When the earth’s mantle takes on water!


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[EN VIDÉO] How does magma form before a volcanic eruption?
Magma is the origin of the formation of volcanoes. This molten rock, which simmers in the crater, comes from a partial melting of the Earth’s mantle. Futura meets Jacques-Marie Bardintzeff, doctor of volcanology, who tells the origin of the magma.

One of the main mechanisms participating in the tectonic plates it’s here subductionwho sees a lithospheric plate pass under another plate to be recycled inside the coat. Subduction zones are generally associated with intense volcanism, often of the explosive type. Furthermore, most of the current mountain ranges, like the Alps or the Himalayas, the result of a continental collision that followed the closure of an ocean by the action of a subduction zone. These mountain ranges therefore contain many clues that allow us to study, indirectly, the complex mechanism of subduction zones.

The mantle absorbs large amounts of water in the subduction zones

Many parameters come into play and their variations make each subduction zone a unique dynamic system.

One of these parameters is water. It is in fact an essential element that will have a direct impact on the fusion capacity of the mantle and the Crustbut also the behavior of the magma during the ascent e formation of various mineral deposits.

It has long been known that subduction zones represent highly hydrated environments. As it plunges into the cloak, the subducting plate carries an enormous amount of sediment oceans submerged in water. This water will significantly hydrate the mantle rocks and directly affect magma formation. The volcanic rocks released from volcanoes on the surface they bear the signature of this one hydrationwhich will also determine the style eruptive. Plus the lavas they are hydrated, more the eruptions are explosive. This explains why the most destructive eruptions are generally found in subduction zones.

Finding the initial water content of a magma, a complicated problem

So far, analysis of volcanic rocks has suggested that the mantle, in this context of subduction, contained about 4% water (per unit of weight). A new study, however, questions this value, advancing the idea that the signature present in volcanic rocks would not be representative of the actual water content of the initial magma. For the authors of the study published in Geosciences of naturein fact, it seems probable that some processes – both at the moment of the rapid rise of the magma and when it reaches the surface – somehow mask the original water content of the magma. liquid magmatic.

To try to find the exact amount of water, the team of researchers looked to a certain type of water igneous rocksThey pluto, which unlike volcanic rocks do not crystallize on the surface but deep inside the crust. A new analytical method was also developed to study the composition of plutonic rock samples from Himalayan mountains and updatessurfacing according to the tectonic deformation that this immense collision zone undergoes.

A more hydrated coat than we thought

The crystals of these rocks therefore show much less signs of alteration than volcanic rocks, exposed to the elements on the surface. Their results ultimately show that it is not 4, but more than 8% water (by weight) that the magma would contain prior to its crystallization.

These results also provide a better understanding of deposit formation coppergold orsilver, which requires a large amount of water. The previously estimated 4% was not sufficient to explain the formation of deposits from minerals. The problem is now solved. These mineral deposits would thus form from magmatic fluids very rich in water (from 12 to 20%) which would have separated from the initial magma at the moment of its crystallization. Understanding the generation of these super-water magmatic liquids is therefore of great interest for the search for new fields.

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