Xenocrysts refer to mineral crystals trapped or caught up in an igneous rock. These crystals are foreign (alien) to the rock they occur in.
By foreign, we mean they didn’t crystallize from the melt that formed the rocks that have them. Thus, their origin is not the same as the igneous rock. Instead, they are incorporated into the melt.
Usually, these crystals and xenoliths are entrained (carried along) and transported by magma. Their origin varies, and they are important to geologists in many ways.
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Related terms
Before proceeding, you must be familiar with terms like xenoliths, phenocryst, inclusions, and megacrysts. You are likely to often come across them when talking about xenocrysts.
Let us define them. They will make the rest of the discussion a breeze.
1. Xenoliths
These are pieces of rocks trapped in different igneous rocks. Most are trapped or embedded in magma as it cools. Xenoliths differ from xenocrysts since they are rock pieces; the latter are mineral crystals.
2. Phenocryst
Phenocrysts are larger mineral crystals embedded in a groundmass. They form from the same melt and have a similar composition as the groundmass.
Usually, they crystallized earlier under conditions that allowed them to grow large. They are what create porphyritic textures.
3. Inclusions
Inclusions refer to materials occurring within a gemstone or mineral. These materials are trapped during crystallization. In gemology, inclusions may include materials reaching the gemstone surface from the inside.
4. Megacrysts
These are grains or crystals larger than the surrounding matrix. They occur both in metamorphic and igneous rocks.
Megacrysts are phenocrysts that occur in igneous rocks and porphyroblasts in metamorphic rock.
Where do xenocrysts come from?
Xenocrysts in igneous rocks are from 1) country rocks, 2) previously crystallized crystals, 3) entrained refractory materials, or 4) mixing of different magmas.
1. Country rock
Xenocrysts may be torn from country rock as magma rises or during emplacement. However, in most cases, they come from the disaggregation of entrained xenoliths or foreign rock bodies.
Disaggregation is the breaking down or fragmenting rock into individual crystals or smaller pieces. These rocks may be igneous, metamorphic, or sedimentary.
2. Previously crystallized part of the same igneous body
Sometimes, xenocrysts may come from previously formed crystals but from the same igneous body. Such are cognate or related genetically to the rocks in which they occur.
These earlier-formed crystals are ripped as fresh magma moves. They may dissolve or remain. This depends on their composition, the magma composition, and prevailing conditions.
3. Refractory materials (restite crystals)
Rising magma may also entrain residue or refractory remaining rock or crystals after partial melting.
4. Magma mixing
Different magma mixing may introduce xenocrysts to rocks. Examining these crystals in a microscope will help geologists distinguish them from phenocrysts.
Characteristics
Xenocrysts will not be in equilibrium with the melt. Also, these crystals may partially melt or undergo alteration and be mechanically modified.
1. Not in equilibrium
Xenocrysts are usually not in equilibrium with the melt or rock they occur in. This makes them different from phenocrysts, which will be in equilibrium with the magma that formed them.
Put differently, their composition is different. This composition is such that you don’t expect them to crystalize from such melts.
However, some may try to equilibrate to varying degrees. This depends on the xenocrysts present, temperature, and magma composition.
2. Are partly melted or altered
Xenocrysts may react, melt, dissolve, resorb, or lose volatiles in magma. Melting will make them lose their properties and occur if prevailing temperatures are favorable.
On the other hand, partial chemical dissolution will cause etch pits and erode them. Thin sections will reveal partial resorption or dissolutions.
For instance, quartz xenocrysts from wall rock will dissolve if in silica-undersaturated magmas like basaltic melt. This is called assimilation. It contaminates the magma, resulting in a diverse composition.
Some may only be partially resorbed. This will have quartz xenocrysts surrounded by rims of clinopyroxene.
3. Undergo mechanical modification
Also, mechanical modification may occur. For instance, attrition from rapid ascending may cause rounding, breakages, pitting, or resurfacing of xenoliths or xenocrysts.
Surface textures that form will confirm these processes. For instance, olivine xenocrysts show transport rounding in kimberlites.
Examples of xenocrysts
Examples of xenocrysts and which rocks or melts they may occur are:
- Diamonds in Kimberlite diatremes
- Quartz in silica-deficient lava
- Sapphire in basaltic rocks
- Zircon from mantle in some granitoids and some mid-ocean ridges.
How are xenocrysts important?
Xenocrysts and xenoliths help geologists better understand the earth’s interior or mantle, which is inaccessible. Mantle-derived rocks include kimberlites, basalts, lamproites, or lamprophyres.
Studying xenoliths and xenocrysts in these rocks can provide information on the mantle’s composition, structure, stability, and thermal state. Also, you will know how the lithosphere formed and the chemical processes inside the mantle.
Furthermore, it is possible to know the age of the mantle using radiometric dating techniques.
That is not all. Studying the chemical properties of xenocrysts and xenoliths can reveal depths and, consequently, the pressure of their formation.
Aluminum-bearing xenocrysts are particularly important in providing the depth of their formation. For instance, calcic plagioclase is stable at depths below 25km (16 mi), spinel at 25-65 km (16-37 mi), and above 65 km, garnet will be the aluminum-bearing mineral.
Also, we can learn about transport conditions, including speeds, by looking at xenoliths and xenocrysts. For instance, rapid upward movement will fragment xenoliths, releasing crystals. Also, rounding will occur in both.
For instance, we know diamond-bearing kimberlites move rapidly to the surface. A slow movement would convert diamonds to graphite.
However, knowing where the xenoliths and phenocrysts come from in the mantle is a challenge. Also, we may not know how they are related to each other genetically.
Lastly, some xenocrysts are valuable gemstones or minerals. Examples are diamonds and peridots.