Xenoliths

Xenoliths

Mafic Xenolith in Granite

ARIZONA

A xenolith is a rock fragment which becomes enveloped in a host rock during the latter's development and solidification. The term xenolith is almost exclusively used to describe inclusions in igneous rock during magma emplacement and eruption. Xenoliths may be engulfed along the margins of a magma chamber, torn loose from the walls of an erupting lava conduit or explosive diatreme or picked up along the base of a flowing lava on Earth's surface. Although the term xenolith is most commonly associated with igneous rock inclusions, a broad definition could include rock fragments which have become encased in sedimentary rock. Xenoliths are sometimes found in recovered meteorites. To be considered a true xenolith, the included rock must be identifiably different from the rock in which it is enveloped; an included rock of similar type is called an autolith or a cognate inclusion. Xenoliths provide important information about the composition of the otherwise inacessible mantle. Basalts, kimberlites, lamproites and lamprophyres, which have their source in the upper mantle, often contain fragments and crystals assumed to be a part of the originating mantle mineralogy. Xenoliths of dunite, peridotite and spinel lherzolite in basaltic lava flows are one example. Kimberlites contain, in addition to diamond xenocrysts, fragments of lherzolites of varying composition. The aluminium-bearing minerals of these fragments provide clues to the depth of origin. Calcic plagioclase is stable to 25 km depth. Between 25 km and about 60 km, spinel is the stable aluminium phase. At depths greater than about 60 km, dense garnet becomes the aluminium-bearing mineral. Some kimberlites contain xenoliths of eclogite, which is considered to be the high-pressure metamorphic product of oceanic basaltic crust, as it descends into the mantle along subduction zones.

Xenoliths in Kimberlite

Peridotite Xenoliths

Xenolith