![Crystal mush](https://upload.wikimedia.org/wikipedia/commons/b/ba/Crystal_mush.jpg)
A crystal mush is
magma
Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
that contains a significant amount of
crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
s (up to 50% of the volume) suspended in the
liquid
A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, a ...
phase (melt).
As the crystal fraction makes up less than half of the
volume
Volume is a measure of occupied three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch). Th ...
, there is no rigid large-scale three-dimensional network as in
solid
Solid is one of the State of matter#Four fundamental states, four fundamental states of matter (the others being liquid, gas, and Plasma (physics), plasma). The molecules in a solid are closely packed together and contain the least amount o ...
s. As such, their
rheological
Rheology (; ) is the study of the flow of matter, primarily in a fluid (liquid or gas) state, but also as "soft solids" or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an appli ...
behavior mirrors that of absolute liquids.
Within a single crystal mush, there is grading to a higher solid fraction towards the margins of the
pluton
In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...
, while the liquid fraction increases towards the uppermost portions, forming a liquid lens at the top.
Furthermore, depending on depth of placement crystal mushes are likely to contain a larger portion of crystals at greater depth in the crust than at shallower depth, as melting occurs from the
adiabatic decompression of the magma as it rises, this is particularly the case for
mid-ocean ridge
A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a diverge ...
s.
Seismic investigation offers strong evidence for the existence of crystal mushes rather than fully liquid magmatic bodies.
Crystal mushes can have a wide range of
mineral
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. ( ...
and
chemical composition
A chemical composition specifies the identity, arrangement, and ratio of the elements making up a compound.
Chemical formulas can be used to describe the relative amounts of elements present in a compound. For example, the chemical formula for ...
s, from
mafic
A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...
(
SiO2-poor,
MgO
Magnesium oxide ( Mg O), or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium (see also oxide). It has an empirical formula of MgO and consists of a lattice of Mg2+ ions and O2− ions ...
-rich) to
felsic (SiO
2-rich, MgO-poor).
Formation
Crystal mushes form at various depths in the
Earth's crust
Earth's crust is Earth's thin outer shell of rock, referring to less than 1% of Earth's radius and volume. It is the top component of the lithosphere, a division of Earth's layers that includes the crust and the upper part of the mantle. The ...
. They result from fractional crystallization of a fluid.
Fractional crystallization is a physical and chemical process that generates a liquid and a solid phase from a specific initial chemical solution. Depending on the initial chemical composition of the liquid, the melt is going to generate different minerals.
The initial fluid can form crystals (solid phase) by cooling down and by adding a certain water's concentration. In
subduction zones, more specific in magmatic arcs, it is possible to transport water into the Earth's
mantle, as the denser
oceanic plate
Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic c ...
subducts under the other – continental or younger oceanic – plate. Water is a key factor for this geochemical process and has a significant impact on the geotherm of the subducting plate. It causes
partial melting
Partial melting occurs when only a portion of a solid is melted. For mixed substances, such as a rock containing several different minerals or a mineral that displays solid solution, this melt can be different from the bulk composition of the soli ...
of the crust, which will then generate a chamber of liquid phase that will later be crystallized and generate minerals.
The source of water stays in minerals that contain H
2O in their chemical compositions.
Another key factor is the concentration of silica in the magma, which leads to the differentiation of magma. At the end of the crystallization is possible to crystallize quartz, but only when the melt contains a high concentration of
SiO2, which is the main component of the mineral.
The rapid increase in the crystal content over a short temperature interval generates ideal
rheological
Rheology (; ) is the study of the flow of matter, primarily in a fluid (liquid or gas) state, but also as "soft solids" or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an appli ...
conditions for melt extraction. The buoyant, lighter magmas extracted from the crystal mush can ascend through the crust and form plutonic complexes.
Extraction
The crystal mush
model
A model is an informative representation of an object, person or system. The term originally denoted the plans of a building in late 16th-century English, and derived via French and Italian ultimately from Latin ''modulus'', a measure.
Models c ...
presents a view of
pluton
In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...
s as crystal graveyards. This implies that the crystals are separated from the silicate liquid where they were crystallised. This model contrasts with the view of
intrusive magma bodies as failed eruptions. Upon cooling, a crystal mush may experience different
Igneous differentiation processes, such as crystal fractionation, mixing, melting.
To create an accumulation of crystals, there has to be a mechanism that extracts the interstitial liquid from the already crystallised solids. There is an increase in the solid portion of the magma chamber with decreasing temperature. This implies that the
permeability lowers with temperature. This also halts
convection
Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...
in the system, and the progressive accumulation of crystals increases the efficiency of expulsion of melt from the underlying parts of the
chamber
Chamber or the chamber may refer to:
In government and organizations
* Chamber of commerce, an organization of business owners to promote commercial interests
*Legislative chamber, in politics
* Debate chamber, the space or room that houses delib ...
due to loading. These mechanisms contribute to the decoupling of behaviour between crystals and liquid, enabling the liquid to percolate upwards.
This extraction mechanism, however, operates in an optimal interval of crystal fraction.
If there is a low crystal fraction, convection still operates in the system, thus halting crystal settling and liquid extraction. However, if the crystal fraction is very high, the system starts behaving like a solid within the timescales of applied stress in the system (
Maxwell time
A Maxwell material is the most simple model viscoelastic material showing properties of a typical liquid. It shows viscous flow on the long timescale, but additional elastic resistance to fast deformations. It is named for James Clerk Maxwell wh ...
).
Eruption
Since magma comprise different compositional fractions, it may undergo different processes like melt extraction, phase separation, water and gas enrichment, and injection of magma from deeper magma chambers (typically within the lower
crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet, dwarf planet, or natural satellite. It is usually distinguished from the underlying mantle by its chemical makeup; however, in the case of icy satellites, it may be distingu ...
). All these may directly or indirectly cause the
eruption
Several types of volcanic eruptions—during which lava, tephra (ash, lapilli, volcanic bombs and volcanic blocks), and assorted gases are expelled from a volcanic vent or fissure—have been distinguished by volcanologists. These are often ...
events.
One of the factors that can initiate magma eruption is phase separation of the liquid and crystal components of the crystal mush. As the magma develops over time and the crystal content of the magma increases, phase separation is taking place and the liquid phase of the magma is pushed up, driven by its buoyancy as a result of its lower density.
Volcano
A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.
On Earth, volcanoes are most often found where tectonic plates are ...
es, as valves of the open system, provide the path for gas release and magma eruption. The amount of dissolved gases may be a further factor that controls the eruption event. The deeper the magma chamber is located, the higher is the amount of gas that can be dissolved in the magma (high pressure conditions), especially in
andesitic
Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomin ...
and
rhyolitic
Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...
magmas. As phase separation occurs and the liquid fraction increases along with decreasing pressure, the emission of gas increases. This process is expressed by a high fraction of bubbles that drive the liquid phase toward the earth surface. In addition, the higher the content of dissolved water and other gases, the more violent the eruption will be.
The last and the most trivial cause for magma eruption is an injection of fresh magma from lower parts of the crust into the issued magma chamber, which increases the content of the liquid phase, and consequently, the pressure inside the chamber, which is concurrently released as a flux of lava onto the earth surface. The “crystal mush” is a leading and most promising model
of magma bodies, that supported by findings (
ignimbrite
Ignimbrite is a type of volcanic rock, consisting of hardened tuff. Ignimbrites form from the deposits of pyroclastic flows, which are a hot suspension of particles and gases flowing rapidly from a volcano, driven by being denser than the surrou ...
s) on the surface, although there are some controversial aspects.
Ore deposits
Magmas containing
volatiles
Volatiles are the group of chemical elements and chemical compounds that can be readily vaporized. In contrast with volatiles, elements and compounds that are not readily vaporized are known as refractory substances.
On planet Earth, the term ' ...
are stable at high
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
s in the deep crust; when this mixture of magma and volatiles rises though the crust the pressure decreases and the volatiles start exsolving from the magma. This leads to
oversaturation of volatiles in magma. Also crystallization of dry minerals within the magma and crystal mush will progressively increase the fluid concentration of the melt, possibly leading to saturation. These fluids, lighter than the magma they were once in, exsolve and rise up to even shallower crust; potentially forming
ore deposit
Ore is natural rock or sediment that contains one or more valuable minerals, typically containing metals, that can be mined, treated and sold at a profit.Encyclopædia Britannica. "Ore". Encyclopædia Britannica Online. Retrieved 7 April ...
s. If these volatiles are sufficiently concentrated to form
ore minerals and if they are trapped by the surrounding host rocks in the
continental crust
Continental crust is the layer of igneous, sedimentary, and metamorphic rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called '' sial'' be ...
within a narrow enough space, they can form economically valuable ore deposits.
Some magmatic chambers are also more predisposed to form large ore deposits, due to regional setting and a combination of factors favorable to ore formation.
A key factor for magma saturation and volatile formation is the
sulphide saturation in the original magma.
High solubility and high concentration of
sulphur
Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
in magma lead to high sulphide saturation and could be an important factor in formation of big ore deposits.
This saturated sulphide in melt can enrich the concentration of metals in the magmatic derived fluids, e.g.,
hydrothermal fluid
Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...
s. These can then rise from the
magmatic chamber and intrude in the continental crust and deposit their dissolved metals in the crust.
Micro-textural and geochemical analyses are interpreted to directly link ore formation to the flow of mineralising fluids through palaeo-
porosity
Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...
within once
permeable
Permeability, permeable, and semipermeable may refer to:
Chemistry
*Semipermeable membrane, a membrane which will allow certain molecules or ions to pass through it by diffusion
*Vascular permeability, the movement of fluids and molecules betwe ...
crystal mush dykes. It is believed these crystal mush dykes acted as conduits for
porphyry copper deposit
Porphyry copper deposits are copper ore bodies that are formed from hydrothermal fluids that originate from a voluminous magma chamber several kilometers below the deposit itself. Predating or associated with those fluids are vertical dikes of ...
mineralising fluids from deep portions of underlying magmas.
References
{{reflist
Igneous petrology