Mantle xenoliths are pieces of rock that originate from the Earth's mantle and are brought to the surface by volcanic activity. These fragments are often found within igneous rocks, such as basalt, and provide valuable insights into the composition and conditions of the mantle beneath the Earth's crust. Studying mantle xenoliths helps geologists understand the processes that shape our planet. They can reveal information about the temperature, pressure, and mineral composition of the mantle, which is crucial for understanding plate tectonics and the formation of volcanoes.