OIB: what is it?

The term OIB, for Ocean Island Basalt (Where basalt of an oceanic island), refers to basalts having the typical chemical composition of those formed at the level of hot spotsor more generally on volcanic islands located far from a plate boundary (ridge or zone of subduction).

OIBs, typical of hotspot volcanism

With MORBs (for Mid-Oceanic Ridge Basalt, in English or mid-ocean ridge basalt in French), OIBs form the second major family of basalts. the volume of the OIBs is however much lower than that of the MORBs on the surface of the Earth. MORB and OIB are differentiated by their chemical composition.

In general, OIBs come from the partial fusion of coat lower, while MORBs are derived from the partial melting of the upper mantle. This origin is “read” in the chemical composition of the basalts, which reflects, to a certain extent, the composition of the mantle from which they come.

The characteristic of OIBs is that they are particularly rich in incompatible elements. During an episode of fusion or crystallization of peridotites of the mantle, these elements tend to concentrate in the phase liquid and not to enter into the composition of the crystallized phase. Overall, the composition of OIBs is quite close to that of Earth’s early mantle, although not exactly similar. Conversely, the composition of MORBs, which are depleted in incompatible elements, is very different from this primitive composition and testifies to the continental crust formation from the upper mantle.

A great variability of composition which testifies to the heterogeneity of the source mantle

The intermediate position of the composition of the OIBs thus reflects their deeper origin and the fact that the source mantle was not (or only slightly) affected by the processes of formation of the continental crust. The hot spots, which originate in the deep mantle, therefore puncture magmatic liquids of the OIB type, which will end up on the surface in the form of basalts with a particular chemical signature. We thus find volcanic rocks of the OIB type in Hawaii or in Iceland, the latter island having the particularity of being a hot spot associated with a ocean ridge.

OIBs, however, show great variability in composition and encompass the full range of volcanic rocks ranging from tholeites to alkaline basalts.

This variability testifies to the process of mineralogical differentiation during the rise of the magma but also to the heterogeneity of the composition of the lower mantle.