D. A. Butterfield

Magmatic events can produce rapid changes in hydrothermal vent chemistry

The Endeavour segment of the Juan de Fuca ridge is host to one of the most vigorous hydrothermal areas found on the global mid-ocean-ridge system, with five separate vent fields located within 15 km along the top of the ridge segment. Over the past decade, the largest of these vent fields, the ‘Main Endeavour Field’, has exhibited a constant spatial gradient in temperature and chloride concentration in its vent fluids, apparently driven by differences in the nature and extent of subsurface phase separation. This stable situation was disturbed on 8 June 1999 by an earthquake swarm. Owing to the nature of the seismic signals and the lack of new lava flows observed in the area during subsequent dives of the Alvin and Jason submersibles (August–September 1999), the event was interpreted to be tectonic in nature. Here we show that chemical data from hydrothermal fluid samples collected in September 1999 and June 2000 strongly suggest that the event was instead volcanic in origin. Volatile data from this event and an earlier one at 9° N on the East Pacific Rise show that such magmatic events can have profound and rapid effects on fluid–mineral equilibria, phase separation, 3He/heat ratios and fluxes of volatiles from submarine hydrothermal systems.

Initial results of the rapid response to the 1993 CoAxial event: Relationships between hydrothermal and volcanic processes

Between June 26 and July 10, 1993, swarms of “T-wave” events occurred over a 40-km portion of the CoAxial segment on the northern Juan de Fuca Ridge. A rapid response utilizing a CTD/rosette/chemical scanner and a remotely operated vehicle occurred in the month following the T-wave swarms. The pattern of T-wave events and water-column anomalies (including several event plumes) are remarkably coincident. The only known eruptive area is at the northern swarm area, where a very fresh pillow lava ridge was discovered, mapped, and sampled with the remotely operated vehicle ROPOS. A vent area about 22 km south of the lava flow was emitting large quantities of bacterially generated floccular material. The temporal pattern of T-wave events and the coincidence between the T-wave swarms, the young lava flows, and hydrothermal plumes suggests that there is a close analogy between this activity and lateral dike injections such as have been closely monitored at Icelandic central volcanoes.

Biological colonization of new hydrothermal vents following an eruption on Juan de Fuca Ridge

Abstract A recent eruption on CoAxial Segment of Juan de Fuca Ridge initiated hydrothermal conditions with rapid changes in water chemistry and growth of microbial communities. Vent animals recruited from distal sources within a year. One site with newly erupted lava attracted no animals to high-iron and low-sulphide conditions. However, sustained release of flocculent material at a second site suggests extensive subterranean microbial production; here, the dissolved sulphide/heat ratio peaked during the first year. The first larval recruits included vestimentiferans, alvinellid polychaetes and nemerteans; despite the small areal extent of venting, one-third of the regional vent species pool had arrived by 2 years. Near-optimal growth conditions and recruitment by many species continued in the centre of the system but several habitats went extinct within 2 years. Rapid response and exploitation by vent animals must be an important adaptation to such ephemeral conditions.

Silica and germanium in Pacific Ocean hydrothermal vents and plumes

Dissolved silica (Si) and inorganic germanium (Ge) concentrations were measured in hydrothermal fluids from black smoker vents on the East Pacific Rise (21°N EPR) and the Southern Juan de Fuca Ridge (45°N SJdFR: North and South Cleft Sites, Axial Volcano). These typically display end-member concentrations ranging from 16 to 23 mM (Si) and 150 to 280 nM (Ge), and end-member Ge/Si ratios clustering between 8 and 14 × 10−6, more than 10-fold greater than the ratio entering the ocean via rivers (0.54 × 10−6) and being recycled in seawater (0.7 × 10−6). ‘Excess’ concentrations of dissolved Si and Ge above oceanic background are observed in mid-water hydrothermal plumes over mid-ocean ridge (MOR) spreading centers on the Southern EPR (SEPR) (10°–20°S) and the SJdFR. The largest Si and Ge concentration anomalies occur over the North Cleft Segment of the SJdFR. These are a factor of three greater than anomalies over the SEPR (10°–20°S). Excess Ge correlates with excess3He in plumes at a Ge/3He molar ratio of about 1 × 104, approximately the same ratio as in black smokers. These observations, combined with low abundances of Ge in FeMn-rich metalliferous sediments, suggest that Ge (and Si) behave conservatively in mid-ocean ridge hydrothermal plumes. A simple ocean Si and Ge balance, constrained by the global river silica flux and Ge/Si ratios in hydrothermal vents, rivers and biogenic silica, suggests that the global hydrothermal silica flux is about 1–4 × 1011 mole yr−1, much lower than that estimated from3He. Either (1) 70–80% of the Ge flux to the ocean is removed in as-yet undiscovered sinks (not opal), or (2) only 10% of the mantle to ocean3He and heat fluxes is associated with MOR hydrothermal convection through the 350°C isotherm (90% is off-ridge), or (3) the oceanic Ge/Si,3He/ (and87Sr86Sr) balances today are far from steady-state.

Boron and halide systematics in submarine hydrothermal systems: Effects of phase separation and sedimentary contributions

Abstract Systematic studies of the distributions of B, δ 11 B, NH 4 , halides (Cl, Br, I) and trace alkalis (Li, Rb, Cs) in vent fluids, combined with experimental data on super- and subcritical phase separation, provide a method for separating the effects of interaction with basalts and/or sediments from those of phase separation. This allows a more general understanding of geochemical processes in submarine hydrothermal systems, especially where a connection with sediment is not otherwise obvious (e.g., Endeavour Segment, Juan de Fuca Ridge). Based on B and δ 11 B corrected for wallrock reactions, all published boron and chloride data from mid-ocean ridge systems (MOR) (e.g., 11°N, 13°N and 21°N of the East Pacific Rise), except for the Endeavour Segment, Juan de Fuca Ridge, are consistent with experimental phase separation data, suggesting a dominant control by the latter process. Fluids from sedimented ridge (SR) (e.g., Escanaba Trough and Guaymas Basin), and from back-arc basins (BAB) (e.g., Mariana Trough, Lau Basin and Okinawa Trough), when compared with mid-ocean ridge data, show expected effects of organic matter and/or sediment contributions. This is particularly noticeable from enhanced levels of Br, I, NH 4 , and trace alkali metal contents (such as Li, Rb and Cs). High B concentrations and elevated δ 11 B in Endeavour Segment can be explained by a small, but distinguishable contribution from sediments, which is confirmed by slightly enhanced levels of Br, I and NH 4 .

A seismic swarm and regional hydrothermal and hydrologic perturbations: The northern Endeavour segment, February 2005

The February 2005 swarm at the overlapping spreading center (OSC) on the northern end of the Endeavour segment is the first swarm on the Juan de Fuca Ridge recorded on a local seafloor seismic network. The swarm included several larger earthquakes and caused triggered seismicity and a hydrothermal response in the Endeavour vent fields as well as regional-scale hydrologic pressure perturbations. The spatial and temporal pattern of over 6000 earthquakes recorded during this seismic sequence is complex. Small-magnitude events dominate, and seismicity rates wax and wane, indicating a magmatic process. The main swarm initiates at the northern end of the Endeavour ridge. However, most of the moment release, including six strike-slip events, occurs in the southwest Endeavour Valley, where the swarm epicenters generally migrate south. The main swarm is contemporaneous with a hydrologic pressure response at four sealed seafloor boreholes, ∼25–105 km away. We infer that the seismic sequence is driven by a largely aseismic magma intrusion at the northern Endeavour axis. Resulting stress changes trigger slip on tectonic faults and possibly dike propagation at the opposing limb of the Endeavour OSC in the southwest Endeavour Valley, consistent with the eventual decapitation of the Endeavour by the West Valley segment. Furthermore, 2.5 days after the start of the main swarm, seismicity is triggered beneath the Endeavour vent fields, and temperature increases at a diffuse vent in the Mothra field. We infer that this delayed response is due to a hydrologic pressure pulse that diffuses away from the main magma intrusion.