DEFINITION OF BLACK SMOKER CHIMNEYS

Hydrothermal vent systems that

form near active magmatic systems along the mid-ocean ridge

system, approximately 2 miles (3 km) below sea level. They

were first discovered by deep submersibles exploring the

oceanic ridge system near the Galapagos Islands in 1979, and

many other examples have been documented since then,

including a number along the Mid-Atlantic Ridge.

Black smokers are hydrothermal vent systems that form

when seawater percolates into fractures in the seafloor rocks

near the active spreading ridge, where the water gets heated to

several hundred degrees Celsius. This hot pressurized water

leaches minerals from the oceanic crust and extracts other elements

from the nearby magma. The superheated water and

brines then rise above the magma chamber in a hydrothermal

circulation system and escape at vents on the seafloor, forming

the black smoker hydrothermal vents. The vent fluids are typically

rich in hydrogen sulfides (H2S), methane, and dissolved

reduced metals such as iron. The brines may escape at temperatures

greater than 680°F (360°C), and when these hot brines

come into contact with cold seawater, many of the metals and

minerals in solution rise in plumes, since the hot fluids are

more buoyant than the colder seawater. The plumes are typically

about 0.6 miles (1 km) high and 25 miles (40 km) wide,

and they can be detected by temperature and chemical anomalies,

including the presence of primitive 3He isotopes derived

from the mantle. These plumes may be rich in dissolved iron,

manganese, copper, lead, zinc, cobalt, and cadmium, which

rain out of the plumes, concentrating these elements on the

seafloor. Manganese remains suspended in the plumes for several

weeks, whereas most of the other metals are precipitated

as sulfides (e.g., pyrite, FeS2; chalcopyrite, CuFeS2; sphalerite,

ZnS), oxides (e.g., hematite, Fe2O3), orthohydroxides (e.g.,

goethite, FeOOH), or hydroxides (e.g., limonite, Fe(OH)3). A

group of related hydrothermal vents that form slightly further

from central black smoker vents are known as white smokers,

which typically have vent temperatures from 500°F–572°F

(260°C–300°C).

On the seafloor along active spreading ridges, the

hydrothermal vent systems form mounds that are typically

164–656 feet (50–200 m) in diameter and some are more

than 66 feet (20 m) high. Clusters of black smoker chimneys

several meters high may occupy the central area of mounds

and deposit iron-copper sulfides. White smoker chimneys

typically form in a zone around the central mound, depositing

iron-zinc sulfides and iron oxides. Some mounds on the

seafloor have been drilled to determine their internal structure.

The TAG hydrothermal mound on the Mid-Atlantic

Ridge is capped by central chimneys made of pyrite, chalcopyrite,

and anhydrite, overlying massive pyrite breccia,

with anhydrite-pyrite and silica-pyrite rich zones found a few

to tens of meters below the surface. Below this, the host

basalts are highly silicified, then at greater depths form a network

of chloritized breccia. White smoker chimneys rim the

central mound, and these are made of pyrite (FeS2) and sphalerite

(ZnS). In addition to the sulfides, oxides, hydroxides,

and orthohydroxides, including several percent copper and

zinc, the TAG mound contains minor amounts of gold.

Seafloor hydrothermal mounds and particularly the

black smoker chimneys host a spectacular community of

unique life-forms, found only in these environments. Lifeforms

include primitive sulfate-reducing thermophyllic bacteria,

giant worms, giant clams, crabs, and fish, all living off

the chemosynthetic metabolism made possible by the

hydrothermal vent systems. Life at the black smokers draws

energy from the internal energy of the Earth (not the Sun),

via oxidation in a reducing environment. Some of the bacteria

living at these vents are the most primitive organisms

known on Earth, suggesting that early life may have resembled

these chemosynthetic thermophyllic organisms.

Black smoker chimneys and the entire hydrothermal

mounds bear striking similarities to volcanogenic massive sulfide

(VMS) deposits found in Paleozoic and older ophiolite

and arc complexes including the Bay of Islands ophiolite in

Newfoundland, the Troodos ophiolite in Cyprus, and the

Semail ophiolite in Oman. Even older VMS deposits are common

in Archean greenstone belts, and these are typically

basalt or rhyolite-hosted chalcopyrite, pyrite, sphalerite, copper-

zinc-gold deposits that many workers have suggested may

be ancient seafloor hydrothermal vents. Interestingly, complete

hydrothermal mounds with preserved black and white

smoker chimneys have recently been reported from the 2.5-

billion-year-old North China craton, in the same belt where

the world’s oldest well-preserved ophiolite is located.

The tectonic setting for the origin of life on the early

Earth is quite controversial. Some favor environments in shallow

pools, some favor deep ocean environments where the

organisms could get energy from the chemicals coming out of

seafloor hydrothermal vents. It is significant that black smoker

types of hydrothermal vents have been discovered in

Archean ophiolite sequences. The physical conditions at these

mid-ocean ridges at more than 2.5 billion years ago permit

the inorganic synthesis of amino acids and other prebiotic

organic molecules. Some scientists think that the locus of precipitation

and synthesis for life might have been in small ironsulfide

globules, such as those that form around black

smokers. By examining black smoker chimneys we may be

effectively looking at a window into the past, and the origin

of life on Earth.

See also BENTHIC; BIOSPHERE; GREENSTONE BELT;

OPHIOLITES.

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