Xenoliths

Xenoliths are blocks of preexisting rocks within magma. Consequently, xenoliths provide a sampling of materials through which the magma has traversed on its rise toward the surface. Some xenoliths originate within the mantle and provide the only means of obtaining samples of this elusive material.

Occurrence of Xenoliths

Xenoliths are fragments of preexisting rocks that have been incorporated in a magma as it makes its way into higher levels of the crust. The term “xenoliths” is derived from the Greek roots xeno and lith, meaning “strange or foreign” and “rock,” respectively. These rock fragments are pieces of previously formed rocks that become incorporated into the magma and perhaps removed from their source as the magma moves. The xenoliths may retain their original identity with minor alteration, or they may be greatly altered by attendant heat and fluids present in the magma. Xenolithic inclusions may be preserved near to their original sources along the borders of an intrusive magma, or they may be carried for great vertical distances from where they originated. In this manner, fragments of deep crust and mantle material from as much as 200 to 300 kilometers below the earth’s surface have been brought to the surface by volcanic eruptions.

Xenoliths represent fragments of the rocks through which a magma has moved to its site of final emplacement and crystallization. They may be found in products of explosive volcanism such as volcanic tuff and breccia, within crystalline igneous rocks as in lava flows, and within shallow and deep-seated igneous rocks. Explosive volcanic materials are ejected by highly gas-charged eruptions that produce diatremes and maar-type volcanoes. These are volcanic craters in the form of inverted conelike or dishlike depressions in the surface surrounded by a rim of ejected deposits. Xenoliths are found as angular or rounded blocks embedded in ash tuffs or volcanic breccia in the rim and the pipelike conduit underlying the crater. In certain types of basalt and related magmas that originate deep in the mantle, such as kimberlite, rare fist-sized fragments of mantle material are transported upward from near the source of the magma origin. Fragments may also be collected from rocks traversed by the magma along its path of vertical ascent through the crust. Xenoliths in crystalline igneous rocks are embedded within the rock and are not exposed until erosion exposes the xenolith by removing overlying material. Granite rocks typically contain large xenoliths of metamorphic or sedimentary rocks. Such xenoliths reflect the typical intrusive process that produces granites. In this process, subsurface magma chambers expand and move upward by physically plucking country rocks from the wall and roof.

Wall-Rock Xenoliths

Three basic varieties of xenoliths are recognized: wall-rock xenoliths, cognate xenoliths, and mantle xenoliths. Wall-rock xenoliths are represented by blocks and pieces of the adjacent country rock that have been incorporated into the magma. Cognate xenoliths are inclusions of the chilled margins of the magma or comagmatic segregations—that is, masses of previously solidified magma that break loose and are later incorporated in more energetic magmatic motions. Mantle xenoliths are presumed to be pieces of the mantle that become incorporated in magmas that are formed by partial melting deep within the earth.

Magmatic intrusions make room for themselves by three processes: forceful injection, stoping, and assimilation. Stoping occurs when the magmatic front advances by injection into fractures and surrounding blocks of country rock. These blocks may sink in the magma, and they may be slightly altered or totally assimilated within the magma depending on the characteristics of the magma and the wall rock. Most wall-rock xenoliths in felsic magmas do not move far from their source. Xenoliths are abundant near the margins of most intrusions. Because the margins are more likely to be losing heat and cooling at rates faster than the interior of the intrusion, the magma is more viscous, and xenoliths are less likely to move very far from the source. In contrast, fast-moving magmas in volcanic conduits often carry a wide variety of xenoliths from country rock traversed by the magma. In this way, a wide variety of crustal rocks cut by the volcanic vent may be brought to the earth’s surface.

Xenoliths usually show effects of the high temperatures to which they are exposed. Preexisting minerals within a xenolith react to form new minerals that are in equilibrium with the magma. Thus, most xenoliths are brought to a high-grade metamorphic state unless they are composed of high-temperature refractory minerals to begin with, or unless they are exposed to high temperatures for short periods of time. The German term Schlieren is used to describe hazy, ill-defined streaks of xenoliths that are almost completely assimilated. Materials caught up in low-temperature felsic magmas are more likely to be altered by reactive assimilation, in which there is an exchange of ions between the xenolith and the magma. Inclusion of large blocks of country rock may alter the composition of magma by enriching it with elements that were not originally abundant.

One process that is not igneous in nature but creates xenoliths as well is the subterranean flow of salt. Buried salt beds are capable of plastic flowage in response to the weight of overlying rocks. The salt is not molten but nevertheless flows under pressure. Often the salt, which has a lower density than the enclosing sedimentary rocks, will rise thousands of feet through overlying sediments to form salt domes or salt plugs. The rise of large masses of salt is very much like the rise of magma. Pieces of wall rocks and sub-salt rocks may be incorporated into the salt as xenoliths. In this manner, salt domes in the Persian Gulf have brought up blocks of sedimentary, igneous, and metamorphic rocks from great depths in the crust. There are even ultramafic igneous xenoliths in the Weeks Island salt dome in southern Louisiana that are thought to be fragments of mantle-derived ultramafic intrusions emplaced along fault zones prior to deposition of the salt.

Cognate Xenoliths

Cognate xenoliths, also called “autoliths,” are xenoliths from parts of the magma that have previously crystallized. Magmas solidify over a wide range of temperatures. Large bodies of magma may require tens of thousands of years to crystallize fully. Material on the outer edge of the magma will cool and crystallize more rapidly than that of the interior, resulting in chilled margins. Elsewhere within the magma, early formed crystals will either float or sink depending on the specific gravity differential. Feldspar crystals tend to float and collect near the top of the magma chamber, and mafic minerals such as olivine or pyroxene tend to sink to the bottom of the chamber. Some magmas undergo energetic degassing because of the reduction in confining pressure as they approach shallow levels in the crust. The rapid evolution of dissolved volatiles may disrupt previously crystallized portions of magma (chilled margins or crystal segregations) and mix solid cognate xenoliths with the mobile fluid phase.

Mantle Xenoliths

Perhaps the most exotic xenoliths are those that originate within the mantle. The mantle lies at depths of five to forty kilometers below the surface and extends down to the top of the outer core nearly three thousand kilometers beneath the surface. No drill has penetrated to the mantle; therefore, these materials are completely inaccessible for direct sampling. The probable composition and mineralogical makeup of the mantle is postulated from calculations of the density, pressure, and temperatures that exist at mantle depths and by comparisons with meteorites, which are pieces of asteroids that have been fragmented by collisions with other asteroids. The interior of asteroids are thought to reproduce conditions similar to those of the mantle. Fortunately, pieces of the upper mantle are delivered to the surface of the earth as xenoliths in some magmas that originate by partial melting deep within the earth. For these rocks to make it to the surface without significant alteration by the host magma, they must be delivered to the surface in a fairly short period of time. Thus, it is not surprising that mantle xenoliths are found in volcanic rocks associated with rift zones and magmatic zones between plates, which allow for the rapid rise of gas-rich magmas to the surface.

Typical mantle xenoliths are composed of peridotite (olivine and pyroxene-rich rocks) incorporated in mafic volcanic rocks such as basalt. Basalts are formed from magma that originates by the partial melting of mantle materials. They often incorporate xenoliths from their place of origin as well as fragments of crustal rocks torn from walls of the conduit along which they are rising. Most notable of these magmas are varieties of peridotites known as “kimberlites” and “lamproites.” Kimberlite (mica peridotite) is a potassic ultramafic rock that occurs in intrusive pipes and plugs, and explosively formed volcanic craters that overlay them. Lamproite is a porphyritic, ultrapotassic ultramafic rock that occurs in dikes and small intrusions. Kimberlite and lamproite magmas typically contain up to 75 percent xenoliths and xenocrysts. Kimberlites contain abundant xenoliths of lherzolite—a mantle peridotite with magnesian olivine, pyroxene, and minor calcium-plagioclase, spinel, or garnet.

The range of xenoliths in basalt is more restricted than that in kimberlites because basalts form at shallower levels of the mantle. Thus, basalts incorporate less of the upper mantle on their way to the surface. Spinel lherzolites are common in alkali basalts such as those found in Hawaii, Arizona, the Rio Grande Rift, and Central Europe. Basalts also contain xenoliths of harzburgite, dunite, and eclogite. Harzburgite (a peridotite with magnetite and spinel) and dunite (an ultramafic rock that consists of mostly olivine with minor chrome-bearing spinel as an accessory mineral) probably represent residual melts following fractionation of lhertzolite.

The occurrence of mantle xenoliths in volcanics is limited to intraplate magmatic environments such as oceanic islands (Hawaii and Tahiti) and continental volcanic provinces or rifts (the southern Colorado Plateau and the Eifel District, Germany). Kimberlite intrusions favor old, stable, thick continental crust (South Africa and central North America). These environments are characterized by simple plumbing systems in which magmas rise rapidly to the surface; otherwise, xenoliths would sink in the host magma. Xenoliths are more rare in complex systems found in interplate environments, such as collisional magmatic provinces or island arcs.

Scientific Value

The chief scientific value of xenoliths rests in their being samples of materials collected as magma ascended through the earth’s mantle and crust. Wall-rock xenoliths provide samples of country rock that remain close to their original source. More importantly, xenoliths in rapidly ascending, volatile-rich volcanic magmas may provide samples of crustal rocks from the walls of the conduit throughout its entire path. These materials are brought to the surface in relatively unaltered states. In addition to mantle xenoliths, some localities—such as Kilbourne Hole, New Mexico, and Williams, Arizona—contain significant xenoliths of granite gneiss representing crustal basement rocks. Cognate xenoliths provide information on the earliest parts of the magma to crystallize. Basalt, kimberlite, and lamproites contain xenoliths from the mantle. Distribution of nodule occurrences is not random. Siliceous basalts rarely contain nodules, whereas alkali basalts commonly contain eclogite and spinel peridotites. Kimberlites contain abundant nodules, including garnet peridotite. These differences suggest different depths of origin for the different magmas. Mantle xenoliths are the only means of obtaining samples of the mantle. Study of these materials can lead not only to determining the composition of the mantle but also to developing an understanding of its physical state and some of its processes.

From a study of these rare rocks, various processes and conditions of the lower crust and upper mantle can be inferred. The mineralogical combinations serve as geobarometers and geothermometers. Controlled crystallization studies at a variety of temperatures and pressures are employed to characterize the stable mineral assemblage within differing crustal and mantle environments. A mixture of minerals or rocks exposed to elevated temperatures, as found in the lower crust and upper mantle, recrystallize into a mineral assemblage that is in equilibrium with the higher temperatures. Alternatively, by examining altered rocks such as some altered xenoliths, it is possible to determine their prior state before metamorphism by the magma.

Economic Value

Wenoliths and xenocrysts find their principal economic value as a source of diamonds formed deep within the mantle and, to a much lesser extent, the olivine gem peridot. Diamonds occur as xenocrysts in kimberlite, lamproite, and alluvial gravels derived from kimberlites. Diamonds form at very high pressures. Minimum conditions required are pressure greater than 40 kilobars, which is equivalent to depths greater than 120 kilometers, and temperatures of approximately 1,000 degrees Celsius. Diamonds are associated with magnesia garnet-bearing lherzolites and coesite.

Diamonds make up only one part in twenty million of a typical diamond-bearing kimberlite, and many kimberlites are devoid of diamonds. By far, most diamond production is from kimberlites in West Africa, South Africa, Canada, and Russia, but diamonds are also mined in Australia, Brazil, and India. Small concentrations of diamonds have been found in kimberlite and lamproite bodies in Arkansas, Wyoming, Montana, and Michigan. After unsuccessful attempts to develop mining operations at the lamproite body at Murfreesboro, Arkansas, the area has been turned into the Crater of Diamonds State Park, where several small diamonds are found by tourists each year.

Principal Terms

assimilation: the absorption of the chemical components of wall rock or xenoliths into a magma

country rock: rocks through which a magma is intruding; also known as “wall rock”

diatreme: a pipelike conduit in the crust of the earth filled with fragmented rock produced by gas-rich volcanic eruptions

eclogite: rock composed principally of garnet and pyroxene that formed at high pressures associated with great depths

felsic rocks: igneous rocks rich in potassium, sodium, aluminum, and silica, including granites and related rocks

mafic rocks: igneous rocks rich in magnesium and iron, including gabbro, basalt, and related rocks

magma: a naturally occurring silicate-rich melt beneath the surface of the earth

mantle: the intermediate zone between the crust and the core of the earth

peridotite: a class of ultramafic rocks made up principally of pyroxene and olivine, with subordinate amounts of other minerals

segregation: the concentration of early-formed minerals in a magma by crystal settling or crystal floating

ultramafic: a term for any rock consisting of more than 90 percent ferromagnesium minerals, including olivine and pyroxene

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