Subduction zone metamorphism

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Melt production and accretion of melt onto continental crust in a subduction zone Metamorphic pathway for subducted crust.jpg
Melt production and accretion of melt onto continental crust in a subduction zone

A subduction zone is a region of the Earth's crust where one tectonic plate moves under another tectonic plate; oceanic crust gets recycled back into the mantle and continental crust gets produced by the formation of arc magmas. Arc magmas account for more than 20% of terrestrially produced magmas [2] and are produced by the dehydration of minerals within the subducting slab as it descends into the mantle and are accreted onto the base of the overriding continental plate. [3] Subduction zones host a unique variety of rock types formed by the high-pressure, low-temperature conditions a subducting slab encounters during its descent. [4] The metamorphic conditions the slab passes through in this process generates and alters water bearing (hydrous) mineral phases, releasing water into the mantle. This water lowers the melting point of mantle rock, initiating melting. [5] Understanding the timing and conditions in which these dehydration reactions occur, is key to interpreting mantle melting, volcanic arc magmatism, and the formation of continental crust. [6]

Contents

Pressure-temperature pathway for subducted crust Metamorphic pathway of pressure-temperature conditions in subduction zones.jpg
Pressure-temperature pathway for subducted crust

A metamorphic facies is characterized by a stable mineral assemblage specific to a pressure-temperature range and specific starting material. Subduction zone metamorphism is characterized by a low temperature, high-ultrahigh pressure metamorphic path through the zeolite, prehnite-pumpellyite, blueschist, and eclogite facies stability zones of subducted oceanic crust. [7] Zeolite and prehnite-pumpellyite facies assemblages may or may not be present, thus the onset of metamorphism may only be marked by blueschist facies conditions. [8] Subducting slabs are composed of basaltic crust topped with pelagic sediments; [9] however, the pelagic sediments may be accreted onto the forearc-hanging wall and not subducted. [10] Most metamorphic phase transitions that occur within the subducting slab are prompted by the dehydration of hydrous mineral phases. The breakdown of hydrous mineral phases typically occurs at depths greater than 10 km. [11] Each of these metamorphic facies is marked by the presence of a specific stable mineral assemblage, recording the metamorphic conditions undergone by the subducting slab. Transitions between facies cause hydrous minerals to dehydrate at certain pressure-temperature conditions and can therefore be tracked to melting events in the mantle beneath a volcanic arc.

Oceanic crust

Arc magmas are produced by partial melting of metasomatic domains in the mantle wedge, which have reacted with liquid phases derived from dehydration melting of minerals contained in the subducting oceanic crust formed at mid-ocean ridges. [2] The subducting oceanic crust consists of four major units. The topmost unit is a thin cap of pelagic sediments up to 0.3 km thick composed of siliceous and calcareous shells, meteoric dusts, and variable amounts of volcanic ash. The next unit is composed of 0.3–0.7 km thick pillow basalts, formed by the quenching of basaltic magma as it erupts into ocean water. Under the pillow basalts is a basaltic sheeted dike complex, that represent cooled magma conduits. The bottom units represent the crystallized magma chamber, feeding the mid-ocean ridge at which the crust was formed. It is composed of 1–5 km thick layered gabbro atop <7 km thick layer of ultramafic rocks (e.g. wehrlite, harzburgite, dunite, and chromite). [12] Oceanic crust is referred to as a metabasite. [13]

Hydrous minerals of a subducting slab

Every year, 1–2 x 10 trillion kilograms of water descends into subduction zones. Approximately 90–95% of that water is contained in hydrous minerals, including mica, phengite, amphibole, lawsonite, chlorite, talc, zoisite, and serpentine. [11] The most significant hydrous minerals are lawsonite (11 wt% H2O), phlogopite (2 wt% H2O) and amphibole (2 wt% H2O). Phlogopite does not release water until approximately 200 km depth whereas amphibole releases water at approximately 75 km depth. Serpentine is also an important hydrous phase (13 wt% H2O) that is only present in oceanic crust formed at a slow spreading ridge where ultramafic rocks are emplaced at shallow levels. Lawsonite does not release water until approximately 300 km depth and is the last hydrous mineral to do so. [1] [11] Metamorphic dehydration reactions are prominent within the subducting slab during subduction, giving rise to liquid phases that contain fluid-mobile trace elements due to the breakdown of hydrous minerals such as phengite, lawsonite and zoisite. [14] This forms a unique type of trace element distribution pattern for arc magma. [3] Arc magmas and the continental crust formed from arc magmas are enriched in boron, lead, arsenic, and antimony derived from the dehydration within the subducting slab. Hydrothermal fluids released from the slab mobilize these elements and allow them to be incorporated into arc magmas, distinguishing arc magmas from those produced at mid-ocean ridges and hotspots. [6] [15]

Facies transitions and dehydration reactions of a subducting slab

Zeolite facies

Basalts may first metamorphose under zeolite facies conditions (50–150 °C and 1–5 km depth) during subduction. Zeolites are microporous silicate minerals that can be produced by the reaction of pore fluids with basalt and pelagic sediments. The zeolite facies conditions typically only affect pelitic sediments undergoing burial, but is commonly displayed by the production of zeolite minerals within the vesicles of vesicular basalt. The glassy rinds on pillow basalts are also susceptible to metamorphism under zeolite facies conditions, which produces the zeolites heulandite or stilbite and hydrous phyllosilicates such as celadonite, smectite, kaolinite, or montmorillonite plus secondary quartz. Crystalline igneous rocks of the subducting slab, such as gabbro and basaltic sheeted dikes, remain stable until greater depth, when the sodium endmember of plagioclase feldspar, albite, replaces detrital igneous plagioclase feldspar. Also at greater depth in the zeolite facies, the zeolite laumontite replaces the zeolite heulandite and the phyllosilicate chlorite is common. [8] [16]

Prehnite-pumpellyite facies

At paths up to 220–320 °C and below 4.5 kbars, subducting slabs may encounter the prehnite-pumpellyite facies, characterized by the presence of the hydrous chlorite, prehnite, albite, pumpellyite, tremolite, and epidote and the loss of the zeolites heulandite and laumontite. Actinolite may occur at higher grade. [17] Aside from albite, these characteristic minerals are water bearing, and may contribute to mantle melting. These minerals are also vital in the formation of glaucophane, which is associated with blueschist facies. The onset of a low-pressure phase of lawsonite is the most significant marker of prehnite-pumpellyite facies metamorphism. The occurrence of lawsonite is significant because lawsonite contains 11 wt.% H2O [18] which is released at higher grade and can initiate significant melting. [8]

Laumontite = Lawsonite + Quartz + H2O [19]

Blueschist facies

Blueschist containing the sodic blue amphibole, glaucophane Blueschist facies rock.jpeg
Blueschist containing the sodic blue amphibole, glaucophane

Blueschist facies is characterized by the formation of a sodic, blue amphibole, namely, glaucophane, for which the blueschist facies is named. Lawsonite is also diagnostic of blueschist facies and occurs in association with glaucophane. [20] Glaucophane forming reactions are listed below. Glaucophane producing reactions are significant because they can either release water or produce the hydrous phase, lawsonite through the breakdown of hydrous phyllosilicates. At high blueschist facies pressures, albite may break down to form jadeite and quartz. Calcite will commonly pseudomorphose into aragonite under blueschist conditions. Other common minerals of blueschist facies metabasites are paragonite, chlorite, titanite, stilpnomelane, quartz, albite, sericite, and pumpellyite.

Tremolite + Chlorite + Albite = Glaucophane + Epidote + H2O

Tremolite + Chlorite + Albite = Glaucophane + Lawsonite

Pumpellyite + Chlorite + Albite = Glaucophane + Epidote + H2O [8]

Eclogite facies

Transition from blueschist to eclogite facies rock, containing glaucophane, omphacitic pyroxene, and garnet Bluechist-ecolgite transition.jpg
Transition from blueschist to eclogite facies rock, containing glaucophane, omphacitic pyroxene, and garnet
Eclogite facies rock, containing omphacitic pyroxene and garnet Eclogite facies.jpg
Eclogite facies rock, containing omphacitic pyroxene and garnet

Eclogite facies is typically encountered around 80–100 km depth and is characterized by the presence of green omphacitic pyroxene and red pyrope garnet. [11] Omphacitic pyroxene is an augite-jadeite solution. At Eclogite facies conditions, plagioclase is no longer stable. The albite component breaks down during glaucophane producing reactions and its sodium becomes incorporated into glaucophane and pyroxene. This reaction is written below. The breakdown of glaucophane is an important water producing reaction at about 600 °C, and over 1 GPa that can trigger significant mantle melting and volcanism. [8]

Glaucophane + Paragonite = Pyrope + Jadeite + Quartz + H2O [8]

Another important water producing reaction that occurs during the eclogite facies is the dehydration of the hydrous phyllosilicate phlogopite by the reaction below. This reaction can also trigger significant mantle melting and volcanism. Aside from triggering mantle melt, this reaction may also trigger partial melting of the subducting slab itself.

Phlogopite + Diopside + Orthopyroxene = H2O + Melt [1]

Lawsonite remains stable up to 1080 °C and 9.4 GPa. The breakdown of lawsonite releases massive amounts of H2O into the mantle that can trigger partial melting of the slab and of the overlying mantle. The breakdown reaction of lawsonite is listed below. [18]

Lawsonite = Grossular + Topaz + Stishovite + H2O [18]

Antigorite Serpentine is another important water bearing phase that breaks down at eclogite facies conditions. Antigorite breaks down at 600–700 °C and between 2–5 GPa. Antigorite contains 13 wt.% water and therefore causes substantial mantle melting. [11] The reaction is listed below.

Antigorite = Forsterite + Enstatite + H2O [21]

Transition into the eclogite facies is proposed to be the source of earthquakes at depths greater than 70 km. These earthquakes are caused by the contraction of the slab as minerals transition into more compact crystal structures. The depth of these earthquakes on the subducting slab is known as the Wadati–Benioff zone. [22]

Paired metamorphic belts

Paired metamorphic belts were envisaged as a set of parallel metamorphic rock units parallel to a subduction zone displaying two contrasting metamorphic conditions and thus two distinctive mineral assemblages. [23] Nearest to the trench is a zone of low temperature, high pressure metamorphic conditions characterized by blueschist to eclogite facies assemblages. This assemblage is associated with subduction along the trench and low heat flow. Nearest the arc is a zone of high temperature-low pressure metamorphic conditions characterized by amphibolite to granulite facies mineral assemblages such as aluminosilicates, cordierite, and orthopyroxenes. This assemblage is associated with high heat flow generated by melting beneath the volcanic arc. [24]

However, further studies show the common occurrence of paired metamorphic belts in continental interiors, resulting in controversy on their origin. [25] Based on inspection of extreme metamorphism and post-subduction magmatism at convergent plate margins, paired metamorphic belts are further extended to two contrasting metamorphic facies series: [7] one is blueschist to eclogite facies series that was produced by subducting metamorphism at low thermal gradients of <10 °C/km, and the other is amphibolite to granulite facies series that was produced by rifting metamorphism at high thermal gradients of >30 °C/km.

Related Research Articles

<span class="mw-page-title-main">Metamorphic rock</span> Rock that was subjected to heat and pressure

Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than 150 to 200 °C and, often, elevated pressure of 100 megapascals (1,000 bar) or more, causing profound physical or chemical changes. During this process, the rock remains mostly in the solid state, but gradually recrystallizes to a new texture or mineral composition. The protolith may be an igneous, sedimentary, or existing metamorphic rock.

<span class="mw-page-title-main">Subduction</span> A geological process at convergent tectonic plate boundaries where one plate moves under the other

Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the heavier plate dives beneath the second plate and sinks into the mantle. A region where this process occurs is known as a subduction zone, and its surface expression is known as an arc-trench complex. The process of subduction has created most of the Earth's continental crust. Rates of subduction are typically measured in centimeters per year, with rates of convergence as high as 11 cm/year.

<span class="mw-page-title-main">Convergent boundary</span> Region of active deformation between colliding tectonic plates

A convergent boundary is an area on Earth where two or more lithospheric plates collide. One plate eventually slides beneath the other, a process known as subduction. The subduction zone can be defined by a plane where many earthquakes occur, called the Wadati–Benioff zone. These collisions happen on scales of millions to tens of millions of years and can lead to volcanism, earthquakes, orogenesis, destruction of lithosphere, and deformation. Convergent boundaries occur between oceanic-oceanic lithosphere, oceanic-continental lithosphere, and continental-continental lithosphere. The geologic features related to convergent boundaries vary depending on crust types.

<span class="mw-page-title-main">Andesite</span> 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.

<span class="mw-page-title-main">Eclogite</span> A dense metamorphic rock formed under high pressure

Eclogite is a metamorphic rock containing garnet (almandine-pyrope) hosted in a matrix of sodium-rich pyroxene (omphacite). Accessory minerals include kyanite, rutile, quartz, lawsonite, coesite, amphibole, phengite, paragonite, zoisite, dolomite, corundum and, rarely, diamond. The chemistry of primary and accessory minerals is used to classify three types of eclogite. The broad range of eclogitic compositions has led to a longstanding debate on the origin of eclogite xenoliths as subducted, altered oceanic crust.

<span class="mw-page-title-main">Volcanic arc</span> Chain of volcanoes formed above a subducting plate

A volcanic arc is a belt of volcanoes formed above a subducting oceanic tectonic plate, with the belt arranged in an arc shape as seen from above. Volcanic arcs typically parallel an oceanic trench, with the arc located further from the subducting plate than the trench. The oceanic plate is saturated with water, mostly in the form of hydrous minerals such as micas, amphiboles, and serpentines. As the oceanic plate is subducted, it is subjected to increasing pressure and temperature with increasing depth. The heat and pressure break down the hydrous minerals in the plate, releasing water into the overlying mantle. Volatiles such as water drastically lower the melting point of the mantle, causing some of the mantle to melt and form magma at depth under the overriding plate. The magma ascends to form an arc of volcanoes parallel to the subduction zone.

<span class="mw-page-title-main">Glaucophane</span>

Glaucophane is the name of a mineral and a mineral group belonging to the sodic amphibole supergroup of the double chain inosilicates, with the chemical formula ☐Na2(Mg3Al2)Si8O22(OH)2.

<span class="mw-page-title-main">Blueschist</span> Type of metavolcanic rock

Blueschist, also called glaucophane schist, is a metavolcanic rock that forms by the metamorphism of basalt and rocks with similar composition at high pressures and low temperatures, approximately corresponding to a depth of 15–30 km (9.3–18.6 mi). The blue color of the rock comes from the presence of the predominant minerals glaucophane and lawsonite.

<span class="mw-page-title-main">Rock cycle</span> Transitional concept of geologic time

The rock cycle is a basic concept in geology that describes transitions through geologic time among the three main rock types: sedimentary, metamorphic, and igneous. Each rock type is altered when it is forced out of its equilibrium conditions. For example, an igneous rock such as basalt may break down and dissolve when exposed to the atmosphere, or melt as it is subducted under a continent. Due to the driving forces of the rock cycle, plate tectonics and the water cycle, rocks do not remain in equilibrium and change as they encounter new environments. The rock cycle explains how the three rock types are related to each other, and how processes change from one type to another over time. This cyclical aspect makes rock change a geologic cycle and, on planets containing life, a biogeochemical cycle.

<span class="mw-page-title-main">Greenschist</span> Metamorphic rock

Greenschists are metamorphic rocks that formed under the lowest temperatures and pressures usually produced by regional metamorphism, typically 300–450 °C (570–840 °F) and 2–10 kilobars (29,000–145,000 psi). Greenschists commonly have an abundance of green minerals such as chlorite, serpentine, and epidote, and platy minerals such as muscovite and platy serpentine. The platiness gives the rock schistosity. Other common minerals include quartz, orthoclase, talc, carbonate minerals and amphibole (actinolite).

Zeolite facies describes the mineral assemblage resulting from the pressure and temperature conditions of low-grade metamorphism.

<span class="mw-page-title-main">Lawsonite</span>

Lawsonite is a hydrous calcium aluminium sorosilicate mineral with formula CaAl2Si2O7(OH)2·H2O. Lawsonite crystallizes in the orthorhombic system in prismatic, often tabular crystals. Crystal twinning is common. It forms transparent to translucent colorless, white, pink, and bluish to pinkish grey glassy to greasy crystals. Refractive indices are nα = 1.665, nβ = 1.672 – 1.676, and nγ = 1.684 – 1.686. It is typically almost colorless in thin section, but some lawsonite is pleochroic from colorless to pale yellow to pale blue, depending on orientation. The mineral has a Mohs hardness of 7.5 and a specific gravity of 3.09. It has perfect cleavage in two directions and a brittle fracture.

<span class="mw-page-title-main">Metamorphic facies</span> Set of mineral assemblages in metamorphic rocks formed under similar pressures and temperatures

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.

Partial melting is the phenomenon that occurs when a rock is subjected to temperatures high enough to cause certain minerals to melt, but not all of them. Partial melting is an important part of the formation of all igneous rocks and some metamorphic rocks, as evidenced by a multitude of geochemical, geophysical and petrological studies.

<span class="mw-page-title-main">Metamorphic zone</span>

In geology, a metamorphic zone is an area where, as a result of metamorphism, the same combination of minerals occur in the bedrock. These zones occur because most metamorphic minerals are only stable in certain intervals of temperature and pressure.

Ultra-high-pressure metamorphism refers to metamorphic processes at pressures high enough to stabilize coesite, the high-pressure polymorph of SiO2. It is important because the processes that form and exhume ultra-high-pressure (UHP) metamorphic rocks may strongly affect plate tectonics, the composition and evolution of Earth's crust. The discovery of UHP metamorphic rocks in 1984 revolutionized our understanding of plate tectonics. Prior to 1984 there was little suspicion that continental rocks could reach such high pressures.

<span class="mw-page-title-main">Eclogitization</span> The tectonic process in which the dense, high-pressure, metamorphic rock, eclogite, is formed

Eclogitization is the tectonic process in which the high-pressure, metamorphic facies, eclogite, is formed. This leads to an increase in the density of regions of Earth's crust, which leads to changes in plate motion at convergent boundaries.

Paired metamorphic belts are sets of parallel linear rock units that display contrasting metamorphic mineral assemblages. These paired belts develop along convergent plate boundaries where subduction is active. Each pair consists of one belt with a low-temperature, high-pressure metamorphic mineral assemblage, and another characterized by high-temperature, low-pressure metamorphic minerals.

A continental arc is a type of volcanic arc occurring as an "arc-shape" topographic high region along a continental margin. The continental arc is formed at an active continental margin where two tectonic plates meet, and where one plate has continental crust and the other oceanic crust along the line of plate convergence, and a subduction zone develops. The magmatism and petrogenesis of continental crust are complicated: in essence, continental arcs reflect a mixture of oceanic crust materials, mantle wedge and continental crust materials.

<span class="mw-page-title-main">Petrogenetic grid</span> Pressure-temperature diagram of mineral stability ranges

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.

References

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  25. ↑ Brown, M., 2006. A duality of thermal regimes is the distinctive characteristic of plate tectonics since the Neoarchean. Geology 34, 961–964.
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