Xenoliths

Xenoliths are fragments of rock enclosed within another rock, typically an igneous one, and are distinct from the host by origin. The term derives from Greek xenos, meaning foreign, and lithos, meaning stone.

They form when solid country rock is incorporated into evolving magma as it moves toward the surface. As the magma ascends, it may engulf chunks of surrounding rock or sample mantle material. Xenoliths are common in volcanic rocks such as basalts and are also found in kimberlites, which sample mantle lithosphere. The enclosed rock may be crustal material or mantle-derived rocks such as peridotite, pyroxenite, or eclogite.

Mantle xenoliths often preserve minerals such as olivine, orthopyroxene, clinopyroxene, garnet, or spinel; crustal xenoliths may be granitoid, metamorphic, or sedimentary rocks. The textures and mineralogy can reveal the pressure and temperature of formation and any history of interaction with the host magma.

Xenoliths provide direct samples of depths inaccessible by drilling and are central to studies of mantle composition, crustal evolution, and metasomatic processes. Analyses typically include petrography, geochemistry, and isotopic dating, which help constrain temperature, pressure, age, and chemical evolution of the sampled reservoir.

In igneous petrology, a xenolith is a rock fragment; a single foreign crystal within a rock is a xenocryst, a distinction that aids interpretation of textures and histories in igneous systems.