Magmatism

The Origin of Magma

Magmas are smelted in the depth range from 15 to 250 km with partial melting of the earth's crust and mantle. Moreover, "... in natural magma foci, the fraction of the liquid phase usually does not exceed 20-30%, and in many cases is only a few percent or less. ... The temperature of silicate magmas at the time of nucleation varies from 1800-1600 to 600-5500 ° C. The maximum estimates relate to the deepest ultrabasic melts that occur during the melting of peridotites of the upper mantle, and the minimum - to the least deep acidic magmas formed in the earth's crust and enriched in water or fluorine, which significantly lower the melting temperature. " ^[one]

There are 3 main mechanisms of magma nucleation:

1. Heating above the melting temperature of the deep substance. The causes of episodic and local heating are possibly: radioactive decay of U, Th, K and / or heat from friction during plastic deformation.

2. The adiabatic rise of deep matter to the line of solidus and above.

3. Dehydration of hydroxyl- containing minerals of the deep substance. "Common minerals of this kind are, for example, mica, which when heated emit up to 4 wt.% Water. If there is water in the magmatic source, the melting point of the silicate substance decreases by tens and hundreds of degrees. ^[1]

The rise and differentiation of magma

In the nucleation regions, due to the lower density and viscosity, the melt is squeezed out of a coherent system of intergranular pores, similar to how water is squeezed out of a loose sediment at the bottom of the sea. Accumulations of a relatively light fluid have some excess pressure and begin to punch upward, independently pushing the walls of previously existing cracks. In this case, the ascent rate of not very viscous magmas can reach kilometers or even tens of kilometers per hour. The depth to which the melt can rise is determined by its total amount, the ratio of the densities of the melt and the host rocks, as well as the ratio between temperature and the content of dissolved water. ^[one]

When magma rises, it evolves towards the enrichment of later smelting with silica and lithophilic elements and depletion of mafic components ( MgO , Fe _n O _m ) as well as other mainly siderophile elements. Evolution is due to the magmatic differentiation of the initially homogeneous melt, in which there is a separation into phases of different composition and properties. This process is complicated by a number of phenomena, among which, perhaps, the main "competitor" is the assimilation by the magma of the lateral rocks of the magmas, the walls and roofs of the magma chambers.

Differentiation Mechanisms

1. crystallization differentiation - the process of phase separation of an initially homogeneous melt due to the sequential precipitation of minerals with a lower binding energy in the crystal lattice from the melt ( Bowen crystallization series ). “Usually, this differentiation occurs when fractioning crystals is carried out as a result of separation of crystalline fractions from magmatic melt (fractional crystallization). In this case, the interaction between crystals and the melt ceases. This process can be accompanied by convection and transfer of crystals to the cold parts. magmatic chambers and their deposition, sometimes rhythmic, at its bottom (convection differentiation). Removal from the melt of crystals changes its chemical composition. Due to the successively discrete formation of minerals, the composition of the melt changes is Busy discrete products and each successive melt crystallization step will vary markedly tend toward the formation of more acid and fusible GP " ^[2] ;

2. gravitational differentiation - the process of separation into phases of an initially homogeneous melt in a gravitational field. Immersion of a denser phase separated from the melt or, conversely, ascent of a less dense phase. It is characteristic of ultrabasic, basic and alkaline magmas, due to their relatively low viscosity, due to the low concentration of SiO ₂ ;

3. diffusion differentiation - the process of separation into phases of an initially homogeneous melt due to the diffusion of ions or molecules in a gravitational field, or in a temperature gradient;

4. emanation differentiation - the process of separation into phases of an initially homogeneous melt due to the emanation of light elements. It is especially characteristic in vertical magmatic columns in the presence of volatile components dissolved in the melt, in particular water;

5. segregation differentiation - the separation of the melt into two immiscible liquid phases.

"The evolution of gas. Phases and the emergence of gas. Bubbles also leads to differentiation of magma, and if crystallization has begun, this process may be accompanied by flotation of crystals." ^[2]

Complicating Phenomena

1. magmatic assimilation - “the absorption and melting by the magma of the rocks of the roof and the walls of the tank, as a result of which the magma is contaminated. M. a. Causes significant local chemical changes in the magma.” ^[2] For example, with the introduction of granite melt into limestones and their assimilation, the calcium content in the melt increases markedly. During crystallization, not acidic plagioclase will form, which is characteristic of normal granites, but more basic. As a result of the assimilation of alumina rocks by granite magma (for example, micaceous schists), high-alumina minerals such as cordierite or andalusite can appear; ^[3]

2. hybridism - the process of mixing two melts of different composition (syntax) or assimilation by a melt of a previously frozen magmatic phase. In hybrid magma, the presence of relicts of the host rocks ( xenoliths ) or their individual, usually refractory minerals ( xenocrystals ) is possible ; ^[four]

3. desiliconization - the extraction of silica from a melt due to the binding of its Mg , Ca , Fe host rocks during the introduction of magmas rich in SiO ₂ into rocks poor in this component (for example, in limestone or ultrabasite ). This leads to depletion of the melt with silica and the violation of the initially normal proportion of SiO ₂ and Al ₂ O ₃ . Alumina is in a forced excess, in connection with which minerals enriched with Al appear, and the amount of quartz decreases until it disappears completely. If the amount of alumina is particularly large, it can stand out in free form, forming corundum . ^[3]

Hardening

During the solidification of the magmatic melt, complete or partial crystallization of the substance occurs and solid bodies of igneous rocks are formed . In cases of near-surface eruptions (volcanism), the formation of rocks with porphyry or porphyry textures is characteristic, which is due to the imbalance of such a process. Cooling is often accompanied by processes of auto-metamorphism and auto-metasomatism , tectonic phenomena (the formation of calderas and ring structures, in connection with the contraction of large intrusions, etc.).

In the spreading zones, the rise and partial melting of the substance of the asthenosphere occurs. In this case, relatively light basaltic magma is melted, which then erupts in the zones of the mid-ocean ridges and back-arc basins , and the relatively heavy residual melt of peridotite sinks back. "Basaltic magma, the different crystallization forms of which are provided by rocks of the II and III layers of the oceanic crust , reveals common compositional features in all spreading zones, which served as the basis for identifying a special geochemical type of basaltoids" - BSOK ( basalts of mid-ocean ridges ) ^[5]

In the zone of the deep-sea trench, heterogeneous, consisting of a mixture of anhydrous basites , green schists , amphibolites and serpentinites , the oceanic crust subduces and undergoes a series of transformations. As they sink, the green shales turn into amphibolites, and the released water reacts with anhydrous basites to form even more amphibolites. According to A. Ringwood's model , the submerging oceanic crust is in such PT conditions that the isobaric transition of amphibolite to eclogite occurs under subsolidus conditions at rather low temperatures (<700 ° C). The released waters rise into the overlapping mantle wedge , contribute to a decrease in viscosity and cause the rise of mantle diapirs , which in turn causes their partial melting. Thus, water tholeiitic magmas are formed, the differentiation of which leads to the appearance of early tholeiitic series of island arcs . ^[6]

At depths of more than 100 km, the oceanic crust is represented by eclogite + serpentine . At a pressure of approximately 50 kbar and a temperature of about 500 ° C, serpentine decomposes into the DHMS + enstatite + water phase. At the same pressure and more and temperatures of 500 ... 1600 ° С, the DHMS phase reacts with enstatite with the formation of forsterite and water. Dehydration reactions are carried out gradually and over a wide interval of depths, because the thickness of the Qu- eclogite is heated unevenly. In the presence of water, Qu eclogite is subject to partial melting with the formation of rhyodacite magma. Coming upstairs, these magmas react with the substance of the mantle wedge and cause the rise of diapirs consisting of Ol- pyroxenite. As a result, basaltic magmas, initial for the calc-alkaline series, arise. As they rise, these magmas experience fractionation, controlled mainly by garnet , pyroxene, and amphibole . ^[6]

The relatively acidic magmas formed during all these processes are transported to the surface and, together with sedimentary rocks, join the margin of the continent, increasing the continental crust . The buildup as a result of the introduction of the material, as well as due to crowding and deformation of the rocks during compression over the subduction zones or in the collision areas, leads to an increase in radiogenic heat generated in situ. This leads to heating and, as a result, to regional metamorphism and partial melting with the formation of secondary granite magmas. The formation of mountain ranges and ridges was timed to this time. ^[6]

There are 3 types of magmatism in the place of its manifestation:

• Continental.
• Outland continental.
• Oceanic.

In their composition, different, more local subtypes are distinguished. For example: magmatism island-arc, rift , plume , hot spots and some others.

In depth of manifestation, Magmatism is divided into 4 classes:

• ultra-abyssal (very deep)
• abyssal (deep),
• hypabyssal (surface),
• surface.

According to the composition of magma into 6 species, corresponding to the silica ranges of igneous rocks .

In the modern geological era, magmatism is especially developed within the Pacific mobile belt, mid-ocean ridges , rift zones of Africa and the Mediterranean , etc. The formation of a large number of various mineral deposits is associated with magmatism.

• Petrogenesis
• Hot spot (geology)
• Igneous rocks
• Magma
• Azonality

• Hain V.E., Lomize M.G. Geotectonics with the basics of geodynamics

• Bogatikov O.A., Kovalenko V.I., Sharkov E.V. Magmatism, tectonics, geodynamics of the Earth. Communication in time and space
• Basis
• Akinini V.V. Late Mesozoic and Cenozoic magmatism and transformation of the lower crust in the northern rim of the Pacific
• Nanoinclusions of the high-barium hydrosilicate Mg3Si4O10 (OH) 2 · nH2O (10a phase) in mantle olivines: mechanisms of formation and transformation