Definition and Formation of Xenoliths
- Xenoliths are inclusions in igneous rock formed during magma ascent, emplacement, and eruption.
- They can be engulfed along the margins of a magma chamber, torn loose from erupting lava conduits or explosive diatremes, or picked up along the base of flowing lava.
- Xenocrysts are individual foreign crystals included within an igneous body.
- Examples of xenocrysts include quartz crystals in silica-deficient lava and diamonds within kimberlite diatremes.
- Xenoliths can vary in composition even within limited areas, such as the rhyolite-dominated lava of Niijima volcano in Japan.
Xenoliths in Relation to Sedimentary Rocks and Meteorites
- The term xenolith can also include rock fragments encased in sedimentary rock.
- Xenoliths have been found in some meteorites.
- To be considered a true xenolith, the included rock must be noticeably different from the enveloping rock.
- An included rock of similar type is called an autolith or a cognate inclusion.
- Xenoliths and xenocrysts provide valuable information about the composition of the inaccessible mantle.
Xenoliths as Indicators of Mantle Composition
- Basalts, kimberlites, lamproites, and lamprophyres often contain fragments and crystals assumed to be from the originating mantle.
- Xenoliths of dunite, peridotite, and spinel lherzolite in basaltic lava flows are examples.
- Kimberlites contain diamond xenocrysts and fragments of lherzolites with varying composition.
- The aluminum-bearing minerals in these fragments provide clues to the depth of origin.
- Some kimberlites contain xenoliths of eclogite, which is a high-pressure metamorphic product of basaltic oceanic crust.
Roof Pendants and Examples of Xenoliths
- The large-scale inclusion of foreign rock strata at the margins of an igneous intrusion is called a roof pendant.
- Examples of xenoliths include those found in granodiorite of the Alta Stock in Utah, sandstone within the Fairlee Pluton in Vermont, and peridotite mantle xenoliths in volcanic bombs from Vulkaneifel, Germany.
- Other examples include weathered peridotite xenoliths in nephelinite lava flows at Kaiserstuhl, SW Germany, and Baltimore Gneiss in the Guilford Quartz Monzonite in Granite, Maryland.
- Lamprophyres with xenoliths can also be found in Ontario, Canada, and granite near Donner Pass, California.
References and External Links
- Hansteen and Troll (2003) studied the oxygen isotope composition of xenoliths from Gran Canaria, providing insights into crustal contamination of ascending magmas.
- Arakawa et al. (2017) examined gabbroic xenoliths from Niijima volcano, contributing to petrological and geochemical constraints.
- Troll et al. (2013) investigated inclusion petrology and oxygen isotopes in xenoliths from Merapi Volcano.
- Additional references include National Geographic Society (2011) and Komov et al. (1994).
- Wikimedia Commons has media related to xenoliths.
Xenolith Data Sources