High-Resolution Surveys Along the Hot Spot–Affected Galapagos Spreading Center: 3. Black Smoker Discoveries and the Implications for Geological Controls on Hydrothermal Activity

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To explore effects of hot spots on mid-ocean ridge hydrothermal systems, we conducted nested sonar, hydrothermal plume, and near-bottom photographic surveys along the portion of the Gala´pagos Spreading Center (GSC) influenced by the Gala´pagos hot spot, from longitude 95o-89.5oW. We report the first active high-temperature black smokers to be found on the GSC, at longitudes 94o4.5'W and 91o56.2'-54.3'W; describe two areas of recently inactive smokers, at longitudes 90o33.4'W and 91o13.8'W; and document an older inactive site, at longitude 90o33.4'W. All imaged vents issue either from dike-induced fissures along linear axial volcanic ridges and collapses or from a caldera. Magmatic control of hydrothermal systems also is revealed by spatial clustering of plumes within the topographically elevated middles of volcanic ridge segments with inferred centralized melt supply. In searched areas, smokers are more typical than diffuse flow vents, but total GSC plume incidence is half of that expected from the spreading rate. Why? Dike-fed fissures provide permeable pathways for efficient hydrothermal extraction of magmatic heat, but cones without calderas do not. Among many point-source cones surveyed, only the two with calderas had detectable plumes. Possibly, dominance of point-source over linear-source melt delivery on the GSC decreases plume incidence. Also, similar maturities of observed vents and their host lava flows indicate that hydrothermally active volcanic segments along the western GSC are contemporaneously in a waning phase of volcanic-hydrothermal activity. Perhaps ridge/hot spot interaction produces melt pulses that drive near-synchronous volcanic-hydrothermal activity on the volcanic segments spanning the hot spot. During active periods, hydrothermally active dike-fed fissures and calderas may be more abundant than we currently observe.

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