Clinopyroxene thermobarometry

Last updated September 05, 2022

Clinopyroxene thermobarometry is a scientific method that uses the mineral clinopyroxene to determine the temperature and pressure of the magma when the mineral crystalized. Clinopyroxene is found in many igneous rocks, so the method can be used to determine information about the entire rock. Many different minerals can be used for geothermobarometry, but clinopyroxene is especially useful because it's a common phenocryst in igneous rocks and easy to identify, and the crystallization of jadeite, a type of clinopyroxene, implies a growth in molar volume, making it a good indicator of pressure.^[1]

The data given by this technique is used for understanding magmatic crystallization, prograde and retrograde metamorphism, and ore deposit formation.^[2] Understanding these processes can aid industries as well as the scientific community. With this data, information about the lithosphere composition can be extrapolated in more detail, and the diamond exploration industry can determine the probability that a kimberlite contains diamonds.^[3]

Methods

Thermobarometry uses equilibrium constants to calculate information about the environmental conditions present during the rocks' formation.^[2] While each rock is forming, it reacts with the surrounding elements until it cools down enough to become inert. Each mineral within the rock will cool and crystalize at different points; a petrogenetic grid is a useful way to visualize each mineral crystalizing in sequence.^[2]

Individual reactions of specific minerals can be used to calculate either the temperature or pressure. Therefore, two different reactions are needed to calculate both the temperature and pressure of the magma for a single rock. Some reactions are better for pressure and others are better for temperature, based on thermodynamics and Le Chatelier's Principle.

This technique requires each reaction to be calibrated, which is done through experimentation and data analysis. Experimentation involves simulating the temperatures and pressures at which these rocks form and observing how the reaction proceeds at those conditions, while data analysis relies on amassing a large database of rock samples with pressure and temperature information. Experimental data tends to have significant variation, so using data from natural formations is more accurate, if it's available.^[2]

Pressure

The reactions best for pressure (geobarometers) are ones that have a large change in molar volume during the reaction. Higher pressures cause the reaction to decrease in total volume, and lighter pressures allow reaction to increase in total volume. Therefore, based on the proportion of minerals that have larger volumes versus the proportion of minerals that have smaller volumes, the pressure of the environment during the reaction can be calculated, as a function of temperature. Experiments must be done to calibrate each reaction and determine the rate at which the volume changes with changes in pressure. ^[2]

Temperature

The reactions best for temperature (geothermometers) are ones that have a large enthalpy of reaction, which means they release or consume a lot of heat. Higher temperatures allow the reaction to consume that heat while lower temperatures cause the reaction to release heat. Similarly to geobarometers, the proportion of minerals that are formed by releasing heat versus consuming heat can be used to calculate the temperature, as long as the reaction is calibrated. ^[2]

Reaction Types

There are three types of reactions that clinopyroxene is involved in and can be used for thermobarometry.

Univariant reactions or displaced equilibria reactions either create or destroy phases within the magma.^[2] Each phase will eventually crystalize as a unique mineral. Based on the temperature and pressure conditions, different proportions of these phases will emerge in the final rock. An example reaction is jadeite and quartz reacting to make analbite.^[2]^[1] Jadeite is a type of pyroxene, so this reaction is used for clinopyroxene barometry.

${\ce {NaAlS2O6 + SiO2 -> NaAlS3O8}}$

This particular reaction involves a large change in volume between the reactants and the products, so the reaction is very sensitive to pressure changes.^[2]

Exchange Reactions occur when there are minerals with similar structures, and ions switch places with each other within that structure.^[2] This is a common method to calculate the temperature because most exchange reactions have a high enthalpy. One example reaction is an exchange of Fe²⁺ and Mg²⁺ within garnet and clinopyroxene.^[2] That causes pyrope and hedenbergite (pyroxene) to change into almandine and diopside (pyroxene).

${\ce {Mg3Al2Si3O12 + 3CaFeSi2O6 -> Fe3Al2Si3O12 + 3CaMgSi2O6}}$

Solvus Equilibria reactions occur when two phases dissolve into each other based on the temperature, so it is usually useful for geothermometry.^[2] One such reaction is when clinopyroxene and orthopyroxene dissolve into each other. This changes the distribution of calcium and magnesium throughout the mineral.^[2]

Applications

Clinopyroxene thermobarometry is usually used by mining industries. It is particularly helpful to the diamond industry, so many stakeholders possess pressure and temperature data regarding the formation of rocks that contain diamonds.^[3] This is important because diamonds are usually found in kimberlites, but kimberlites do not always contain diamonds. Instead of mining every kimberlite found, they can be sampled to see if they formed in an environment that would have favored the crystallization of diamonds.

Other applications are largely scientific; pressure and temperature data about magma can be used to propose detailed models of the lithosphere and mantle.^[3] These models enhance understanding of geological and volcanic activity, which may contribute to scientists' ability to predict events such as eruptions or earthquakes.

Related Research Articles

Hornblende Complex inosilicate series of minerals

Hornblende is a complex inosilicate series of minerals. It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole. Hornblende minerals are common in igneous and metamorphic rocks.

Amphibole is a group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain SiO^₄ tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol is Amp. Amphiboles can be green, black, colorless, white, yellow, blue, or brown. The International Mineralogical Association currently classifies amphiboles as a mineral supergroup, within which are two groups and several subgroups.

<span class="mw-page-title-main">Pyroxene</span> Group of inosilicate minerals with single chains of silica tetrahedra

The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula XY(Si,Al)₂O₆, where X represents calcium (Ca), sodium (Na), iron or magnesium (Mg) and more rarely zinc, manganese or lithium, and Y represents ions of smaller size, such as chromium (Cr), aluminium (Al), magnesium (Mg), cobalt (Co), manganese (Mn), scandium (Sc), titanium (Ti), vanadium (V) or even iron or. Although aluminium substitutes extensively for silicon in silicates such as feldspars and amphiboles, the substitution occurs only to a limited extent in most pyroxenes. They share a common structure consisting of single chains of silica tetrahedra. Pyroxenes that crystallize in the monoclinic system are known as clinopyroxenes and those that crystallize in the orthorhombic system are known as orthopyroxenes.

Peridotite Coarse-grained ultramafic igneous rock type

Peridotite ( PERR-ih-doh-tyte, pə-RID-ə-) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg²⁺), reflecting the high proportions of magnesium-rich olivine, with appreciable iron. Peridotite is derived from Earth's mantle, either as solid blocks and fragments, or as crystals accumulated from magmas that formed in the mantle. The compositions of peridotites from these layered igneous complexes vary widely, reflecting the relative proportions of pyroxenes, chromite, plagioclase, and amphibole.

Forsterite (Mg₂SiO₄; commonly abbreviated as Fo; also known as white olivine) is the magnesium-rich end-member of the olivine solid solution series. It is isomorphous with the iron-rich end-member, fayalite. Forsterite crystallizes in the orthorhombic system (space group Pbnm) with cell parameters a 4.75 Å (0.475 nm), b 10.20 Å (1.020 nm) and c 5.98 Å (0.598 nm).

Jadeite is a pyroxene mineral with composition NaAlSi₂O₆. It is hard (Mohs hardness of about 6.5 to 7.0), very tough, and dense, with a specific gravity of about 3.4. It is found in a wide range of colors, but is most often found in shades of green or white. Jadeite is formed only in subduction zones on continental margins, where rock undergoes metamorphism at high pressure but relatively low temperature.

Enstatite Pyroxene: magnesium-iron silicate with MgSiO3 and FeSiO3 end-members

Enstatite is a mineral; the magnesium endmember of the pyroxene silicate mineral series enstatite (MgSiO₃) – ferrosilite (FeSiO₃). The magnesium rich members of the solid solution series are common rock-forming minerals found in igneous and metamorphic rocks. The intermediate composition, (Mg,Fe)SiO^₃, has historically been known as hypersthene, although this name has been formally abandoned and replaced by orthopyroxene. When determined petrographically or chemically the composition is given as relative proportions of enstatite (En) and ferrosilite (Fs) (e.g., En₈₀Fs₂₀).

Ultramafic rock Type of igneous and meta-igneous rock

Ultramafic rocks are igneous and meta-igneous rocks with a very low silica content, generally >18% MgO, high FeO, low potassium, and are composed of usually greater than 90% mafic minerals. The Earth's mantle is composed of ultramafic rocks. Ultrabasic is a more inclusive term that includes igneous rocks with low silica content that may not be extremely enriched in Fe and Mg, such as carbonatites and ultrapotassic igneous rocks.

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.

Melilite refers to a mineral of the melilite group. Minerals of the group are solid solutions of several endmembers, the most important of which are gehlenite and åkermanite. A generalized formula for common melilite is (Ca,Na)₂(Al,Mg,Fe²⁺)[(Al,Si)SiO₇]. Discovered in 1793 near Rome, it has a yellowish, greenish-brown color. The name derives from the Greek words meli (μέλι) "honey" and lithos (λίθους) "stone".The name refers to a group of minerals (melilite group) with chemically similar composition, nearly always minerals in åkermanite-gehlenite series.

Omphacite is a member of the clinopyroxene group of silicate minerals with formula: (Ca, Na)(Mg, Fe²⁺, Al)Si₂O₆. It is a variably deep to pale green or nearly colorless variety of clinopyroxene. It normally appears in eclogite, which is the high-pressure metamorphic rock of basalt. Omphacite is the solid solution of Fe-bearing diopside and jadeite. It crystallizes in the monoclinic system with prismatic, typically twinned forms, though usually anhedral. Its space group can be P2/n or C2/c depending on the thermal history. It exhibits the typical near 90° pyroxene cleavage. It is brittle with specific gravity of 3.29 to 3.39 and a Mohs hardness of 5 to 6.

Pigeonite is a mineral in the clinopyroxene subgroup of the pyroxene group. It has a general formula of (Ca,Mg,Fe)(Mg,Fe)Si₂O₆. The calcium cation fraction can vary from 5% to 25%, with iron and magnesium making up the rest of the cations.

Normative mineralogy is a calculation of the composition of a rock sample that estimates the idealised mineralogy of a rock based on a quantitative chemical analysis according to the principles of geochemistry.

Geothermobarometry is the science of measuring the previous pressure and temperature history of a metamorphic or intrusive igneous rocks. Geothermobarometry is a combination of geobarometry, where a pressure of mineral formation is resolved, and geothermometry where a temperature of formation is resolved.

The endmember hornblende tschermakite (☐Ca₂(Mg₃Al₂)(Si₆Al₂)O₂₂(OH)₂) is a calcium rich monoclinic amphibole mineral. It is frequently synthesized along with its ternary solid solution series members tremolite and cummingtonite so that the thermodynamic properties of its assemblage can be applied to solving other solid solution series from a variety of amphibole minerals.

A metamorphic facies is a set of mineral assemblages in metamorphic rocks formed under similar pressures and temperatures. The assemblage is typical of what is formed in conditions corresponding to an area on the two dimensional graph of temperature vs. pressure. Rocks which contain certain minerals can therefore be linked to certain tectonic settings, times and places in the geological history of the area. The boundaries between facies are wide because they are gradational and approximate. The area on the graph corresponding to rock formation at the lowest values of temperature and pressure is the range of formation of sedimentary rocks, as opposed to metamorphic rocks, in a process called diagenesis.

Igneous petrology is the study of igneous rocks—those that are formed from magma. As a branch of geology, igneous petrology is closely related to volcanology, tectonophysics, and petrology in general. The modern study of igneous rocks utilizes a number of techniques, some of them developed in the fields of chemistry, physics, or other earth sciences. Petrography, crystallography, and isotopic studies are common methods used in igneous petrology.

Dorrite is a silicate mineral that is isostructural to the aenigmatite group. Although it is most chemically similar to the mineral rhönite [Ca₂Mg₅Ti(Al₂Si₄)O₂₀], the lack of titanium (Ti) and presence of Fe³⁺ influenced dorrite's independence. Dorrite is named for Dr. John (Jack) A. Dorr, a late professor at the University of Michigan that researched in outcrops where dorrite was found in 1982. This mineral is sub-metallic resembling colors of brownish-black, dark brown, to reddish brown.

A petrogenetic grid is a geological phase diagram that connects the stability ranges or metastability ranges of metamorphic minerals or mineral assemblages to the conditions of metamorphism. Experimentally determined mineral or mineral-assemblage stability ranges are plotted as metamorphic reaction boundaries in a pressure–temperature cartesian coordinate system to produce a petrogenetic grid for a particular rock composition. The regions of overlap of the stability fields of minerals form equilibrium mineral assemblages used to determine the pressure–temperature conditions of metamorphism. This is particularly useful in geothermobarometry.

Garnet-Biotite Geothermometry is a method used to evaluate the peak temperature at which metamorphic rocks have formed. Geothermometry makes up one component of geothermobarometry, which also includes the evaluation of pressure (geobarometry). There are many geothermometers, but garnet-biotite is particularly useful because of the frequent occurrence of biotite and garnet together in medium grade metamorphic rocks. The garnet biotite thermometer correlates temperature with the partitioning of Fe and Mg in coinciding garnet and biotite. The garnet-biotite thermometer has been "calibrated" many times since the 70's by both experimental and empirical methods, however Ferry and Spear's 1978 experimental calibration study is reported thoroughly and commonly cited. Given a rock containing both garnet and biotite, an equilibrium constant (K_D) can be found simply by using microprobe analysis. Then, by comparing the found K_D value to the calculated garnet-biotite geothermometer, the peak temperature of rock formation can be determined.

References

1 2 Putirka, Keith; Johnson, Marie; Kinzler, Rosamond; Longhi, John; Walker, David (1996). "Thermobarometry of mafic igneous rocks based on clinopyroxene-liquid equilibria, 0-30 kbar". Contributions to Mineralogy and Petrology. 123: 92–108.
1 2 3 4 5 6 7 8 9 10 11 12 13 Misra, Kula C. (2012). Introduction to Geochemistry Principles and Applications. Pondicherry, India: Wiley-Blackwell. pp. 107–128. ISBN 9781444347197.
1 2 3 Grütter, Herman S. (2009). "Pyroxene xenocyst geotherms: Techniques and Application". Lithos. 112: 1167–1178. doi:10.1016/j.lithos.2009.03.023.

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[:12-1] 1 2 Putirka, Keith; Johnson, Marie; Kinzler, Rosamond; Longhi, John; Walker, David (1996). "Thermobarometry of mafic igneous rocks based on clinopyroxene-liquid equilibria, 0-30 kbar". Contributions to Mineralogy and Petrology. 123: 92–108.

[:02-2] 1 2 3 4 5 6 7 8 9 10 11 12 13 Misra, Kula C. (2012). Introduction to Geochemistry Principles and Applications. Pondicherry, India: Wiley-Blackwell. pp. 107–128. ISBN 9781444347197.

[:22-3] 1 2 3 Grütter, Herman S. (2009). "Pyroxene xenocyst geotherms: Techniques and Application". Lithos. 112: 1167–1178. doi:10.1016/j.lithos.2009.03.023.

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