MORB: what is it?

The term MORB, for Mid-Oceanic Ridge Basalt (basalt mid-ocean ridge, in French) refers to basalts with the typical chemical composition of those formed at ridges. In particular, they make up the upper layer of the oceanic crust.

MORBs, one of the two major families of basalts

In general, basalts are volcanic rocks resulting from the partial melting of the Earth’s mantle. In details, the chemical composition of basalts varies according to the composition of the coat which is at the origin. Two major types of basalt are thus distinguished, which show significant variations in composition: MORBs and OIBs. These differences indicate that there are different mantle magmatic reservoirs, which do not mix. This is an argument in favor of a convective system two separate layers.

MORBs are the most common basalts on the surface of the globe. They are formed from liquids magmatic produced by the upper parts of the asthenospheric mantle. Their chemical composition and the isotopic ratios show that they derive from a so-called “depleted” mantle, that is to say that the mantle which is the source has a reduced concentration in certain chemical elements compared to a so-called primitive mantle. The composition of this primitive mantle is estimated from the analysis of chondriteswhich are meteorites whose composition is close to that of the planet shortly after its formation, before any modification produced by the processes of differentiation.

MORBs, witnesses to the formation of continents

These chemical elements missing in the upper mantle are not just any. These are so-called incompatible elements, which tend, during an episode of melting or crystallization, to concentrate in the liquid phase and therefore not to enter the crystallized phase.

The fact that the MORBs show a chemical signature associated with an incompatible element-depleted mantle therefore suggests that this mantle has already, during its geological history, known and undergone episodes of melting and extraction of magmatic liquid. The composition of the MORBs and the depleted upper mantle actually testifies to “extraction” of continental crust of the primitive mantle, the missing elements in the upper mantle being found in igneous rocks forming the continental crust.