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What peridotite!

The first session was very successful dredging. 5 dredges, more than one ton of rock was collected and classified, numbered and described on board, before being carefully packed to be studied more thoroughly to the ground.

All rocks collected, apart from some gaps and sediment (\u0026lt;5%) are peridotites which show strong variations linked to many circulations of fluids (mainly sea water) that have crossed. But what exactly is a peridotite?

A peridotite is a rock composed of a mineral assemblage dominated by olivine, also known as peridot, and accompanied in varying proportions, pyroxene and some% other accessory minerals (oxides). This rock is the Earth's mantle on his first 400 km - the cloak representing approximately 70% of the mass of the Globe and extending over 2900 km between the crust and the Earth's core.

From a chemical point of view, the peridotite is characterized by a high magnesium and iron, compared rocks typically encountered in the earth, richer in silica and aluminum. Indeed, olivine consists of a network of silicon atoms and oxygen (ie This is called a silicate) which organized the magnesium atoms, and a little less quantity of iron. Its formula is (Mg, Fe) SiO ₂ 4. Pyroxenes are also iron-magnesium silicates (+ / - aluminum-calcium).

When "fresh", that is to say not altered by fluids, its constituent minerals are easily identified by their optical properties. On the left image, olivine is easily distinguished by its pale green to yellowish and transparent. As for the pyroxenes, minerals are brilliant, with darker shades on the image.

However, there has never been sampled by ocean domain peridotites also fresh. Indeed, in the absence of basaltic magma flows, the extension to the axis of the ridges can be accommodated by large faults of the game that plunge deep into the crust to mantle depths, allowing the recovery of peridotites but also the penetration of seawater Thus, even before reaching the surface, the peridotites are beginning to respond with deep sea water which has warmed during its descent. The mineralogy of the peridotite was then changed and it acquires a dark green color, more homogeneous, with some areas remains more "fresh" (clearer), as observed on dredged samples (right image). This is called the process of serpentinization. But that's another story! A follow?