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
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.
Further Reading
Scott, Steven. “Minerals on Land, Minerals in the Sea.” Geotimes
47, no. 12 (2002): 19–23.
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