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When minerals tell us about water-rock reactions deep in the ocean

17-06-2008

A team from the universities of Montpellier, Marseille, Grenoble and the ESRF has just unveiled the structure of a newly-discovered mineral located in ocean ridges. This kind of mineral, called polyhedral serpentine, has provided valuable information about hydrothermal processes on the ocean floor. The results of this research are published in the European Journal of Mineralogy.

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Serpentines are rock forming minerals that can provide information about the history of water circulation in rocks and related processes in different geological settings. Depending on their location, serpentines play different roles: they trigger earthquake and volcanoes in subduction zones, they are soft “aseismic” materials in continental active fault zones and they are at the basis of alteration processes which help support primitive organisms at mid-ocean ridges (underwater volcanic ranges).

Serpentine in ocean ridges originates from the penetration of seawater into bottom rocks stemming from the earth mantle. The water, heated by volcanic activity, modifies the rocks to form serpentine during reactions that take place at high temperature (100-400°C). Then, the modified water is expelled via hydrothermal vents, which are metres-high submarine geysers. Depending on the conditions (temperature, fluid chemistry, water amount…) different types of serpentine minerals can form.

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Image of the polyhedral serpentine displaying its peculiar facetted morphology (Scanning electron microscopy).

A new form of serpentine, polyhedral serpentine, has recently been discovered. The researchers that came to ESRF beamline ID24 studied several samples from the fault area of California and mid-Atlantic ridge. They showed that this mineral forms from a gel, partly preserved in the samples. It marks the main ancient conduits for water circulation in the rocks during ocean plate alteration before it sinks to the subduction zone.

According to the researchers, when the polyhedral serpentine originates in a fault, the precursor gel should have helped to propagate the displacement along the fault. Therefore it could also have implications for fault movements and seismicity. This type of serpentine can also be found in meteorites.

Muriel Andreani, the main author of the publication, from the University of Montpellier, explains that “it is essential to describe these minerals in a fundamental way to search for past chemical and mechanical processes that are potentially recorded in their structure and chemistry”.

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µ-XANES mapping at the iron K-edge on a selected area of a thin section (Californian sample) containing a polyhedral serpentine vein (V2) and an older fibrous serpentine vein (chrysotile + polygonal serpentine: V1). A) Total iron content distribution in the sample, proportional to the height of the edge -jump of the XANES spectra. B) Distribution of the energy-position of the FeK-edge. Note the electron-volt shift of the edge energy that can be attributed to changes in the oxidation state of iron and/or to structural changes (iron local geometry). C) Averaged XANES spectra for the V1 and V2 regions,as well as almandine and andradite standard compounds used for pre-edge calibration (the zoom inset shows the deconvolution of V1 and V2 pre-edges). Credits: Eur. J. Mineral., www.schweizerbart.de

 

The ESRF ID24 beamline allowed scientists to track the amount and the state of iron that serpentines contain. “The state of the iron in the serpentine is linked to the production of iron oxides and hydrogen during serpentine formation. This is relevant because ocean hydrothermal fields where serpentine form are natural examples of efficient hydrogen production that could be copied in industrial processes”.

The next step for the team is to make a full study of the iron state in serpentine from samples collected during an oceanographic cruise. The samples come from a hydrothermal field called Rainbow in the Atlantic Ocean.

MONTSERRAT CAPELLAS

 

Reference:
Andreani M. et al, Occurrence, composition and growth of polyhedral serpentine, European Journal of Mineralogy, 2008, 20(2): p. 159-171.

 

Top image: Sampling of the serpentine-rich rocks, near the Rainbow hydrothermal field located in the North Atlantic Ocean, with the articulated arm of the Nautile, a submarine belonging to the IFREMER (French Research Institute for Exploitation of the Sea).