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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.

The fact that vigorous hydrothermal discharge occurs on ridge segments with a variety of spreading rates and morphological characteristics makes it difficult to discern the large‐scale geological controls on the distribution of hydrothermal venting along the mid‐ocean ridge. We undertook to simplify this problem by mapping the distribution of hydrothermally produced thermal and light attenuation anomalies along the crest of the Juan de Fuca Ridge, a medium‐rate spreading center with segments of markedly differing morphology. Hydrothermal anomalies were detected by towing a high‐precision conductivity‐temperature‐depth‐transmissometer (CTDT) package in a sawtooth pattern through the lowermost 1000 m of the water column above each segment. These surveys, conducted between 1985 and 1989, have produced the first quantitative and continuous visualizations of hydrothermal plumes along a multisegment portion of the mid‐ocean ridge. Hydrothermal discharge is strongest on those segments, or portions of segments, where the apparent magmatic budget is highest, as indicated by the degree of along‐axis inflation and other morphological characteristics. This correspondence implies that magma is episodically delivered in small and spatially discontinuous bodies of melt rather than by the constant resupply of a steady state magma chamber. We propose that the probability of discharge occurring at any point on the mid‐ocean ridge is proportional to the spreading rate. This probability, scaled by the observation that significant plumes presently emanate from ∼20% of the Juan de Fuca Ridge, is expected to range from ∼5% on the slowest‐spreading ridge to ∼50% on the fastest‐spreading ridge.

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