Black and White Smokers
KeywordsHydrothermal Fluid Spreading Center Black Smoker Volcanogenic Massive Sulfide Lost City
Black and white smokers. Chimney-like edifices composed of mixtures of copper-, iron-, and zinc-sulfide minerals and calcium- and barium-sulfate minerals. They form as very hot (up to ~400 °C or 750 °F) fluids exit very young seafloor and mix with cold seawater, with the high-temperature fluids passing through channels within the edifices into the deep ocean.
Venting of hydrothermal fluids from the youngest portions of the seafloor along the mid-ocean ridges was first observed in 1977 along the Galapagos Rift where unusual biological communities were found associated with warm vent fluids (17 °C, much warmer than the surrounding 2 °C seawater); the presence of these warm fluids had been predicted based on measurements of heat flow (see “Oceanic Heat Flow: Thermal Models of the Lithosphere”) and bottom seawater thermal anomalies close to the mid-ocean ridge (Corliss et al., 1979). In 1978, massive sulfide deposits that likely formed from much higher temperature fluids were found near 21°N latitude on the East Pacific Rise about 650 m west of the spreading axis (Francheteau et al., 1979). The first actively venting black and white smokers were subsequently discovered in 1979, along the spreading axis of the East Pacific Rise (Spiess et al., 1980) not far from where the deposits were found a year earlier.
The hot fluids were observed exiting “stacks” or “chimneys” that were 1–5 m tall, composed of copper-, iron-, and zinc-sulfide minerals and the mineral anhydrite (calcium sulfate) (Spiess et al., 1980). The observed black chimneys, or black smokers, resembled organ pipes ≤30 cm in diameter and emitted hot (>350 °C) fluid with dark-colored (black) precipitates suspended within the exiting fluid (Spiess et al., 1980). The white chimneys, or white smokers, were covered with worm tubes (making them light-colored) and emitted cooler (<330 °C) fluids at slower flow rates with light-colored precipitates suspended within exiting waters (Spiess et al., 1980; Haymon and Kastner, 1981).
Formation of Black Smoker Chimneys
Rapid formation of the Stage 1 chimney wall (it can form at rates up to 30 cm per day; Goldfarb et al., 1983) is in part a consequence of anhydrite being an unusual mineral that is more soluble at low temperatures than at high temperatures; if seawater is heated to ~150 °C or greater, anhydrite precipitates (Bischoff and Seyfried, 1978). When the very hot vent fluid exits at meter-per-second velocities into seawater, mixtures above ~150 °C will be saturated in anhydrite, with the sulfate coming from seawater and the calcium from both the vent fluid and seawater (Styrt et al., 1981; Albarède et al., 1981). Metal sulfides and oxides (zinc sulfide, iron sulfide, copper-iron sulfide, manganese oxide, and iron oxide) also precipitate from the vent fluid and vent fluid/seawater mixtures as fine-grained particles. Some of these particles become trapped within and between grains of anhydrite within the Stage 1 chimney walls, giving the anhydrite, which is clear to white in its pure form, a gray to black color (Goldfarb et al., 1983; Haymon, 1983). The remaining particles form a plume of “smoke” above the chimney. Because bottom seawater is denser than the mix of seawater and hydrothermal fluid in the plume, the plume rises a few 100 m above the seafloor to a depth where it is of the same buoyancy as the surrounding ocean water (see “Hydrothermal Plumes”).
Formation of White Smoker Chimneys
Very different types of chimneys are found at the Lost City vent field, located 15 km from the axis of the Mid-Atlantic Ridge. The Lost City hydrothermal system is hosted in mantle rocks (peridotite and serpentinite – see “Peridotite”), and the venting fluids have a higher pH than seawater, low metal and sulfide concentrations, and low temperature (<100 °C) relative to black smoker and white smoker fluids (Kelley et al., 2005); the very tall spires being deposited from these fluids are composed of calcium carbonate and magnesium hydroxide minerals (calcite and/or aragonite (CaCO3) and brucite (MgOH2)) that are saturated in the high pH, carbonate- and hydroxide-rich fluids.
Summary and Conclusions
The morphologies of actively venting seafloor hydrothermal deposits and the composition and appearance of their hydrothermal plumes reflect the compositions of the fluids (hydrothermal fluid and seawater) from which they form, and the styles of flow and mixing of these fluids. “Black smoker” chimneys with conduit diameters of <2–10 cm form from very high-temperature (>330–400 °C) fluids that flow at rates of meters per second, and the turbulent mixing that results when this fluid exits into seawater results in the formation of particle-laden plumes reminiscent of black smoke. “White smoker” chimneys form from lower-temperature (~200–330 °C) vent fluids that flow less vigorously through anastomosing narrow conduits that can become blocked resulting in diversion of flow; a greater percentage of metals are trapped and deposited within these chimneys than within black smokers, and they tend to be richer in zinc because of the lower temperatures that result in saturation and precipitation of zinc-sulfide minerals.
Over time, collapse and incorporation of these different types of chimneys and flanges onto and into mounds, and subsequent reworking of this material as hot fluids flow through the mounds, forms larger deposits. These larger deposits (see “Volcanogenic Massive Sulfides” and “Marine Mineral Resources”) found along the mid-ocean ridges and along the spreading centers within back-arc basins are analogs for some types of ore deposits found on land (e.g., Cyprus-type massive sulfide ore deposits; Francheteau et al., 1979).
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