|Hydrothermal activity on the southern, ultrafast-spreading segment of the East Pacific Rise|
Marchig, V. (2000). Hydrothermal activity on the southern, ultrafast-spreading segment of the East Pacific Rise, in: Cronan, D.S. (Ed.) Handbook of marine mineral deposits. pp. 309-325
In: Cronan, D.S. (Ed.) (2000). Handbook of marine mineral deposits. CRC Marine Science Series, 17. CRC Press: Boca Raton. ISBN 0-8493-8429-X. 406 pp., more
In: Kennish, M.J.; Lutz, P.L. (Ed.) CRC Marine Science Series., more
The southem part of theEast Pacific Rise is an ultra-fast spreading zone characterized by vigorous volcanism. Hydrothermal activity is consequently also high, producing large anomalies in sea water, high density of hydrothermal fauna, and elevated contents of hydrothermal precipitates in the surrounding sediment. Spreading proceeds in cycles, whereby every cycle begins with a slow volcanic phase (less differentiated pillow lava forming a steep, narrow ridge) and then develops a vigorous volcanic phase (sheet lava of higher differentiation forming a large, rounded ridge). After volcanic activity slows down or ceases, the tectonic activity forms first a small, and later a large and deep, axial summit graben. In the course of further spreading a new cycle begins consequent on newly initiated volcanic activity. During the vigorous volcanic phase strong degasing takes place, the methane from this serving as food for hydrothermal fauna. This phase is therefore characterized by maximum hydrothermal fauna and maximum water turbidity caused by floating organisms, and registers as light attenuation anomalies. Hydrothermal circulation is initiated and developed during tectonic phases; black smokers and massive sulfide chimneys are formed in the course of discharge of ore-forming solutions from the seafloor. The frequency of massive sulfides depends on seafloor permeability, therefore the best indication for finding massive sulfides is a high coverage of talus in the axial summit graben. Massive sulfide chimneys are distinct in the manner of their growth from those described from the northem part of the East Pacific Rise. The first friable pipe is built of idiomorphic pyrite, which subsequently gets cemented by layered colloform marcasite, pyrite, and sphalerite. Anhydrite is seldom observed; there is no indication of a first pipe formed from anhydrite, as described in the north. The second stage of chimney growth is clogging of the central conduit with sulfides rich in idiomorphic chalcopyrite. The third stage of ore precipitation occurs on the outside of the chimney wall, from the solution which cannot use the clogged central conduit and therefore wells out from the seafloor around the chimney. This is quenched, producing colloform sphalerite and marcasite. Characteristic of this layer are numerous embedded calcitic worm tubes. Although hydrothermal activity is high in the southern part of the East Pacific Rise, massive sulfide ores do not occur in large amounts or as big edifices, first, because most of the precipitates from the hydrothermal solutions get dispersed to the surrounding sediment, and second, because the edifices formed become covered with fresh lava flows.