Oceanic hydrothermal fluids

Hydrothermal circulation occurs along the Mid-Oceanic ridges, continuous volcanic chain stretching over about 60 000 kilometers, when the sea-water seeps through the fractured oceanic crust. This permeable structure of the oceanic crust, along with the presence of a deep heat source, allows the circulation of the sea-water through the substratum.

The hydrothermal circulation then occurs in three steps:

• The cold water penetrates in the crust (recharge phase) and is progressively heated. Exchanges with the rocks start with a modification of the chemical composition. The fluid becomes less concentrated in magnesium and sulfate, and gets enriched in hydrogen sulfide.
• The fluid then reaches a maximal temperature in the rhigh-temperature eaction zone, close to the magma chamber. Under these conditions, the composition of the fluid is strongly influenced by the temperature, the pressure, the water/rock ratio, the type of the attacked rock, and the reaction time.
• The high-temperature fluid then very rapidly rises adiabatically (discharge phase), while still reacting with the rocks. As time goes by, the fluid slowly cools down by conduction and by mixing with sea-water, colder near the surface, affecting its final composition.

Despite their similar appearance, hydrothermal fluids spread over a wide range of temperature and are characterized by very contrasted chemical compositions, depending on the reaction conditions.

Fluid collection with a titanium syringe
© Ifremer / Serpentine 2007

Why do we study hydrothermal fluids?

The job of a geochemist is to collect these high temperature fluids, to analyze their chemical composition when they come out of the sea-floor and to infer their history during hydrothermal circulation in order to understand les thermodynamic and geochemical processes that controls them during their transit through the oceanic crust.

From the chemical oceanography stand point, it is important to know the influence of hydrothermal fluids on the elements'cycle in the ocean and, more generally, on the global chemistry of oceans at short and long term.

From the biological oceanography stand point, we find new ecosystems based on chemosynthesis around these hydrothermal vents.

In this process, all the vital elements are brought by the fluid (sulfur, methane, hydrogen, hydrogen sulfide, and other metal elements). The flows of matter and energy brought by the fluids also play an important role in the "oceanic chemical budget". From the geologic point of view, the chemistry of fluids helps estimate the depth of the magma chamber, to better understand the production of the new oceanic crust and the formation of massive sulfide deposits.

How do we discover these hydrothermal fluids?

This delicate task can only be done by submarine and/or ROV, using sampling systems that were developed for this goal. A variety of systems now exists, all based on basic suction principles (like syringes) or by pumping. The fluid having a high temperature (350-400°C), the syringes made of titanium (see top picture) are often used. The geochemist must prepare these samplers before each dive.

The submarine or the ROV take care of the sampling at depth in the studied active areas. Upon return on the ship, the cherished fluid is recovered. The lengthy work of conditioning the gases and onboard analyses then starts (see picture below).

After qualification of the samples, gases (H₂, CH₄, CO₂...) are extracted with a gas extractor, collected, and analyzed on board using analytical instruments installed in a dedicated embarked laboratory van. Some minerals (Mg, SO₄, Cl) are also analyzed on board.

In these fluids, some compounds are found in higher concentration and some in lower concentration than in sea-water. Numerous specific conditionings are also done for the analysis of trace elements, isotopic analyses, and to look for organic molecules (hydrocarbons, carboxylic acids, amino acids...). The chemical composition of a fluid coming out of the sea-floor is a recording of the pressure and temperature conditions, and of the series of chemical reactions that it experienced during its journey through the fractured oceanic crust.

Together, these pieces of data give us information about the depth of the hydrothermal circulation in the substratum, on the residence time, and contribute to understand the way the associated metal sulfide deposits are formed.

In summary, a variety of conditionings and analyses are used to establish the chemical signature of the fluid, to know its history through the oceanic crust, to evaluate the consequences of these flows on the surrounding environment.

Conditioning of the gases and on-board analyses
© Ifremer / Serpentine 2007

What are the processes that control the chemical composition of the fluids?

Hydrothermal fluids can come out as high-temperature vents (350 to 400°C) and as low-temperature diffusion (few tens of degrees). The higher the temperature, the higher the rate of reaction with the rocks. Based on experimental work and on thermodynamic models approach, we now think that in the reaction zone, the equilibrium between the water and the rock's minerals is reached.

The phase separation process occurs as soon as the salted fluid is exposed to high temperature and pressure. We know that the critical point of sea-water is characterized by a pressure of 298 bars and a temperature of 405°C. Depending on whether we are higher or lower than these values, we will obtain a fluid considered sub-critical or super-critical, and in these two cases, the fluid will have very different thermodynamic and chemical properties.
This way, fluids with very variable compositions have been collected to date on the mid-oceanic ridges (Pacific, Atlantic, Indian ocean), and this a various depths, from a few hundreds of meters, to over 4000 meters, such the Ashadze site, deepest hydrothermal site known to date, studied and sampled with success during this cruise.

Do these hydrothermal fluids change with time?

In the mid-1970's, shortly after their first discovery on the Galapagos ridge, we thought that these fluids should have a relatively stable composition. Since, thanks to the progressive discovery of new active sites in various emission conditions, and in different tectonic and geologic settings, it is now certain that hydrothermal fluids exhibit very variable chemical compositions, that they can change from one area to another or event from one vent to another in the same area, and that they can stay stable over more or less long periods of time (scale of about 10 years), or slowly or abruptly change in time.

 

They are controlled by the tectonic and volcanic activity that can vary and make the fluid's composition change. It is clear that volcanic eruptions, and earthquakes have dramatic consequences and an influence on the temperature, the chemical composition, creating flows of matter and energy more or less strongly expelled in the surrounding water column.

At a given site, the chemical composition of a fluid can vary under the influence of thermodynamic variations such the phase separation that is controlled by the pressure and the temperature, of tectonic of volcanic events, or remain stable over a long time, thus favoring the progressive establishment of habitats of biological and bacterial communities based on chemosynthesis.

We know that high-spreading rate ridges (>12 cm/year, as for the East Pacific Rise), the slow progressive movements leading to faulting of the crust, associated with brutal magmatic events, rapidly affect the composition of the fluids derived from a reaction between sea-water and the basaltic crust.

In contrast, on the slow-spreading ridges such as the Mid-Atlantic ridge, the fluids being more controlled by the tectonic movements, will tend to remain more stable over time. But the sea-water, by going deeper, will reach and react with the mantle rocks (peridotite) to produce a reaction called "serpentinization", and produce fluids of very unusual composition.