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What are upper mantle xenoliths?

The word “xenolith” has Greek origin, it means foreign rock. This term is used for certain inclusions in igneous rocks. These foreign inclusions have been “picked up” by the melt during its travel to the Earth’s surface. 

How does this happen?

Many of the volcanic rocks are originated from the mantle of the Earth. The mantle can partially melt (this means that it is not completely molten, so do not imagine a sea of magma down there), to produce basalt for example. This melting happens due to the heating effect of mantle convection or of mantle plumes originated from the core-mantle boundary.

This figure shows shematically the structure and composition of the Earth. We distinguish inner and outer core, D´´ layer between the core and mantle, lower mantle (here in orange) upper mantle (here in green) and crust.  Between the upper and lower mantle there is a transition zone (here in yellow). There is convection in both the outer core and the mantle. These layers are distinguished based on the propagation of seismic waves.  At the borders between 2 units the velocity of wave propagation changes abruptly. These changes happen due to change in composition or change of structure of the dominant minerals. Compositions are assumed based on high pressure and temperature experimental data.

During partial melting first small pockets of melts form at the junctions of different minerals. At sufficient degree of melting these small melt pockets will be connected and start to migrate in the mantle. As the density of the melt is lower than that of the surrounding rock, the melt will migrate towards lower pressures i.e. to the surface. Large amount of melt will form wider channels in the surrounding rock during its migration. Bigger rock pieces might fall into these melt channels and if the melt comes to the surface fast enough; these pieces do not have time to melt. They are preserved as xenoliths in volcanic rocks. The xenoliths will represent the whole rock column which was crossed by the melt before the volcanic eruption.

This figure shows what happens in great depth beneath a volcano.

These rocks are interesting because they give us information about rocks and geological processes from much larger depths that can be reached by drilling.

Composition of upper mantle xenoliths?

As every rock, this rock is also built up by minerals. The picture below shows a close look on a mantle xenolith. These rocks, and the uppermost mantle of the Earth composed dominantly of 3 minerals. Orthopyroxene (brown), clinopyroxene (green) and olivine (yellowish).  These are all Fe-Mg-silicates (with variable composition and crystal structure).

Besides these three major minerals plagioclase, spinel (small black minerals in the picture above) and garnet are also present. These latter minerals constitute only a few % of the rock, but they are very important.  Their stability depends on pressure, thus simply looking at the rock we have an idea which depth it is coming from.

How do we know these? The answer is complex . By the combination of geophysical and geochemical data and the help of high pressure and temperature experimental petrology and geochemistry.

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