A xenolith is an inclusion of foreign rock within an igneous rock. Plutons
sometimes contain xenoliths of nearby country rock, but xenoliths within some
basalts may have come from the earth's mantle.
Numerous basalts contain xenoliths of the relatively rare rocks peridotite
and dunite. Peridotite is an igneous rock composed mostly of olivine and pyroxene;
it lacks feldspar, which is common in most igneous rocks. Dunite is a related
rock that is almost entirely olivine.
Because basalts appear to be generated in the mantle's asthenosphere, and
because peridotite and dunite are uncommon at the earth's surface, most geologists
suspect that these xenoliths may be solid pieces of the earth's mantle brought
to the surface by the erupting basalt. The density, seismic velocity, and
chemical composition of peridotite and dunite make them reasonable candidates
for mantle rock.
Peridotites (usually serpentinized by metamorphism) also occur at the base
of ophiolite sequences on land (see next chapter), which are widely interpreted
to be slivers of oceanic crust and mantle rock attached to continents at convergent
plate boundaries.
Even though the mantle extends to a depth of nearly 2900 kilometers, the
only direct evidence that we have of its composition comes from xenoliths
which rarely rise from depths greater than 200 kilometers. Most of the mantle
can only be understood by seismic studies and high pressure laboratory experiments.
