Editing Subduction (section)

===Metamorphism===
{{main|Subduction zone metamorphism}}
Subduction zones host a unique variety of rock types created by the high-pressure, low-temperature conditions a subducting slab encounters during its descent.<ref>Zheng, Y.-F., Chen, Y.-X., 2016. Continental versus oceanic subduction zones. National Science Review 3, 495-519.</ref> The metamorphic conditions the slab passes through in this process create and destroy water bearing (hydrous) mineral phases, releasing water into the mantle. This water lowers the melting point of mantle rock, initiating melting.<ref name=sdsu>{{cite web|title=How Volcanoes work – Subduction Zone Volcanism|url=http://www.geology.sdsu.edu/how_volcanoes_work/subducvolc_page.html|publisher=San Diego State University Department of Geological Science|access-date=2021-04-11|archive-date=2018-12-29|archive-url=https://web.archive.org/web/20181229025459/http://www.geology.sdsu.edu/how_volcanoes_work/subducvolc_page.html|url-status=dead}}</ref> 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.<ref name=Mibe>{{cite journal|last1=Mibe|first1=Kenji|title= Slab melting versus slab dehydration in subduction zones|doi=10.1073/pnas.1010968108|journal=Proceedings of the National Academy of Sciences|date=2011|volume=108|issue=20|pages=8177–8182|display-authors=etal|pmid=21536910|pmc=3100975|doi-access=free}}</ref>

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, [[Ultra-high-pressure metamorphism|high-ultrahigh pressure metamorphic]] path through the [[zeolite]], prehnite-pumpellyite, [[blueschist]], and [[eclogite]] facies stability zones of subducted oceanic crust.<ref name="Zheng, Y 2017">Zheng, Y.-F., Chen, R.-X., 2017. Regional metamorphism at extreme conditions: Implications for orogeny at convergent plate margins. Journal of Asian Earth Sciences 145, 46–73.</ref> [[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.<ref name="Winter 541">{{cite book|last1=Winter|first1=John D.|title=Principles of Igneous and Metamorphic Petrology|publisher=Prentice Hall|isbn=978-0-321-59257-6|pages=541–548|year=2010}}</ref> Subducting slabs are composed of basaltic crust topped with [[pelagic sediments]];<ref>{{cite book|last1=Reynolds|first1=Stephen|title=Exploring Geology|publisher=McGraw-Hill|isbn=978-0073524122|pages=124|date=2012-01-09}}</ref> however, the pelagic sediments may be accreted onto the forearc-hanging wall and not subducted.<ref>{{cite journal|last1=Bebout|first1=Grey E.|title=Metamorphic Chemical Geodynamics of Subduction|journal=Earth and Planetary Science Letters|date=May 31, 2007|volume=260|issue=3–4 |pages=375|doi=10.1016/j.epsl.2007.05.050|bibcode=2007E&PSL.260..373B}}</ref> 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&nbsp;km.<ref name="Peacock 12-15">{{cite book |last1=Peacock|first1=Simon M.|chapter=Thermal Structure and Metamorphic Evolution of Subducting Slabs |pages=12–15 |editor-last1=Eiler |editor-first1=John |title=Inside the subduction factory |date=1 January 2004 |series=Geophysical Monograph Series |volume=138 |publisher=American Geophysical Union |isbn=9781118668573}}</ref> Each of these metamorphic facies is marked by the presence of a specific stable mineral assemblage, recording the metamorphic conditions undergone but 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.