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[1] Samples were collected from hydrothermal plumes along the East Pacific Rise (EPR) from 28° to 32°S during the Ridge Axis Plume and Neotectonic Unified Investigation (RAPANUI) cruise (5 March to 12 April 1998). Forty-five vertical casts and tow-yos were conducted: 3 off axis and 42 over the axes of two overlapping propagating ridges, the West and East ridges. These ridges are composed of several nontransform offset ridge segments. Spreading rates range from 149 mm/yr in the segment interiors to 0 mm/yr at the propagating rifts. These maximum spreading rates are considered the fastest on the mid-ocean ridge system and affect the structure of the ridge. Segment-averaged methane plume maxima ranged from 1.7 to 7.2 nM. Mean methane concentrations on the West Ridge were nearly double those of the East Ridge. Westwardly advecting hydrothermal methane persisted to our most distal station, nearly 480 km west of the East Pacific Rise. Background concentrations were less than 1 nM. The highest methane concentration measured was 50 nM in a buoyant plume. Methane did not covary with manganese or any other hydrothermal tracer in plumes over portions of a segment that exhibited recent magmatic activity, possibly as a result of the hydrothermal system's recovery from a phase separation event. In contrast, methane/manganese ratios on the other segments ranged from 0.077 to 0.091. Methane δ13C values in plume maxima ranged from −27 to −33‰ versus Peedee belemnite; background values were around −40‰. These data are compared with hydrothermal plume methane data from slower spreading ridges and illustrate similarities in hydrothermal processes and sources between these systems.

A two-stage scavenging model is used to describe the transport of hydrothermal Mn to the sediments adjacent to ridge crests. Dissolved Mn is hypothesized to be scavenged by slowly settling metal-depositing capsuled bacteria which, in turn, are incorporated into rapidly settling macroaggregates. Upon reaching the seafloor, the Mn is subject to resuspension in particulate form and to remobilization within the sediment column and release back into the water column as dissolved Mn. Measured Mn distributions in the vicinity of the southern Juan de Fuca Ridge and estimated values of process rate constants are used to limit the range of possible model outcomes. The results present a picture of water column distributions and fluxes of dissolved, fine particulate, and large-particle associated Mn in a plume advecting off axis. The model and best available parameter values suggest that more than 80% of the hydrothermal Mn is deposited within several hundred kilometers of the ridge crest, though dissolved Mn concentrations beyond that distance exceed background levels by many times. The residence time of hydrothermal Mn in the water column is of the order of several years. An off-axis component of advection of the order of 0.1–0.3 cm/s is needed to make similar the model and measured distributions of Mn depositing in the sediments.

Hydrothermal fluids collected from the ASHES vent field in 1986, 1987, and 1988 exhibit a very wide range of chemical composition over a small area (∼60 m in diameter). Compositions range from a 300°C, gas-enriched (285 mmol/kg CO2), low-chlorinity (∼33% of seawater) fluid to a 328°C, relatively gas-depleted (50 mmol/kg CO2), high-chlorinity (∼116% of seawater) fluid. The entire range of measured compositions at ASHES is best explained by a single hydrothermal fluid undergoing phase separation while rising through the ocean crust, followed by partial segregation of the vapor and brine phases. Other mechanisms proposed to produce chlorinity variations in hydrothermal fluids (precipitation/dissolution of a chloride-bearing mineral or crustal hydration) cannot produce the covariation of chlorinity and gas content observed at ASHES. There is good agreement of the measured fluid compositions with compositions generated by a simple model of phase separation, in which gases are partitioned according to Henry's law and all salt remains in the liquid phase. Significant enrichments in silica, lithium and boron in the low-chlorinity fluids over levels predicted by the model are attributed to fluid-rock interaction in the upflow zone. Depletions in iron and calcium suggest that these elements have been removed by iron-sulfide and anhydrite precipitation at some time in the history of the low-chlorinity fluids. The distribution of low- and high-chlorinity venting is consistent with mechanisms of phase segregation based on differential buoyancy or relative permeability. The relatively shallow depth of the seafloor (1540 m) and the observed chemistry of ASHES fluids are consistent with phase separation in the sub-critical or near-critical region.

We have mapped the distribution and intensity of submarine hydrothermal emissions from the summit caldera of Axial Volcano by means of hydrographic and chemical sampling of the water column during annual cruises in four consecutive years (1985–1988). These investigations were undertaken to ascertain the strength of hydrothermal venting relative to other vent fields on the Juan de Fuca Ridge and to gauge the importance of Axial Volcano as a source of hydrothermal emissions to the northeast Pacific Ocean. Measurable temperature anomalies are mostly restricted to a 200-m-thick water column within the caldera, where they range from a background level of ∼0.01°C to local maxima of 0.02–0.1°C above known high-temperature vent fields. The temperature anomaly plume is significantly smaller in both extent and intensity than plumes emitted by vent fields on other segments of the Juan de Fuca Ridge. A maximum estimate of the total heat flux from the summit, based on the total excess heat and advective velocity of the plume, is 8×108 W. Hydrothermal venting seems surprisingly small for an edifice that testifies to a long-term oversupply of magma at a single axial location. We speculate that additional heat may be lost from Axial Volcano by diffuse percolation of sea water over broad areas of its flanks and by magma eruption and venting along flank rift zones.

In 1986 and 1987, buoyant and neutrally buoyant hydrothermal plume particles from the ASHES vent field within Axial Volcano were sampled to study their variations in composition with height above the seafloor. Individual mineral phases were identified using standard X ray diffraction procedures. Elemental composition and particle morphologies were determined by X ray fluorescence spectrometry and scanning electron microscopy/X ray energy spectrometry techniques. The vent particles were primarily composed of sphalerite, anhydrite, pyrite, pyrrhotite, chalcopyrite, barite, hydrous iron oxides, and amorphous silica. Grain size analyses of buoyant plume particles showed rapid particle growth in the first few centimeters above the vent orifice, followed by differential sedimentation of the larger sulfide and sulfate minerals out of the buoyant plume. The neutrally buoyant plume consisted of a lower plume, which was highly enriched in Fe, S, Zn, and Cu, and an upper plume, which was highly enriched in Fe and Mn. The upper plume was enriched in Fe and Mn oxyhydroxide particles, and the lower plume was enriched in suspended sulfide particles in addition to the Fe and Mn oxyhydroxide particles. The chemical data for the water column particles indicate that chemical scavenging and differential sedimentation processes are major factors controlling the composition of the dispersing hydrothermal particles. Short-term sediment trap experiments indicate that the fallout from the ASHES vent field is not as large as some of the other vent fields on the Juan de Fuca Ridge.

Evidence of large-scale episodic venting of hydrothermal fluids was initially discovered in August 1986 in the form of a 130-km3 radially symmetric “megaplume” over the southern Juan de Fuca Ridge. We report here on the discovery in September 1987 of a second, smaller megaplume about 45 km north of the location of the first megaplume. The 3He/heat, 3He/dissolved Mn, and 3He/dissolved silica ratios in both megaplumes were typical of high-temperature vent fluids. Evidence from long-term records of current flow over the southern Juan de Fuca Ridge, and from the mineralogy and Mn chemistry of megaplume particles, makes it unlikely that the second megaplume was a reencounter of the first. A plume model that relates the heat flux to the observed plume rise height of ∼1000 m finds that the total heat content of the fluids that formed the megaplumes was 1016–1017 J, or equivalently a fluid volume of 3–8 × 107 m3 at 350°C. The geometry and suspended particle population of the first megaplume imply that such features are formed within a few days time. The extraordinary heat and volume fluxes associated with megaplumes (102–103 greater than ordinary vent fields), as well as their typical hydrothermal chemistry, suggest that they resulted from tectonic or hydraulic fracturing that suddenly increased the permeability of the hydrothermal fluid reservoir in the axial crust. The flux of hydrothermal heat from continuous venting and episodic megaplumes on the southern Juan de Fuca Ridge is presently 4 − 10 × 109 W, a factor of 5–10 greater than various geophysical model calculations for this ridge segment. This imbalance may be symptomatic of a recent surge in the local cycle of magmatic activity.