science

Subduction zone metamorphism

Subduction zone metamorphism is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Subduction zone metamorphism rather than just read about it. In short: 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 and are produced by the dehydration of minerals within the subducting slab as it descends into the mantle and are accret…

Key takeaways

Subduction zone metamorphism belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Subduction zone metamorphism to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Subduction zone metamorphism from memory before moving on to harder problems.

Reference excerpt

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 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. Subduction zones host a unique variety of rock types formed by the high-pressure, low-temperature conditions a subducting slab encounters during its descent. 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. 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. 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. 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. Subducting slabs are composed of basaltic crust topped with pelagic sediments; however, the pelagic sediments may be accreted onto the forearc-hanging wall and not subducted. 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. 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. 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). Oceanic crust is referred to as a metabasite.

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. 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. 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. This forms a unique type of trace element distribution pattern for arc magma. 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.

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.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Subduction zone metamorphism

Start with the simplest possible case. Write down what Subduction zone metamorphism claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Subduction zone metamorphism before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Subduction zone metamorphism ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Subduction zone metamorphism

In research: Subduction zone metamorphism appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Subduction zone metamorphism in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Subduction zone metamorphism is common in secondary-school and first-year university syllabi. It links to neighbouring topics Metamorphic petrology, Subduction zones, so understanding it makes those chapters shorter.
In everyday life: Look for Subduction zone metamorphism outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Subduction zone metamorphism in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Subduction zone metamorphism means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Subduction zone metamorphism out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Subduction zone metamorphism in simple terms?

Why does Subduction zone metamorphism matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Subduction zone metamorphism?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Subduction zone metamorphism.