Xenolith

Xenocrysts are "foreign" minerals incorporated into the magma during magma scent or during xenoliths fragmentation. There are three principal types:

1) Restite material, the refractory residue brought up from the site of mantle ore crustal melting.
2) Totally exotic material, ripped off the walls of conduits or the walls or roof of a magma chamber.
3) Cognate material, representing perhaps an accumulation of crystals already formed in a magma chamber, later ripped up during fresh magma movement.

Xenolith that are not in equilibrium with the magma may melt, dissolve or react, and these processes may lead to their eventual assimilation. Therefore the absence of xenolith does not necessarily indicate that no contamination of magma occurred. It may in fact mean that numerous xenoliths were totally assimilated. On the other hand, the presence of numerous xenoliths does not necessarily imply the magma has been severely contaminated. They may have been incorporated without much reaction just before magma solidification.

Xenoliths can be assimilated in three ways:

Melting: Take for example a quartz-feldspar sandstone incorporated in an intermediate magma at 800°C and with a water pressure of 2Kbar; quartz, plagioclase and Orthose individually melt at temperature in excess of 800°C, but minimum-melting-point mixtures of quartz with either one or both feldspar, or feldspar with feldspar all form at temperature <800°C. this means that melting could occur everywhere quartz and feldspar or two feldspar are in contact (See image 1) The important point to note is that melting occurs through the entire xenolith at appropriate sites (in contrast solution can occur at the margins of a xenolith). Evidence of melting is sometime preserved as a film of glass along grain boundaries, and if reaction take place in association with the melting , a spongy (Sieve) texture also develops along xenolith margins.

Solution: Solution differs from melting in that it takes place only where the xenolith contacts magma (at its margins). The process will not in itself lead to a disaggregation of a xenolith. Xenocrysts wich undergo such solution become rounded just like some phenocrysts during magma mixing.

Reaction: Like solution, sometime take place only where xenolith contacts magma. For example, the reaction of quartz with basic magma produces pyroxenes, usually as a fine-grained aggregate encrusting the margins of xenolith or xenocrysts.

Depending on foreign material and magma composition and temperature and available time, the foreign material chemically equilibrates with the melt to varying degree. The thermal and chemical principles of assimilation were enounced by bowen (1928) many decades ago.

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Melting Phenomena in a Xenolith containing Quartz (Qz) and Albite (Ab). Melt along grain boundaries is stippled. From Shelley




Assimilation require thermal energy, the source of which can only be the magma itself. Heat from the magma has two sources:

• That released during cooling to lower T
• The latent heat of crystallization

The fate of assimilated crystals depend on their composition and that of the parent melt. Provided sufficient heat is available, a crystalline phase dissolves if the silicate melt is not already saturated with respect to that phase. Thus, quartz xenocrysts can dissolve on a time scale of day in basaltic melts in which the activity of silica in < 1, the basalt become so enriched in silica. The presence of xenocrysts or xenoliths in a magmatic rock may suggest they are contaminants, but xenocrysts can also originate by mixing of dissimilar magmas, and foreign material can be incorporated late into the magma with minimal contamination.

Some basaltic magmas are extruded on the Earth’s surface carrying pieces of the mantle, called mantle xenoliths. These xenoliths make it possible to estimate how rapidly these magmas ascended from depth to the surface, because the upward flow of magma must exceed the settling velocity of the xenoliths. Two magma types, alkali basalt and kimberlite, commonly carry such mantle xenoliths. Both originate by small degrees of partial melting in the upper mantle near the base of the lithosphere.

The common occurrence of dense mantle xenoliths in tephra and lava flow eruptions of alkali basalt indicates that this magma carried them from depth. An average settling rate for xenoliths of different sizes can be calculated by balancing the different forces on the xenoliths during ascent, and by making certain assumptions about the rheologic properties of the magma. Calculation indicates that a 20-cm-wide xenolith will settle at a rate of 0.1 m/s. Obviously, the magma ascent rate would have to be greater in order to carry the xenolith to the surface; the xenolith settling velocity gives a minimum estimate of magma ascent rate.

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Dunitic Xenoliths in a basalto, San Carlos Indian Reservation, Arizona. From Natalie Teager, Arizona State University.


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Sandstone Xenolith in Basalt, Coconino County, Arizona. From Natalie Teager, Arizona State University.




Bibliography



• E. WM. Heinrich (1956): Microscopic Petrografy. Mcgraw-hill book company,inc
• David Shelley (1983): Igneous and metamorphic rocks under the microscope. Campman & Hall editori.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press
• Shelley D (1992): Igneous and Metamorphic Rocks under the Microscope: Classification, textures, microstructures and mineral preferred orientation
• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.

Photo
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Peridotitic xenolith in a basalt from Iblei (Sicily, Italy). XPL image, 2x (Field of view = 7mm)
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Peridotitic xenolith (with sieve texture due to reaction) in a basalt from Iblei (Sicily, Italy). XPL image, 2x (Field of view = 7mm)
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Peridotitic xenolith (with sieve texture due to reaction) in a basalt from Iblei (Sicily, Italy). PPL image, 2x (Field of view = 7mm)
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Peridotitic xenolith (with sieve texture due to reaction) in a basalt from Iblei (Sicily, Italy). PPL image, 2x (Field of view = 7mm)
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Peridotitic xenolith (with sieve texture due to reaction) in a basalt from Iblei (Sicily, Italy). XPL image, 2x (Field of view = 7mm)
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Peridotitic xenolith (with kink bands in olivine) in a basalt from Iblei (Sicily, Italy). XPL image, 2x (Field of view = 7mm)
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Peridotitic xenolith in a alkali basalt from Sardinia (Italy). PPL image, 2x (Field of view = 7mm)
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Peridotitic xenolith in a alkali basalt from Sardinia (Italy). XPL image, 2x (Field of view = 7mm)
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Peridotitic xenolith in a alkali basalt from Sardinia (Italy). XPL image, 2x (Field of view = 7mm)
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Peridotitic xenolith in a alkali basalt from Sardinia (Italy). PPL image, 2x (Field of view = 7mm)
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Peridotitic xenolith in a alkali basalt from Sardinia (Italy). XPL image, 2x (Field of view = 7mm)
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Peridotitic xenolith in a alkali basalt from Sardinia (Italy). XPL image, 2x (Field of view = 7mm)
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Volcanic xenolith in an ignimbrite. PPL image, 10x (Field of view = 2mm)
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Volcanic xenolith in an ignimbrite. PPL image, 10x (Field of view = 2mm)
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Volcanic xenolith in an ignimbrite. PPL image, 10x (Field of view = 2mm)
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Volcanic xenolith in an ignimbrite. PPL image, 10x (Field of view = 2mm)
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Volcanic xenolith in an ignimbrite. XPL image, 10x (Field of view = 2mm)
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Volcanic xenolith in an ignimbrite. XPL image, 10x (Field of view = 2mm)
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Volcanic xenoliths in an ignimbrite. PPL image, 10x (Field of view = 2mm)
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Volcanic xenoliths in an ignimbrite. XPL image, 10x (Field of view = 2mm)
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Volcanic xenolith in an ignimbrite. PPL image, 10x (Field of view = 2mm)
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Peridotitic xenolith in a vescicolated basalt from Etna volcano (Italy). PPL image, 2x (Field of view = 7mm)
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Peridotitic xenolith in a vescicolated basalt from Etna volcano (Italy). XPL image, 2x (Field of view = 7mm)
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Peridotitic xenolith in a vescicolated basalt from Etna volcano (Italy). PPL image, 2x (Field of view = 7mm)
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Peridotitic xenolith in a vescicolated basalt from Etna volcano (Italy). XPL image, 2x (Field of view = 7mm)
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Peridotitic xenolith in a vescicolated basalt from Etna volcano (Italy). PPL image, 2x (Field of view = 7mm)
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Peridotitic xenolith in a vescicolated basalt from Etna volcano (Italy). XPL image, 2x (Field of view = 7mm)
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Peridotitic xenolith in a vescicolated basalt from Etna volcano (Italy). PPL image, 2x (Field of view = 7mm)
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Peridotitic xenolith in a vescicolated basalt from Etna volcano (Italy). XPL image, 2x (Field of view = 7mm)
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Deformad olivine crystals in a peridotitic xenolith from Etna volcano (Italy). XPL image, 2x (Field of view = 7mm)
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Deformad olivine crystals in a peridotitic xenolith from Etna volcano (Italy). XPL image, 2x (Field of view = 7mm)
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Deformad olivine crystals in a peridotitic xenolith from Etna volcano (Italy). XPL image, 2x (Field of view = 7mm)
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Deformad olivine crystals in a peridotitic xenolith from Etna volcano (Italy). XPL image, 2x (Field of view = 7mm)
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Deformad olivine crystals in a peridotitic xenolith from Etna volcano (Italy). XPL image, 10x (Field of view = 2mm)
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Deformad olivine crystals in a peridotitic xenolith from Etna volcano (Italy). XPL image, 10x (Field of view = 2mm)
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Deformad olivine crystals in a peridotitic xenolith from Etna volcano (Italy). XPL image, 10x (Field of view = 2mm)