Xenoliths are foreign rock pieces or fragments trapped in an igneous rock body (host). However, a broader definition will include foreign rocks trapped in sedimentary rocks or meteorites.

They are foreign rocks because they were incorporated into fluid melt or magma bodies. Therefore, these alien fragments existed at the time of incorporation. This means they didn’t crystalize or solidify from the melt.

An example is basalt having rock fragments of peridotite. In this case, peridotite is a xenolith as it didn’t crystallize from basaltic magma. Instead, it was entrained by basalt magma and brought to the surface.

Xenoliths can be igneous, metamorphic, or sedimentary rocks. Most are unrelated to the host rocks. However, some are genetically related.

For instance, earlier-formed gabbro fragments may be trapped in later-formed basaltic magma. These two forms have a genetic relationship.

However, gabbro didn’t crystallize from the melt that crystallized basalt. Instead, the ascending magma entrained (incorporated) it.  

The term xenolith comes from two ancient Greek words: xeno = foreign and lithos = rock. It means foreign rocks.  

Why are they important? Geologists use them to understand the inaccessible mantle and deeper crust. Also, some may contain precious minerals or gemstones.

How do you identify xenoliths?

We identify xenoliths using their name and the rock in which they occur. These rock fragments can be from sand grain-sized to several meters in size.

You can easily spot xenoliths because they often have a different color, texture, or density from the host rock.

Origin or xenolith types

Xenoliths may be from 1) country rocks, 2) refractory rocks that remain after partial melting, or 3) cognate.

Here is a bit more on each of these types or origins.

1. Torn country rock (accidental)

Accidental xenoliths are torn along the pipe vent as magma violently rises during an explosive eruption. Also, lava may pick them as it flows. These rock fragments may be metamorphic, sedimentary, or igneous.

They are genetically unrelated or have no link to the magma that entrains them. However, they may have similar compositions. For instance, rhyolitic magma can tear pieces of granite.

2. Cognate inclusions

Cognate or autoliths xenoliths are entrained earlier formed rock fragments from the same magma. They are genetically related to the host rocks.

For instance, basaltic magma may incorporate solidified gabbros formed earlier as it rises. In such a case, these two have a genetic relationship. They both originated from the same magma but at different times.

3. Residual mantle rocks

Rising magma can also incorporate residual mantle source rocks left after partial melting. These remnant, refractory, or restite rocks may include garnet lherzolite or harzburgites.

You can also call them inclusions

Sometimes, xenoliths and other materials hosted in another rock are known as inclusions. However, in gemology and mineralogy, inclusions mean material in a gemstone or minerals.

Usually, inclusions are older than the rocks that host them. This is according to the principle of inclusions.

Those not genetically related can be much older. However, this is not always true with cognate. Some may have a small age difference.

Xenolith assimilation may occur

Xenoliths can occur through melting, dissolution, or reaction. Assimilation contaminates magma.

Melting will occur if the melt temperature exceeds the minimum melting point of the combined two or more rock constituents. It can occur anywhere within the rock.

Evidence of melting will include glassy film at grain boundaries. When it occurs together with a reaction, a sieve or spongy texture may appear on the margins of the xenoliths.

Dissolution, on the other hand, occurs in areas where magma touches the xenoliths. It will not result in the disaggregation of the whole xenolith. However, it will cause rounding. For instance, mafic magma will dissolve less mafic xenoliths.

What about reaction? It affects where magma touches the foreign rock. Usually, a reaction will form a new mineral around the margins. For instance, pyroxene from quartz reaction with basaltic or basic magmas may create such margins.

Sometimes, total assimilation may occur, leaving no xenolith. This doesn’t mean it didn’t exist. Similarly, too many xenoliths don’t mean severe contamination. Incorporation may occur just before solidification.

Examples

Examples include kimberlite, basalts, nephelinites, or lamproite, which may have mantle peridotite xenoliths like garnet lherzolite.

Also, basalt may have dunite xenocrysts or nodules, while granitoids may have metamorphic, igneous, or sedimentary xenoliths.

Why are xenoliths important?

Xenoliths and xenocrysts provide geologists with information about the mantle or interior of Earth, which is inaccessible.

The great mantle is 40-280 km deep from the surface. Digging to such great depth is cumbersome. Also, high temperatures and pressure will make the job impossible.

Some of the information xenoliths provide include:  

• Mantle chemical composition, mineral, structure, thermal state, stability, and age. Also, they can help us better understand the origin of the lithosphere.
• Considering various mineral phases in xenoliths can help us know the depths in which they occur. A good example is aluminum-bearing minerals. They form calcic plagioclase, spinel, or garnet at different depths. This is possible because they retain composition and mineralogy. Thus, they will give you pressure where they are formed.
• Understand how magma transportation occurs, including speeds. Fast speeds will disaggregate any rock fragments present and cause rounding.
• Know the origin and evolution of host rocks

Additionally, some valuable minerals like diamonds are hosted in eclogite, or garnet pyroxenite, garnet harzburgites entrained by kimberlites, not kimberlite itself.

References

• Russell, J. K., & Jones, T. J. (2023). Transport and eruption of mantle xenoliths creates a lagging problem. Communications Earth & Environment, 4(1). https://doi.org/10.1038/s43247-023-00843-0