 24 ‰) imply precipitation from slab-derived fluids at temperatures between ∼130 and 300 ∘C. These temperature estimates are consistent with the presence of blueschist facies phases such as lawsonite coexisting with the carbonates in some veins. Incorporated C is inorganic (δ13CVPDB = −1 ‰ to +4 ‰) and likely derived from the decarbonation of calcareous sediment and/or oceanic crust. These findings provide evidence for the mobilization of C in the downgoing slab at depths of < 20 km. Our study shows for the first time in detail that a portion of this C forms carbonate precipitates in the subduction channel of an active convergent margin. This process may be an important asset in understanding the deep carbon cycle since it highlights that some C is lost from the subducting lithosphere before reaching greater depths.
Complex subsurface hydrothermal fluid mixing at a submarine arc volcano supports distinct and highly diverse microbial communities

(National Academy of Sciences, 2020-12-04) Reysenbach, Anna-Louise ; St. John, Emily ; Meneghin, Jennifer ; Flores, Gilberto ; Podar, Mircea ; Dombrowski, Nina ; Spang, Anja ; L’Haridon, Stephane ; Humphris, Susan E. ; de Ronde, Cornel E. J. ; Tontini, F. Caratori ; Tivey, Maurice A. ; Stucker, Valerie ; Stewart, Lucy C. ; Diehl, Alexander ; Bach, Wolfgang

Hydrothermally active submarine volcanoes are mineral-rich biological oases contributing significantly to chemical fluxes in the deep sea, yet little is known about the microbial communities inhabiting these systems. Here we investigate the diversity of microbial life in hydrothermal deposits and their metagenomics-inferred physiology in light of the geological history and resulting hydrothermal fluid paths in the subsurface of Brothers submarine volcano north of New Zealand on the southern Kermadec arc. From metagenome-assembled genomes we identified over 90 putative bacterial and archaeal genomic families and nearly 300 previously unknown genera, many potentially endemic to this submarine volcanic environment. While magmatically influenced hydrothermal systems on the volcanic resurgent cones of Brothers volcano harbor communities of thermoacidophiles and diverse members of the superphylum “DPANN,” two distinct communities are associated with the caldera wall, likely shaped by two different types of hydrothermal circulation. The communities whose phylogenetic diversity primarily aligns with that of the cone sites and magmatically influenced hydrothermal systems elsewhere are characterized predominately by anaerobic metabolisms. These populations are probably maintained by fluids with greater magmatic inputs that have interacted with different (deeper) previously altered mineral assemblages. However, proximal (a few meters distant) communities with gene-inferred aerobic, microaerophilic, and anaerobic metabolisms are likely supported by shallower seawater-dominated circulation. Furthermore, mixing of fluids from these two distinct hydrothermal circulation systems may have an underlying imprint on the high microbial phylogenomic diversity. Collectively our results highlight the importance of considering geologic evolution and history of subsurface processes in studying microbial colonization and community dynamics in volcanic environments.
Ultra-diffuse hydrothermal venting supports Fe-oxidizing bacteria and massive umber deposition at 5000 m off Hawaii

(Nature Publishing Group, 2011-05-05) Edwards, Katrina J. ; Glazer, Brian T. ; Rouxel, Olivier J. ; Bach, Wolfgang ; Emerson, David ; Toner, Brandy M. ; Chan, Clara S. ; Tebo, Bradley M. ; Staudigel, Hubert ; Moyer, Craig L.

A novel hydrothermal field has been discovered at the base of Lōihi Seamount, Hawaii, at 5000 mbsl. Geochemical analyses demonstrate that ‘FeMO Deep’, while only 0.2 °C above ambient seawater temperature, derives from a distal, ultra-diffuse hydrothermal source. FeMO Deep is expressed as regional seafloor seepage of gelatinous iron- and silica-rich deposits, pooling between and over basalt pillows, in places over a meter thick. The system is capped by mm to cm thick hydrothermally derived iron-oxyhydroxide- and manganese-oxide-layered crusts. We use molecular analyses (16S rDNA-based) of extant communities combined with fluorescent in situ hybridizations to demonstrate that FeMO Deep deposits contain living iron-oxidizing Zetaproteobacteria related to the recently isolated strain Mariprofundus ferroxydans. Bioenergetic calculations, based on in-situ electrochemical measurements and cell counts, indicate that reactions between iron and oxygen are important in supporting chemosynthesis in the mats, which we infer forms a trophic base of the mat ecosystem. We suggest that the biogenic FeMO Deep hydrothermal deposit represents a modern analog for one class of geological iron deposits known as ‘umbers’ (for example, Troodos ophilolites, Cyprus) because of striking similarities in size, setting and internal structures.

Bach Wolfgang

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