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Solid earth sciences
202412202412
Role of water in dynamics of slabs and surrounding mantle
Eiji Ohtani, Takayuki IshiiEiji Ohtani, Takayuki Ishii
Water partitioning, NAMs, DHMS, Hydration, Dehydration, Slabs, Fluid, Seismic scatterers, Intermediate-depth earthquakes, Deep-focus earthquakes
Processes associated with slab dehydration in the deep mantle. (A) The release of water through the decomposition of hydrous minerals of the slabs in the upper mantle produces earthquake swarms in the mantle wedge above the slab. Olivine may remain metastable to the transition zone depths in “wet” slabs, because water partitions into dense hydrous magnesium silicates resulting in formation of dry olivine. (B) Seismic reflectors and scatterers in the lower mantle may be created by fluids or melts released from the slabs due to decomposition of DHMSs (Dense Hydrous Magnesium Silicates) or the stisohivite-CaCl2-type silica transformation of the hydrous SiO2 phase in the basalt layer of the slab descending into the lower mantle.
Water bound to various hydrous minerals can be transported deep into the mantle by slab subduction. Serpentine is one of most important hydrous minerals in the crust and shallow upper mantle. A partially serpentinized slab mantle limits the amount of water that can enter deep into the mantle. The partitioning of water between hydrous minerals and nominally anhydrous minerals (NAMs) is a crucial factor in controlling the physical properties and dynamics of slabs. Recent experiments on water partitioning have revealed that water strongly partitions to coexisting hydrous minerals. NAMs, such as olivine and its high-pressure polymorphs, have limited water content in water-undersaturated wet slabs. Metastable olivine wedges are not a feature of dry slabs, but can be of wet slabs that are not saturated with water. The transformation kinetics of the dry state, which generates deep-focus earthquakes and produces significant deformation in the slab, can work under wet slabs. Water bound to hydrous minerals is transported by the slab to the mantle transition zone and lower mantle. Hydrous minerals in stagnant slabs over 660 km depth release water as the slab warms, producing locally hydrated mantle transition zones and dense water-bearing magmas at the base of the upper mantle, and generating intraplate volcanism, which are referred to as the big mantle wedge model. Seismic scatterers are observed in the lower mantle at depths from 700 to 1900 km. These scatterers may be caused by water release at the top of the lower mantle by dehydration of hydrous minerals such as dense hydrous magnesium silicates. The shear instability due to the second order phase transformation from stishovite to CaCl2-type phase in hydrous aluminous SiO2 also causes the depth variation of seismic scatterers in the lower mantle. The high-pressure polymorphs of aluminous SiO2 contain a large amount of water more than 1 wt%, which can be important water carriers under lower mantle conditions.
