Submarine Magmatic-Hydrothermal Systems at the Monowai Volcanic Centre, Kermadec Arc

Authors listed on this Accepted Manuscript vary slightly from those listed on the Version of Record. Harold L. Gibson is an additional author on the published version.The Monowai volcanic centre (MVC) is located at the mid-point along the ~2530 km long Tonga-Kermadec arc system, is probably the most hydrothermally active submarine volcanic system globally. The MVC is comprised of a large elongate caldera (Monowai caldera, 7.9 x 5.7 km; 35 km²; depth to caldera floor is 1590 m), which has formed within an older caldera some 84 km² in area. To the south of the nested caldera system is a large composite volcano, Monowai cone, which rises to within ~ 100 m of the sea surface and has been volcanically active for at least several decades. Despite the large size, mafic volcanic rocks dominate the MVC; basalts are the most common rock type recovered; less common are basaltic andesites and andesites. Hydrothermal plume mapping during the 2004 NZAPLUME III cruise showed at least three major hydrothermal systems associated with the caldera and cone. Monowai cone has hydrothermal venting from the summit. This summit plume is gas-rich and acidic; plume samples show a pH shift of -2.00 pH units, δ³He up to 358 ‰, H₂S concentrations up to 32 μM and CH₄ concentrations up to 900 nM. The summit plume is also metal-rich with elevated total dissolvable Fe (TDFe up to 4200 nM), TDMn (up to 412 nM), and TDFe/TDMn (up to 20.4). Monowai caldera has a major hydrothermal vent system with plumes extending from ~ 1000 to 1400 m depth. The caldera plume has lower values for TDFe, although ranges to higher TDMn concentrations than the summit plume, and is relatively gas-poor (no H₂S detected, pH shift of -0.06 pH units, CH₄ concentrations up to 26 nM). Hydrothermal vents have been observed associated with prominent basaltic andesite ridges (Mussel Ridge) proximal to the southwest wall of the caldera (1025 – 1171 m depth). However, the composition of the hydrothermal plumes in the caldera are different to the vents, indicating that the source of the caldera plumes is at greater depth and is more metal-rich and therefore likely higher temperature. Minor plumes detected as light scattering anomalies down the northern flank of Monowai caldera most likely represent resuspension of volcanic debris. Particulate samples from both the cone sites and the caldera site are enriched in Al, Ti, Ca, Mg, Si, and S, with the cone summit plume especially enriched in K, As, W and Cu, Pb, Zn. The elevated Ti and Al suggest acidic water-rock reactions and intense high-sulfidation alteration of the host volcanic rocks. Observations from submersible dives with Pisces V in 2005 and the remotely operated vehicle ROPOS in 2007 of Mussel Ridge indicate numerous low temperature vents (< 60°C), with a large biomass of vent-associated fauna, in particular large accumulations of the mussel Bathymodiolus sp. and the tubeworm Lamellibrachia sp. We interpret the Monowai volcanic centre as possessing a robust high-sulfidation magmatic-hydrothermal system, with significant differences in the style and composition of venting at the cone and caldera sites. At Monowai cone, the large shifts in pH, elevated TDFe and TDFe/TDMn, and H₂S-, CH₄- and ³He-rich nature of the plume fluids coupled with elevated Ti, P, V, S and Al in the particulates indicates significant magmatic volatile ± metal contributions to the hydrothermal system and aggressive acidic water-rock interaction. By contrast, Monowai caldera has low TDFe/TDMn in hydrothermal plumes; however, end-member vent fluid compositions, combined with presence of alunite, sulfide minerals and native sulfur in samples from Mussel Ridge suggest recent acid volatile-rich venting and active Fe-sulfide formation in the subsurface, and the potential for the presence of significant SMS mineralization

Tracking the Evolution of a Hydrothermal Event Plume with a RAFOS Neutrally Buoyant Drifter

The migration and evolution of a deep ocean hydrothermal event plume were tracked with a neutrally buoyant RAFOS float. The float remained entrained in the plume for 60 days, and the plume vorticity was calculated directly from the anticyclonic motion of the float. Concentrations of suspended particles, particulate iron, and dissolved manganese in the plume did not decay significantly during the 60 days, which indicates that event plumes would be easily detectable a year after formation

Hydrothermal activity in the Northwest Lau Backarc Basin: Evidence from water column measurements

The Northwest Lau Backarc Basin, consisting of the Northwest Lau Spreading Center (NWLSC) and the Rochambeau Rifts (RR), is unique in having elevated 3He/ 4He ratios (up to 28 R a) in the erupted lavas, clearly indicating a hot spot or ocean island basalt (OIB)-type signature. This OIB-type helium signature does not appear in any other part of the Lau Basin. Water column plume surveys conducted in 2008 and 2010 identified several sites of active hydrothermal discharge along the NWLSC-RR and showed that the incidence of hydrothermal activity is high, consistent with the high spreading rate of ∼100 mm/year. Hydrocasts into the Central Caldera and Southern Caldera of the NWLSC detected elevated 3He/ 4He (δ 3He = 55% and 100%, respectively), trace metals (TMn, TFe), and suspended particles, indicating localized hydrothermal venting at these two sites. Hydrocasts along the northern rift zone of the NWLSC also had excess δ 3He, TMn, and suspended particles suggesting additional sites of hydrothermal activity. The RR are dominated by Lobster Caldera, a large volcano with four radiating rift zones. Hydrocasts into Lobster Caldera in 2008 detected high δ 3He (up to 239%) and suspended particle and TMn signals, indicating active venting within the caldera. A repeat survey of Lobster in 2010 confirmed the site was still active two years later. Plumes at Lobster Caldera and Central Caldera have end-member 3He/ 4He ratios of 19 R a and 11 R a, respectively, confirming that hot spot-type helium is also present in the hydrothermal fluids

Composition and Dissolution of Black Smoker Particulates from Active Vents on the Juan De Fuca Ridge

During two Atlantis II/Alvin cruises to the Juan de Fuca Ridge in 1984 active high temperature (140°–284°C) vents were sampled for black smoker particulates using the Grassle Pump. Individual mineral phases were identified using standard X ray diffraction and petrographic procedures. In addition, elemental compositions and particle morphologies were determined by X ray energy spectrometry and scanning electron microscope/X ray energy spectrometry techniques. The vent particulates from the southern Juan de Fuca Ridge vent sites were highly enriched in S, Si, Fe, Zn, and Cu and were primarily composed of sphalerite, wurtzite, pyrite, pyrrhotite, barite, chalcopyrite, cubanite, hydrous iron oxides, and elemental sulfur. Two additional unidentified phases which were prevalent in the samples included an Fe-Si phase and a Ca-Si phase. The grain sizes of the individual particle phases ranged from \u3c 2 μm for the sphalerite and Fe oxide particles to \u3e 100 μm for the Fe-Si particles. Grain size and current meter data were used in a deposition model of individual phase dispersal. For many of the larger sulfide and sulfate particles, the model predicts dispersal to occur over length scales of only several hundreds of meters. The high-temperature black smokers from the more northerly Endeavour Segment vents were highly enriched in Fe, S, Ca, Cu, and Zn and were primarily composed of anhydrite, chalcopyrite, sphalerite, barite, sulfur, pyrite, and other less abundant metal sulfide minerals. The grain sizes of the individual particles ranged from \u3c 10 μm to slightly larger than 500 μm. The composition and size distributions of the mineral phases are highly suggestive of high-temperature mixing between vent fluids and seawater. A series of field and laboratory studies were conducted to determine the rates of dissolution of several sulfate and sulfide minerals. The dissolution rates ranged over more than 3 orders of magnitude, from 3.2 × 10−8 cm s−1 for anhydrite to 1.2 × 10−12 cm s−1 for chalcopyrite. The results indicate that for some minerals, particularly anhydrite and marcasite, total dissolution occurs within a few hours to a few weeks of their formation. For other more stable minerals, including pyrite, sphalerite and chalcopyrite, the time required for total dissolution is much longer, and consequently, individual crystals may be expected to persist in the sediments for considerable periods of time after deposition

Submarine hydrothermal activity and gold-rich mineralization at Brothers Volcano, Kermadec Arc, New Zealand

Brothers volcano, of the Kermadec intraoceanic arc, is host to a hydrothermal system unique among seafloor hydrothermal systems known anywhere in the world. It has two distinct vent fields, known as the NW Caldera and Cone sites, whose geology, permeability, vent fluid compositions, mineralogy, and ore-forming conditions are in stark contrast to each other. The NW Caldera site strikes for ∼600 m in a SW–NE direction with chimneys occurring over a ∼145-m depth interval, between ∼1,690 and 1,545 m. At least 100 dead and active sulfide chimney spires occur in this field and are typically 2–3 m in height, with some reaching 6–7 m. Their ages (at time of sampling) fall broadly into three groups: <4, 23, and 35 years old. The chimneys typically occur near the base of individual fault-controlled benches on the caldera wall, striking in lines orthogonal to the slopes. Rarer are massive sulfide crusts 2–3 m thick. Two main types of chimney predominate: Cu-rich (up to 28.5 wt.% Cu) and, more commonly, Zn-rich (up to 43.8 wt.% Zn). Geochemical results show that Mo, Bi, Co, Se, Sn, and Au (up to 91 ppm) are correlated with the Cu mineralization, whereas Cd, Hg, Sb, Ag, and As are associated with the dominant Zn-rich mineralization. The Cone site comprises the Upper Cone site atop the summit of the recent (main) dacite cone and the Lower Cone site that straddles the summit of an older, smaller, more degraded dacite cone on the NE flank of the main cone. Huge volumes of diffuse venting are seen at the Lower Cone site, in contrast to venting at both the Upper Cone and NW Caldera sites. Individual vents are marked by low-relief (≤0.5 m) mounds comprising predominately native sulfur with bacterial mats. Vent fluids of the NW Caldera field are focused, hot (≤300°C), acidic (pH ≥ 2.8), metal-rich, and gas-poor. Calculated end-member fluids from NW Caldera vents indicate that phase separation has occurred, with Cl values ranging from 93% to 137% of seawater values. By contrast, vent fluids at the Cone site are diffuse, noticeably cooler (≤122°C), more acidic (pH 1.9), metal-poor, and gas-rich. Higher-than-seawater values of SO4 and Mg in the Cone vent fluids show that these ions are being added to the hydrothermal fluid and are not being depleted via normal water/rock interactions. Iron oxide crusts 3 years in age cover the main cone summit and appear to have formed from Fe-rich brines. Evidence for magmatic contributions to the hydrothermal system at Brothers includes: high concentrations of dissolved CO2 (e.g., 206 mM/kg at the Cone site); high CO2/3He; negative δD and δ18OH2O for vent fluids; negative δ34S for sulfides (to −4.6‰), sulfur (to −10.2‰), and δ15N2 (to −3.5‰); vent fluid pH values to 1.9; and mineral assemblages common to high-sulfidation systems. Changing physicochemical conditions at the Brothers hydrothermal system, and especially the Cone site, occur over periods of months to hundreds of years, as shown by interlayered Cu + Au- and Zn-rich zones in chimneys, variable fluid and isotopic compositions, similar shifts in 3He/4He values for both Cone and NW Caldera sites, and overprinting of “magmatic” mineral assemblages by water/rock-dominated assemblages. Metals, especially Cu and possibly Au, may be entering the hydrothermal system via the dissolution of metal-rich glasses. They are then transported rapidly up into the system via magmatic volatiles utilizing vertical (∼2.5 km long), narrow (∼300-m diameter) “pipes,” consistent with evidence of vent fluids forming at relatively shallow depths. The NW Caldera and Cone sites are considered to represent stages along a continuum between water/rock- and magmatic/hydrothermal-dominated end-members