Download Enigma 9 resolved : Olivine … what will happen to you

Enigma 9 resolved : Olivine … what will happen to you ?
The weekly question focused on the difference between the black smokers and white smokers, all
these hydrothermal fields encountered during oceanographic explorations.
Let's begin by presenting what is a hydrothermal system!
Three important areas
- The recharge area:
Seawater (3 ° C) infiltrates the oceanic crust fractures. It can gi deep enough through the oceanic
crust. The expedition IODP 345 shows the gabbros are already altered by the seawater
- Zone reactive:
The sea water interacts with the surrounding, and becomes a hydrothermal fluid. The composition of
this fluid is directly in relation with the crust. In Hess, the rocks are rich in olivine (mafic rocks).
- Zone of discharge:
The hydrothermal fluid is expelled into the sea water with which it interacts. This fluid is now very
different from the seawater (its temperature and its chemical composition).
In the oceans, we are interested in hydrothermal systems on basic bedrock (basalt, gabbro,
peridotite).
In this context we can distinguish:
1. Black smokers: they are in the form of chimneys. Emission of sulphurous water at temperatures
above 300 º C. High levels of transition metals. Sulphide deposits in chimneys.
2. White smokers: They reject barium sulfate and calcium lower temperatures (below 300 º C). Much
less rich in transition metals as black smokers. Carbonate deposits.
There are also:
3. Diffuse emissions: There have no chimneys. Fluids are diluted and extremely low temperature (350 degrees). Under these conditions, their emissions are characterized by a special effect, like a
mirage on a paved road in the heat!
Some examples studied in the Atlantic:
The sea water flows near magma chambers. It interacts with the surrounding high temperature. In
the discharge zone, metal-rich fluids which are deposited along the chimneys.
In all cases, water and olivine do not mix!
Olivines are altered to serpentine, chlorite, amphibole, talc. These minerals are following perfectly
the Goldich's law : "minerals are even more vulnerable than their genesis conditions differ more
than those prevailing on the surface"
Serpentinization reactions take place: olivine in the presence of water, even at low temperature gives
quickly serpentine and magnetite. The green color of the olivine will become darker shades soon
with the magnetite.
Thus, the energy measured in some hydrothermal systems comes from the energy released during
serpentinization of olivine!
An equivalent site of Lost City was discovered in ophiolites obducted onto continental crust
Ophiolites represent fragments of oceanic crust, which after the disappearance of an ocean, is found
on the continents. Among the various known examples, the territory of Oman is particularly
spectacular. Aboard the JOIDES, the petrologists work on these Oman's outcrops.
On the site of 'Mamy Nova' we discovered in this old oceanic crust, watered by the rain today, hot
springs (40 ° c), with carbonate deposits and highly alkaline pH . No magma chamber may explain the
current discharge zone. We are in the case of a flow of rainwater that alters this oceanic crust olivine
(peridotite), and this reaction is causing the temperature of the water!.
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To go further
Hydrothermal vents, is a home to a fauna very original but also very abundant. Biomass is 500 to
1000 times higher than estimated abyssal environment. This density can be explained by the
presence of many bacteria using the energy of oxidation of sulphides present in the hydrothermal
fluid and form the basis of this ecosystem. They are then directly consumed by gasteropodes (snails)
themselves the target of secondary consumers (fish and shellfish). But bacteria can also establish
symbioses with livings such as Riftia pachyptia and some bivalves.
Animal communities vary with the temperature of the water in the discharge zone.