Subduction zone metamorphism
[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.
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.
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 bottom units represent the crystallized magma chamber, feeding the mid-ocean ridge at which the crust was formed.
Approximately 90–95% of that water is contained in hydrous minerals, including mica, phengite, amphibole, lawsonite, chlorite, talc, zoisite, and serpentine.
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.
Zeolites are microporous silicate minerals that can be produced by the reaction of pore fluids with basalt and pelagic sediments.
[8][16] 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.
Other common minerals of blueschist facies metabasites are paragonite, chlorite, titanite, stilpnomelane, quartz, albite, sericite, and pumpellyite.
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.
[18] Lawsonite = Grossular + Topaz + Stishovite + H2O[18] Antigorite Serpentine is another important water bearing phase that breaks down at eclogite facies conditions.
Antigorite = Forsterite + Enstatite + H2O[21] Transition into the eclogite facies is proposed to be the source of earthquakes at depths greater than 70 km.
[23] Nearest to the trench is a zone of low temperature, high pressure metamorphic conditions characterized by blueschist to eclogite facies assemblages.
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.
[24] However, further studies show the common occurrence of paired metamorphic belts in continental interiors, resulting in controversy on their origin.
