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Review key Xenoliths: Mantle Inclusions, Magmatic Entrapment & Earth Interior exam facts and rate your mastery to track revision.
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#1
A xenolith is a foreign rock fragment trapped inside an igneous rock that did not crystallize from the host magma.
#2
The term originates from Greek xenos meaning stranger or guest and lithos meaning stone.
#3
Xenoliths originate when ascending magma tears fragments from the conduit walls through thermal stress and mechanical stoping.
#4
Host magmas must ascend rapidly, often at meters per second, to prevent xenoliths from melting or chemically assimilating.
#5
Kimberlites, lamproites, and alkali basalts serve as the primary volcanic transport agents for deep mantle xenoliths.
#6
Mantle xenoliths consist predominantly of peridotite varieties, including lherzolite, harzburgite, dunite, and pyroxenite.
#7
Spinel lherzolite xenoliths represent upper mantle rocks originating at depths between 30 and 70 kilometers.
#8
Garnet lherzolite xenoliths indicate deeper mantle origins where pressures exceed approximately two gigapascals.
#9
Eclogite xenoliths contain omphacite pyroxene and pyrope garnet, representing metamorphosed oceanic crust subducted into the mantle.
#10
Xenocryst designates an individual foreign mineral crystal trapped in magma, such as a mantle diamond carried in kimberlite.
#11
Autolith or cognate xenolith refers to an inclusion that crystallized from an earlier phase of the same parent magma system.
#12
Mineral geothermometers measure temperature-dependent element exchanges between coexisting minerals like orthopyroxene and clinopyroxene.
#13
Mineral geobarometers determine crystallization pressure based on pressure-sensitive reactions such as aluminum solubility in pyroxene.
#14
Xenoliths provide physical evidence confirming that Earth's upper mantle is compositionally peridotitic rather than basaltic.
#15
Reaction rims or coronas frequently form around xenolith boundaries where host magma chemically reacts with the foreign rock.
#16
Diamonds found within kimberlites are xenocrysts formed in the mantle keel under temperatures between 900 and 1300 degrees Celsius.
#17
Crustal xenoliths found in granitic batholiths reveal partial melting and assimilation processes during continental crust formation.
#18
Volatile degassing of carbon dioxide and water in host magmas generates the explosive velocity required to propel xenoliths to the surface.
#19
Isotopic analysis of xenoliths reveals the age and chemical depletion events of ancient continental lithospheric roots.
#20
Xenolith studies complement seismic tomography by providing ground-truth petrological specimens of deep Earth layers.
Subject Specialist Commentary
Analytical perspective & practical exam advice from the Master10 academic board
A xenolith is nature's geological messenger from the deep Earth. Because humans have only drilled roughly twelve kilometers into the crust, our understanding of the upper mantle relies heavily on rock fragments swept up by rising magma. When fast-moving volcanic eruptions shoot upward like natural elevators, they rip off pieces of mantle and deep crust, cooling quickly at the surface before the foreign rocks can melt.
For competitive exams in geography and geology, examiners frequently test the terminology separating xenoliths from xenocrysts and autoliths. Remember that a xenolith is a foreign rock fragment, a xenocryst is a foreign single crystal, and an autolith is a related piece of the same magma family. When evaluating diamond deposits, note that diamonds are xenocrysts transported by kimberlite, not formed by it. Use the mnemonic X-E-N-O: eXternal origin, Entrained in melt, Non-melted, and Old mantle sample.
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