xenolith

This article concerns the geologic term, for other uses see: Xenolith (disambiguation)

Gabbroic xenolith in a granite; eastern Sierra Nevada, Rock Creek Canyon, California.

A xenolith (Greek: 'foreign rock') is a rock fragment which becomes enveloped in a larger rock during the latter's development and hardening. In geology, 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. A xenocryst is an individual foreign crystal included within an igneous body. Examples of xenocrysts are quartz crystals in a silica-deficient lava and diamonds within kimberlite diatremes.

Although the term xenolith is most commonly associated with igneous 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 and xenocrysts 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. (Blatt, 1996)

Xenoliths in granodiorite of the Alta Stock, Little Cottonwood Canyon, Utah

Large and rather unusual xenolith of sandstone (probably from the Albee Fm.) within the Fairlee Pluton in Vermont

Peridotite (green) mantle xenolith within a (dark) volcanic bomb from the Vulkan-Eifel, Germany. Coin of one euro for scale.

Rounded, yellow, weathered peridotite xenolith in a nephelinite lava flow. Kaiserstuhl, SW Germany

References

Wikimedia Commons has media related to: Xenoliths

• Blatt, Harvey, and Robert J. Tracy (1996) Petrology, W. H. Freeman, 2nd ed. ISBN 0-7167-2438-3

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