black smokers and white smokers

black smokers and white smokers At the mid-oceanic ridges (MOR) sea water is heated and circulated at depth, forming localized convective systems similar to geothermal systems on land (see geothermal energy). The bedrocks are basaltic and are very similar in composition to sea-water-based geothermal systems in south-west Iceland. The main differences are that the MOR systems are located at depths of about 3000 m and are hence under a hydrostatic pressure of about 300 bar, whereas geothermal systems in Iceland are at 1 atm (or about 1 bar pressure) at the Earth's surface. This difference causes MOR geothermal systems to be rich in hydrogen sulphide gas (H₂S); the pH of the circulating water is thus low (about 3).

The first hydrothermal vents to be discovered on mid-ocean ridges were found near the Galopagos Islands in the Pacific in 1979. It was not until 1985 that hydrothermal vents were found on the Mid-Atlantic Ridge. The vents were first discovered with the submarine Alvin, owned by the US government. As seen from Alvin, the vents were of two different types, one black, the other white. The vents have since been referred to as ‘black smokers’ and ‘white smokers’ (Fig. 1).

Black smokers have fluid temperatures of more than 360 °C, whereas white smoker fluids are generally lower in temperature (260–300 °C). Once the vent water enters the sea, the hot fluid mixes with sea water and the dissolved constituents rise because of the buoyancy resulting from the higher temperature. These anomalies are generally referred to as plumes. They can nowadays easily be traced by temperature and chemical anomalies. The latter consist of trace concentrations of metals such as manganese (Mn) or a rare isotope of helium (³He). The plumes are typically about 40 km wide and 1 km high (Fig. 2). Tens of such systems have now been discovered around the globe.

Active hydrothermal vents typically have diameters of 50–200 m, and some are over 20 m high. These structures are generally referred to as mounds. In general, mounds have both black and white smokers. The chemistry of the sea water has been altered by reaction with basalts at depth. As mentioned above, the pH of the water is low (3–4) and it also has elevated concentrations of hydrogen sulphide gas (H₂S) as well as the metals manganese (Mn), iron (Fe), copper (Cu), zinc (Zn), and to a lesser extent cobalt (Co), lead (Pb), and cadmium (Cd).

The so-called TAG hydrothermal mound on the Mid-Atlantic Ridge (26° 08′ N) has been studied by researchers from the Woods Hole Oceanographic Institute in the USA. The field was drilled in order to decipher the structure of mounds. The TAG mound is typical of mounds found on the MOR in having both black and white smokers. The top of the mound is composed of massive pyrite breccia. At depth, first anhydrite and then amorphous silica are also found. At still greater depth, the basalt wall-rock is rich in silica; the basalts then become rich in the alteration mineral chlorite. The mound itself is a 20-m cone with several black smoker chimney clusters (Fig. 3). The minerals that form these chimneys are primarily pyrite (FeS₂), with some chalcopyrite (CuFeS₂) and anhydrite (CaSO₄). The fluids have a relatively low concentration of zinc (45 mmol l⁻¹). White smokers are located at the edge of the mound. The minerals forming the vents are primarily pyrite and sphalerite (ZnS). The fluids have elevated concentrations of zinc (300–400 mmol l⁻¹) in comparison with those of the black smokers. The fluid is thought to evolve, in that primary black smoker fluid flows through the mound, precipitating metal sulphides (e.g. FeS₂). This reaction causes the pH to decrease and dissolve the sphalerite. The white smokers thus represent secondary fluids that are rich in zinc. This process can be represented by the following chemical reactions:Fe²⁺ + 2HS⁻ = FeS₂ + H⁺
ZnS + H⁺ = Zn²⁺ + HS⁻.


Anhydrite (CaSO₄) is precipitated when the hot vent fluid (at a temperature of more than 150 °C) mixes with sulphate-rich sea water. This is because anhydrite is unusual in having retrograde solubility (its solubility decreases as the temperature rises).

In the hot sea-water plume above the vents metals are found in the form of dissolved constituents and small particles. Manganese remains in the plume for up to two weeks (at concentrations up to 14 nmol l⁻¹), whereas other metals such as iron, cobalt, copper, and zinc are precipitated from the plume at plume height (Fig. 4). 70 per cent of the metals are precipitated as sulphides and the remainder as oxides (e.g. haematite; Fe₂O₃) and oxyhydroxides (e.g. goethite; FeOOH) and hydroxides (limonite; Fe(OH)₃).

The TAG mound contains 1–2 weight per cent (wt per cent) of copper. The black smoker deposits contain 700 parts per million (ppm) of zinc and 250 ppb (parts per billion) of gold, whereas the white smoker deposits contain 1–4 wt per cent of zinc and 3 ppm of gold. The TAG mound has been compared to ancient massive sulphide deposits such as the deposits in Cyprus, Oman, and Newfoundland. The total amount of copper in the TAG mound is about 3.9 × 10⁶ tonnes: more or less identical to that of the Cyprus deposit (3 × 10⁶ t). The major difference between the Cyprus and the TAG metal deposits is that no anhydrite is found in Cyprus whereas the TAG mound has about 10⁵ m³ of that mineral. This is due to the retrograde solubility of the anhydrite, which has been dissolved out by cold waters that have percolated through the Cyprus formation over a long period of time. This has caused the Cyprus mound, to collapse and form a breccia. The only other difference is that the TAG mound contains amorphous silica (SiO₂), whereas the Cyprus mound has jasper—a microcrystalline form of silica, SiO₂. Jasper is a typical ageing product of amorphous silica.

The fallout from plumes forms sediments that are very rich in iron hydroxides. They are found up to tens of kilometres away from the vents (Fig. 4). The vent fluid chemistry affects the global elemental cycle. The plumes are also hosts to lush populations of gigantic clams and worms. (see biosphere)

A. Michard and and K. Vala Ragnarsdottir