Crystal mush

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During cooling, crystals will form from the melt. Thus, the crystal/melt ratio increases, generating a magma, a crystal-mush, and finally a cumulate rock. Crystal mush.jpg
During cooling, crystals will form from the melt. Thus, the crystal/melt ratio increases, generating a magma, a crystal-mush, and finally a cumulate rock.

A crystal mush is magma that contains a significant amount of crystals (up to 50% of the volume) suspended in the liquid phase (melt). [1] As the crystal fraction makes up less than half of the volume, there is no rigid large-scale three-dimensional network as in solids. [2] As such, their rheological behavior mirrors that of absolute liquids.

Contents

Within a single crystal mush, there is grading to a higher solid fraction towards the margins of the pluton, while the liquid fraction increases towards the uppermost portions, forming a liquid lens at the top. [1] 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 ridges. [3]

Seismic investigation offers strong evidence for the existence of crystal mushes rather than fully liquid magmatic bodies. [1]

Crystal mushes can have a wide range of mineral and chemical compositions, from mafic (SiO2-poor, MgO-rich) to felsic (SiO2-rich, MgO-poor).

Formation

Crystal mushes form at various depths in the Earth's crust. 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. [4]

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 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 of the crust, which will then generate a chamber of liquid phase that will later be crystallized and generate minerals. [5]

The source of water stays in minerals that contain H2O 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. [5]

The rapid increase in the crystal content over a short temperature interval generates ideal rheological conditions for melt extraction. The buoyant, lighter magmas extracted from the crystal mush can ascend through the crust and form plutonic complexes. [5]

Extraction

The crystal mush model presents a view of plutons as crystal graveyards. [6] 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. [7] Upon cooling, a crystal mush may experience different Igneous differentiation processes, such as crystal fractionation, mixing, melting. [8]

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 in the system, and the progressive accumulation of crystals increases the efficiency of expulsion of melt from the underlying parts of the chamber 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. [9] 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).

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)). All these may directly or indirectly cause the eruption events. [10]

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. Volcanoes, 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 and rhyolitic 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 [9] [11] of magma bodies, that supported by findings (ignimbrites) on the surface, although there are some controversial aspects. [12] [13]

Ore deposits

Magmas containing volatiles are stable at high pressures 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. [14] 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 deposits. If these volatiles are sufficiently concentrated to form ore minerals and if they are trapped by the surrounding host rocks in the continental crust within a narrow enough space, they can form economically valuable ore deposits. [15] 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. [15]

A key factor for magma saturation and volatile formation is the sulphide saturation in the original magma. [15] High solubility and high concentration of sulphur in magma lead to high sulphide saturation and could be an important factor in formation of big ore deposits. [15] This saturated sulphide in melt can enrich the concentration of metals in the magmatic derived fluids, e.g., hydrothermal fluids. 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 within once permeable crystal mush dykes. It is believed these crystal mush dykes acted as conduits for porphyry copper deposit mineralising fluids from deep portions of underlying magmas. [16]

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Granite Common type of intrusive, felsic, igneous rock with granular structure

Granite is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies underground. It is common in the continental crust of Earth, where it is found in igneous intrusions. These range in size from dikes only a few centimeters across to batholiths exposed over hundreds of square kilometers.

Magma Hot semifluid material found beneath the surface of Earth

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 satellites. Besides molten rock, magma may also contain suspended crystals and gas bubbles.

Pegmatite Igneous rock with very large interlocked crystals

A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than 1 cm (0.4 in) and sometimes greater than 1 meter (3 ft). Most pegmatites are composed of quartz, feldspar, and mica, having a similar silicic composition to granite. However, rarer intermediate composition and mafic pegmatites are known.

Andesite Type of volcanic rock

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 predominantly of sodium-rich plagioclase plus pyroxene or hornblende.

Anorthosite Mafic intrusive igneous rock composed predominantly of plagioclase

Anorthosite is a phaneritic, intrusive igneous rock characterized by its composition: mostly plagioclase feldspar (90–100%), with a minimal mafic component (0–10%). Pyroxene, ilmenite, magnetite, and olivine are the mafic minerals most commonly present.

Magma chamber Accumulation of molten rock within the Earths crust

A magma chamber is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma, in such a chamber is less dense than the surrounding country rock, which produces buoyant forces on the magma that tend to drive it upwards. If the magma finds a path to the surface, then the result will be a volcanic eruption; consequently, many volcanoes are situated over magma chambers. These chambers are hard to detect deep within the Earth, and therefore most of those known are close to the surface, commonly between 1 km and 10 km down.

Komatiite Ultramafic mantle-derived volcanic rock

Komatiite is a type of ultramafic mantle-derived volcanic rock defined as having crystallised from a lava of at least 18 wt% MgO. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. Komatiite was named for its type locality along the Komati River in South Africa, and frequently displays spinifex texture composed of large dendritic plates of olivine and pyroxene.

Layered intrusion

A layered intrusion is a large sill-like body of igneous rock which exhibits vertical layering or differences in composition and texture. These intrusions can be many kilometres in area covering from around 100 km2 (39 sq mi) to over 50,000 km2 (19,000 sq mi) and several hundred metres to over one kilometre (3,300 ft) in thickness. While most layered intrusions are Archean to Proterozoic in age, they may be any age such as the Cenozoic Skaergaard intrusion of east Greenland or the Rum layered intrusion in Scotland. Although most are ultramafic to mafic in composition, the Ilimaussaq intrusive complex of Greenland is an alkalic intrusion.

Cumulate rock

Cumulate rocks are igneous rocks formed by the accumulation of crystals from a magma either by settling or floating. Cumulate rocks are named according to their texture; cumulate texture is diagnostic of the conditions of formation of this group of igneous rocks. Cumulates can be deposited on top of other older cumulates of different composition and colour, typically giving the cumulate rock a layered or banded appearance.

In geology, igneous differentiation, or magmatic differentiation, is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process, cooling, emplacement, or eruption. The sequence of magmas produced by igneous differentiation is known as a magma series.

Compatibility (geochemistry) Partitioning of elements in a mineral

Compatibility is a term used by geochemists to describe how elements partition themselves in the solid and melt within Earth's mantle. In geochemistry, compatibility is a measure of how readily a particular trace element substitutes for a major element within a mineral.

Fractional crystallization (geology) Process of rock formation

Fractional crystallization, or crystal fractionation, is one of the most important geochemical and physical processes operating within crust and mantle of a rocky planetary body, such as the Earth. It is important in the formation of igneous rocks because it is one of the main processes of magmatic differentiation. Fractional crystallization is also important in the formation of sedimentary evaporite rocks.

Merensky Reef Layer of igneous rock in the Bushveld igneous complex, South Africa

The Merensky Reef is a layer of igneous rock in the Bushveld Igneous Complex (BIC) in the North West, Limpopo, Gauteng and Mpumalanga provinces of South Africa which together with an underlying layer, the Upper Group 2 Reef (UG2), contains most of the world's known reserves of platinum group metals (PGMs) or platinum group elements (PGEs)—platinum, palladium, rhodium, ruthenium, iridium and osmium. The Reef is 46 cm thick and bounded by thin chromite seams or stringers. The composition consists predominantly of cumulate rocks, including leuconorite, anorthosite, chromitite, and melanorite.

Melt inclusion

A melt inclusion is a small parcel or "blobs" of melt(s) that is entrapped by crystals growing in magma and eventually forming igneous rocks. In many respects it is analogous to a fluid inclusion within magmatic hydrothermal systems. Melt inclusions tend to be microscopic in size and can be analyzed for volatile contents that are used to interpret trapping pressures of the melt at depth.

Magmatic water, also known as juvenile water, is an aqueous phase in equilibrium with minerals that have been dissolved by magma deep within the Earth's crust, and is released to the atmosphere during a volcanic eruption. It plays a key role in assessing the crystallization of igneous rocks, particularly silicates, as well as the rheology and evolution of magmatic chambers. Magma is composed of minerals, rocks and volatile organic compounds (VOCs) in varying relative abundances. Magmatic differentiation varies significantly based on various factors, most notably the presence of water. An abundance of VOCs within magma chambers decreases viscosity and leads to the formation of minerals bearing halogens, including Cl−, and OH− groups. In addition, the relative abundance of VOCs varies within basaltic, andesitic, and rhyolitic magma chambers, leading to some volcanoes being exceedingly more explosive than others. Magmatic water is practically insoluble in silicate melts, but has demonstrated the highest solubility within rhyolitic melts. An abundance of magmatic water has been shown to lead to high-grade deformation, altering the amount of δ18O and δ2H within host rocks.

Igneous rock Rock formed through the cooling and solidification of magma or lava

Igneous rock, or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava.

Tectonic–climatic interaction is the interrelationship between tectonic processes and the climate system. The tectonic processes in question include orogenesis, volcanism, and erosion, while relevant climatic processes include atmospheric circulation, orographic lift, monsoon circulation and the rain shadow effect. As the geological record of past climate changes over millions of years is sparse and poorly resolved, many questions remain unresolved regarding the nature of tectonic-climate interaction, although it is an area of active research by geologists and palaeoclimatologists.

Magmatic underplating Trapping of basaltic magmas within the crust

Magmatic underplating occurs when basaltic magmas are trapped during their rise to the surface at the Mohorovičić discontinuity or within the crust. Entrapment of magmas within the crust occurs due to the difference in relative densities between the rising magma and the surrounding rock. Magmatic underplating can be responsible for thickening of the crust when the magma cools. Geophysical seismic studies utilize the differences in densities to identify underplating that occurs at depth.

Mark S. Ghiorso American geochemist

Mark S. Ghiorso is an American geochemist who resides in Seattle, Washington. He is best known for creating MELTS, a software tool for thermodynamic modeling of phase equilibria in magmatic systems.

The lower oceanic crust is the lower part of the oceanic crust and represents the major part of it. It is generally located 4–8 km below the ocean floor and the major lithologies are mafic which derive from melts rising from the earth's mantle. This part of the oceanic crust is an important zone for processes such as melt accumulation and melt modification. And the recycling of this part of the oceanic crust, together with the upper mantle has been suggested as a significant source component for tholeiitic magmas in Hawaiian volcanoes. Although the lower oceanic crust builds the link between the mantle and the MORB, and can't be neglected for the understanding of MORB evolution, the complex processes operating in this zone remain unclear and there is an ongoing debate in Earth Sciences about this. It is 6KM long.

References

  1. 1 2 3 Cooper, Kari M. (February 2017). "What Does a Magma Reservoir Look Like? The 'Crystal's-Eye' View". Elements . 13: 23–28. doi:10.2113/gselements.13.1.23.
  2. ↑ Winter, John D. (2014). Principles of Igneous and Metamorphic Petrology. Essex: Pearson Education Limited. pp. 213, 217. ISBN   978-1-292-02153-9.
  3. ↑ Philpotts, Anthony R.; Ague, Jay J. (2009). Principles of Igneous and Metamorphic Petrology. Cambridge: Cambridge University Press. p. 16. ISBN   978-0-521-88006-0.
  4. ↑ Emeleus, C. H.; Troll, V. R. (August 2014). "The Rum Igneous Centre, Scotland". Mineralogical Magazine. 78 (4): 805–839. doi: 10.1180/minmag.2014.078.4.04 . ISSN   0026-461X.
  5. 1 2 3 Müntener, Othmar; Ulmer, Peter; Nandedkar, Rohit H. (2014-06-01). "Fractional crystallization of primitive, hydrous arc magmas: an experimental study at 0.7 GPa" (PDF). Contributions to Mineralogy and Petrology. 167 (6): 1015. doi:10.1007/s00410-014-1015-5. hdl: 20.500.11850/87224 . ISSN   1432-0967.
  6. ↑ Gelman, Sarah; Deering, Chad; Bachmann, Olivier; Huber, Christian; Gutiérrez, Francisco (2014-10-01). "Identifying the crystal graveyards remaining after large silicic eruptions". Earth and Planetary Science Letters. 403: 299–306. doi:10.1016/j.epsl.2014.07.005. ISSN   0012-821X.
  7. ↑ Glazner, Allen F.; Coleman, Drew S.; Mills, Ryan D. (2018), "The Volcanic-Plutonic Connection", Physical Geology of Shallow Magmatic Systems, Springer International Publishing, pp. 61–82, doi:10.1007/978-3-319-14084-1_11, ISBN   9783319140834
  8. ↑ J. Leuthold, J. C. Lissenberg, B. O'Driscoll, O. Karakas; T. Falloon, D.N. Klimentyeva, P. Ulmer (2018); Partial melting of the lower oceanic crust at spreading ridges. Frontiers in Earth Sciences: Petrology: 6(15): 20p; doi : 10.3389/feart.2018.00015
  9. 1 2 Bachmann, Olivier; Bergantz, George W. (2004). "On the Origin of Crystal-poor Rhyolites: Extracted from Batholithic Crystal Mushes". Journal of Petrology. 45 (8): 1565–1582. doi: 10.1093/petrology/egh019 . ISSN   0022-3530.
  10. ↑ Troll, V. R.; Mattsson, T.; Upton, B. G. J.; Emeleus, C. H.; Donaldson, C. H.; Meyer, R.; Weis, F.; Dahrén, B.; Heimdal, T. H. (2021-02-23). "Fault-Controlled Magma Ascent Recorded in the Central Series of the Rum Layered Intrusion, NW Scotland". Journal of Petrology. 61 (10). doi: 10.1093/petrology/egaa093 . ISSN   0022-3530.
  11. ↑ Hildreth, W.S. (2004), https://doi.org/10.1016/j.jvolgeores.2004.05.019
  12. ↑ Glazner, A. F., Coleman, D. S., & Bartley, J. M. (2008), The tenuous connection between high-silica rhyolites and granodiorite plutons. Geology, 36(2), 183–186. https://doi.org/10.1130/G24496A.1
  13. ↑ Simakin, A.G., and Bindeman, I.N. (2012), Remelting in caldera and rift environments and the genesis of hot, recycled, rhyolites. Earth and Planetary Science Letters, 337–338, 224–235. 10.1016/j.epsl.2012.04.011
  14. ↑ "Volatiles: oversaturation and magma movements". Big Idea : Magma moves in fits and starts. 2011-12-16. Retrieved 2018-12-17.
  15. 1 2 3 4 Wilkinson, Jamie J. (2013). "Triggers for the formation of porphyry ore deposits in magmatic arcs". Nature Geoscience. 6 (11): 917–925. doi:10.1038/ngeo1940. hdl: 10141/622499 . ISSN   1752-0908.
  16. ↑ Carter, Lawrence C.; Williamson, Ben J.; Tapster, Simon R.; Costa, Catia; Grime, Geoffrey W.; Rollinson, Gavyn K. (2021-03-17). "Crystal mush dykes as conduits for mineralising fluids in the Yerington porphyry copper district, Nevada". Communications Earth & Environment. 2 (1): 1–11. doi: 10.1038/s43247-021-00128-4 . ISSN   2662-4435.
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