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Appendix 1 Landmarks in Studies of Hydrothermal

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Appendix 1 Landmarks in Studies of Hydrothermal APPENDIX 1 LANDMARKS IN STUDIES OF HYDROTHERMAL PROCESSES AT SEAFLOOR SPREADING CENTERS The process of seafloor spreading was hypothesized (Holmes, 1931; Hess, 1962; Dietz, 1961) and verified by application of the magnetic polarity-reversal time scale (Cox et al., 1963) to interpretation of the sequence of seafloor magnetic anomalies (Morley, unpublished manuscript; Vine and Matthews, 1963; Vine and Wilson, 1965; Pitman and Heirtzler, 1966) and dating of samples recovered in transects across the seafloor by the Deep Sea Drilling Project (Maxwell ~~., 1970). Seafloor spreading centers were incorporated into the theory of plate tectonics as divergent plate boundaries where lithosphere (oceanic crust and upper mantle) is generated by the process of seafloor spreading (McKenzie and Parker, 1967; Morgan, 1968; Le Pichon, 1968; Isacks ~ ale, 1968). The existence of subseafloor hydrothermal convection systems in ocean basins was inferred from the presence of the components of such systems at oceanic ridges comprising magmatic heat sources to drive convection, seawater as a fluid medium, and fractured volcanic rocks as a permeable solid medium (Elder, 1965; Deffayes, 1970). The hypersaline hydrothermal solutions in the vicinity of the Atlantis II Deep of the Red Sea appeared as an anomaly on the hydrographic data of the Swedish Deep Sea Expedition at a water sampling station of the research vessel ALBATROSS while transiting the Red Sea in 1948. (Bruneau ~ al., 1953). The anomaly was not recognized at that time. The Red Sea represents an early stage of opening of an ocean basin about a seafloor spreading center. The combination of stratified high temperature and high salinity solutions ponded in certain basins along the axis of the Red Sea was recognized in hydrographic data of water sampling 771 772 LANDMARKS IN STUDIES OF HYDROTHERMAL PROCESSES stations made by ships transiting the Red Sea between 1963 and 1965 during the International Indian Ocean Expedition (Charnock, 1964; Miller, 1964; Swallow and Crease, 1965). Metalliferous sediments were found in certain basins along the axis of the Red Sea and their origin related to the metal-rich hypersaline hydrothermal solutions present (Miller et al., 1966; Hunt ~.!!.., 1967; Bischoff, 1969). -- Anomalously high contents of Fe, Mn, Cu, Cr, Ni, and Pb were identified in sediments of the East Pacific Rise and attributed to hydrothermal activity alternatively related to volcanism (Skornyakova, 1964; Arrhenius and Bonatti, 1965), or to exhala­ tions from mantle-level magmatic processes (Bostrom and Peterson, 1966). The first sample of solid hydrothermal material reported from an oceanic ridge was recovered in the form of a fractionated ferromanganese encrustation from a seamount located off the axis of the East Pacific Rise (Bonatti and Joensuu, 1966). From the vantage point of Iceland, Palmason (1967) suggested that hydrothermal circulation may play a significant role in the distribution of oceanic heat flow, and Talwani et al. (1971) attributed low heat flow measured at the axis o~the Reykjanes Ridge to dissipation of heat by convective circulation of seawater. An excess of the primordial inert isotope 3He was detected at mid-depth in the Pacific Ocean and attributed to mantle degassing at sites along the crest of the East Pacific Rise (Clarke et al., 1969), providing the basis for use of 3He as an index of hydr~ thermal discharge (Craig ~.!!.., 1975). On the basis of the mineralogy of altered basalts, Hart (1970) suggested that much of the basalt-seawater interaction occurs by low-temperature reaction near the rock-seawater interface, but some basalt-seawater reaction takes place at depths of up to 5 km within the oceanic crust. Recognition of the widespread occurrence of hydrated metamor­ phosed oceanic crust and upper mantle led to realization that a large volume of water was required to react with the rocks (Melson et al., 1968; Miyashiro et al., 1971; Christensen, 1972); analysis Ci:f oxygen isotopes indicate~that a low 0 18 0 isotopic source such as seawater had interacted with the hydrated rocks (Muehlenbachs and Clayton, 1972; Spooner ~~., 1974). Various lines of evidence from crustal sections interpreted as oceanic crust exposed on land (ophiolites) and in ocean basins LANDMARKS IN STUDIES OF HYDROTHERMAL PROCESSES 773 were consolidated into a unified model for subseafloor metamor­ phism, heat and mass transfer at seafloor spreading centers (Spooner and Fyfe, 1973). The observation that theoretical values of conductive heat flow calculated from models of heat produced by the generation of lithosphere at oceanic ridges (McKenzie, 1967; Ie Pichon and Langseth, 1969; Sclater and Francheteau, 1970) exceeded the average measured values of conductive heat flow (Langseth and Von Herzen, 1970) led to the interpretation that the major portion of heat dissipation at oceanic ridges must be due to convective heat transfer by deeply circulating hydrothermal fluids (Lister, 1972). The observed discrepency between theoretical and average measured conductive heat flow was used to estimate the global large magnitude of heat transfer by hydrothermal convection at seafloor spreading centers (Williams and Von Herzen, 1974; Wolery and Sleep, 1976). A basal layer of metalliferous sediments was identified overlying basaltic basement in the Pacific Ocean Basin similar to the sediment forming at the crest of the East Pacific Rise, which demonstrated that metalliferous sediments have formed more or less continuously related to hydrothermal activity during seafloor spreading (von der Borch and Rex, 1970). "Volcano-exhalative" and hydrogenous metal-bearing components of East Pacific Rise sediments were distinguished on the basis of radiogenically determined metal accumulation rates and distinctive lead isotopic composition (Bostrom, 1970; Bender ~~., 1971; Dasch ~~., 1971). The observation that transition metals were depleted in the slowly cooled interior relative to the rapidly quenched exterior of extrusive basalts from the Mid-Atlantic Ridge was the basis for the inference that metals were mobilized from the interior by dissolution as chloride complexes in seawater that produced ore­ forming hydrothermal solutions adequate to form metallic minerals observed at seafloor spreading centers (Corliss, 1971). Discovery of the first active submarine hydrothermal field on an oceanic ridge in an open ocean basin by the NOAA Trans-Atlantic Geotraverse (TAG) project in 1972, the TAG Hydrothermal Field on the Mid-Atlantic Ridge, confirmed that hydrothermal processes could act more or less continuously from early to advanced stages in the opening of an ocean basin about a seafloor spreading center represented, respectively, by the Red Sea and the Atlantic Ocean (Rona, 1973; Rona and R. B. Scott, 1974; M. R. Scott et al., 1974; R. B. Scott ~~., 1974; Rona, 1980). -- 774 LANDMARKS IN STUDIES OF HYDROTHERMAL PROCESSES The first direct measurements of the magnitude and gradient of near-bottom water temperature anomalies with the characteris­ tics of hydrothermal discharge at an oceanic ridge were made in 1973 at the TAG Hydrothermal Field on the Mid-Atlantic Ridge (Rona et al., 1974, 1975) and at the Galapagos Spreading Center (Williams ~.!!.., 1974). Stockwork-type copper-iron sulfides were recovered from oceanic crust exposed in large-offset ridge-ridge transform fault segments of major fracture zones in the Atlantic Ocean (Bonatti et al., 1976) and Indian Ocean (Dmitriev et al., 1970; Rosanova and--­ Baturin, 1976), and penetrated in a drillhole through the upper portion of oceanic crustal Layer 2 (Deep Sea Drilling Project Hole 504B south of the Costa Rica Rift; Anderson ~ a1., 1982). Laboratory experiments reacting basalt with seawater in closed containers simulating pressures and temperatures of subsea­ floor hydrothermal systems produced elemental exchanges analogous to those determined for oceanic crust from studies of basalt alteration and indicated chemical conditions for generating ore­ forming hydrothermal solutions (Bischoff and Dickson, 1975; Hajash, 1975). Simultaneous measurements of increased temperature and excess 3He in a near-bottom water sample recovered at the Galapagos Spreading Center in 1976 confirmed the existence of hydrothermal discharge from an oceanic ridge (Weiss ~.!!.., 1977). The first direct observation of active submarine hydrothermal vents at an oceanic ridge and the discovery of a biological community associated with the vents was made diving with DSRV ALVIN at the Galapagos Spreading Center in 1977 where four groups of vents discharging hydrothermal solutLons at temperatures up to 17°C were investigated (Corliss et al., 1979). Clams in the vent biota were photographed by the Scripps Deep Tow instrumentation system (Lonsdale, 1977). Global fluxes of certain elements extrapolated from their concentrations in dilute hydrothermal solutions sampled at the Galapagos Spreading Center were determined to equal or exceed input of those elements to the ocean by rivers transporting material derived from weathering of continents (Corliss et al., 1979; Edmond ~~., 1979). ------ The global magnitude of convective heat flux at seafloor spreading centers calculated at between 4.0 and 6.4 x 1019 cal y-1 from the discrepency between theoretical heat flow and average measured conductive heat flow (Williams and Von Herzen, 1974; Wolery and Sleep, 1976) was experimentally verified on the basis of extrapolation of the measured ratio between 3He and water LANDMARKS IN STUDIES OF HYDROTHERMAL PROCESSES
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