Venting of a separate CO2-rich gas phase from submarine arc volcanoes: Examples from the Mariana and Tonga-Kermadec arcs

Submersible dives on 22 active submarine volcanoes on the Mariana and Tonga-Kermadec arcs have discovered systems on six of these volcanoes that, in addition to discharging hot vent fluid, are also venting a separate CO2-rich phase either in the form of gas bubbles or liquid CO2 droplets. One of the most impressive is the Champagne vent site on NW Eifuku in the northern Mariana Arc, which is discharging cold droplets of liquid CO2 at an estimated rate of 23 mol CO2/s, about 0.1% of the global mid-ocean ridge (MOR) carbon flux. Three other Mariana Arc submarine volcanoes (NW Rota-1, Nikko, and Daikoku), and two volcanoes on the Tonga-Kermadec Arc (Giggenbach and Volcano-1) also have vent fields discharging CO2-rich gas bubbles. The vent fluids at these volcanoes have very high CO2 concentrations and elevated C/3He and δ 13C (CO2) ratios compared to MOR systems, indicating a contribution to the carbon flux from subducted marine carbonates and organic material. Analysis of the CO2 concentrations shows that most of the fluids are undersaturated with CO2. This deviation from equilibrium would not be expected for pressure release degassing of an ascending fluid saturated with CO2. Mechanisms to produce a separate CO2-rich gas phase at the seafloor require direct injection of magmatic CO2-rich gas. The ascending CO2-rich gas could then partially dissolve into seawater circulating within the volcano edifice without reaching equilibrium. Alternatively, an ascending high-temperature, CO2-rich aqueous fluid could boil to produce a CO2-rich gas phase and a CO2-depleted liquid. These findings indicate that carbon fluxes from submarine arcs may be higher than previously estimated, and that experiments to estimate carbon fluxes at submarine arc volcanoes are merited. Hydrothermal sites such as these with a separate gas phase are valuable natural laboratories for studying the effects of high CO2 concentrations on marine ecosystems

Multiple hydrothermal sources along the south Tonga arc and Valu Fa Ridge

Quantifying hydrothermal venting at the boundaries of tectonic plates is an outstanding geoscience problem. Considerable progress has been made by detailed surveys along mid-ocean ridges (MORs), but until recently little was known about fluid venting along volcanic arcs. We present the first systematic survey for hydrothermal venting along the 425-km-long south Tonga arc and new chemistry data for particle and thermal plumes previously reported along an adjacent 88-km-long section of the back-arc Valu Fa Ridge (VFR). Eleven hydrothermal plumes, recognized by their anomalous light backscattering, Eh, temperature, pH, dissolved 3He, CH4, and total dissolvable Fe and Mn, were identified arising from seven volcanic centers along the arc. Five plumes on the VFR were characterized chemically. Vent field density for the south Tonga arc was 2.6 sites/100 km of arc front, comparable to that found by surveys of the Kermadec arc (1.9 to 3.8 sites/100 km) and to MORs in the eastern Pacific (average value for 2280 km of surveyed ridgecrest: 3.2 sites/100 km). A "vent gap" occurs along a 190 km section of the arc closest to the VFR, and a site density twice the average for MORs on the eastern edge of the Pacific plate was found on this part of the VFR (6.6 sites/100 km). We suggest magmas ascending under the adjacent south Tonga arc have been captured by the VFR. While chemical enrichments of plumes on the south Tonga arc were, in general, slightly less than those on the Kermadec arc, several instances of excessive anomalies in pH suggest a similar presence of fluids enriched in magmatic volatiles (CO2-SO2-H2S). Locally, venting on the VFR has contributed to accumulations of 3He, Fe, and Mn within the southern Lau basin. On a broader scale, our results provide considerable support for the notion that venting from intraoceanic arcs on the convergent margin of the Pacific plate adds significantly to the total hydrothermal input into the Pacific Ocean

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

Hydrothermal activity on near-arc sections of back-arc ridges: Results from the Mariana Trough and Lau Basin

The spatial density of hydrothermal venting is strongly correlated with spreading rate on mid-ocean ridges (with the interesting exception of hot spot-affected ridges), evidently because spreading rate is a reliable proxy for the magma budget. This correlation remains untested on spreading ridges in back-arc basins, where the magma budget may be complicated by subduction-induced variations of the melt supply. To address this uncertainty, we conducted hydrothermal plume surveys along slow-spreading (40-60 mm/yr) and arc-proximal (10-60 km distant) sections of the southern Mariana Trough and the Valu Fa Ridge (Lau Basin). On both sections we found multiple plumes overlying ∼15-20% of the total length of each section, a coverage comparable to mid-ocean ridges spreading at similar rates. These conditions contrast with earlier reported results from the two nearest-arc segments of a faster spreading (60-70 mm/yr) back-arc ridge, the East Scotia Ridge, which approaches no closer than 100 km to its arc. There, hydrothermal venting is relatively scarce (∼5% plume coverage) and the ridge characteristics are distinctly slow-spreading: small central volcanic highs bookended by deep median valleys, and axial melt lenses restricted to the volcanic highs. Two factors may contribute to an unexpectedly low hydrothermal budget on these East Scotia Ridge segments: they may lie too far from the adjacent arc to benefit from near-arc sources of melt supply, and subduction-aided migration of mantle from the Bouvet hot spot may reduce hydrothermal circulation by local crustal warming and thickening, analogous to the Reykjanes Ridge. Thus the pattern among these three ridge sections appears to mirror the larger global pattern defined by mid-ocean ridges: a well-defined trend of spreading rate versus hydrothermal activity on most ridge sections, plus a subset of ridge sections where unusual melt delivery conditions diminish the expected hydrothermal activity

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