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Superplumes – Subduction Zone – the Water Channel to the Mantle

In this paper Omori & Komabayashi review the role of subduction zones as water carriers to the mantle from the surface of the Earth. In order to discuss a mode of water transportation by way of the subduction zone a phase diagram for the hydrous peridotite in the MgO-Al₂O₃-SiO₂-H₂O system and a model distribution of the dehydration reaction in the subducting peridotite are shown. With the exception of locations where a slab is observed to penetrate into the lower mantle, stagnation of the slab is of significance with regard to the dehydration of the mantle transition zone, therefore the transport of water into the lower mantle is limited in most subduction zones. To allow estimation of sites of dehydration in the mantle without thermal modelling the dehydration induction model for intraslab earthquakes was introduced. There are 2 organised structures that are shown in subduction-zone earthquakes in their depth distribution down to a depth of 700 km and in the hypocentre distribution geometry at the intermediate depths, 50-300 km. The former is represented by 3 types of depth-distribution of earthquakes in the subduction zones globally; and the latter has been shown as the double seismic zone (DSZ). Omori & Komabayashi review the dehydration-induced earthquake model to show a close link between the locations of metamorphic dehydration reactions in the oceanic peridotite that is being subducted and hypocentre distribution. The transport of water into the mantle by the subducting slab is suggested to possibly be a reflection of the phenomenon of earthquakes in the subduction zone. The hypothesis involves hydration of the oceanic peridotite to 50 km below the floor of the ocean, and the authors have proposed a possible mechanism of the dehydration process. The hydration of the oceanic peridotite implies a new water channel to the mantle which is more suitable to carrying water than mantle wedge material that was proposed previously. The transport of water has a significant role in the dynamics of the Earth. A decline of material circulation in the solid Earth results from irreversible cooling of the interior of the Earth. Instead a cooler mantle allows much water to be carried deeper in to the mantle, and rheological activation will be expected. A negative feedback to stabilise the mantle-surface material activity is provided by the subduction of hydrous material.

In the plate tectonics paradigm the subduction zone is the most important subsystem. In terms of their temperature and chemical composition plates that are subducting are distinct. A plate continues to cool as it moves from an oceanic ridge to a subduction zone. Relatively low-temperature domains are generated in the mantle by the subduction of such a cold plate, which have roles in making a down-flow and the storage of water molecules in the mantle. The main constituents of oceanic crust are herzbergitic-Iherzoritic peridotite, which are differentiated at the oceanic ridge by partial melting. Hydroxyl components brought into hydrous minerals in the oceanic crust by hydrothermal alteration near the oceanic ridge, and probably in the peridotitic lithosphere. As the hydroxyl minerals can survive subduction metamorphism, depending on thermal conditions of the subduction zone, and thereby convey water into the deep mantle. It is known that the mechanical properties of mantle rocks can be changed by water (Karato et al., 1986; Hirth & Kohlstedt, 1996; Chen et al., 1998; Karato & Jung, 1998): As a result of this thermal, chemical and mechanical heterogeneity are brought to the mantle by subducting plates from the subduction zone. It is suggested that for mantle plumes or superplumes to be generated it is essential to consider such heterogeneities in the mantle (Karato, 2007; Maruyama et al., 2007; in source 1). For this reason the generation and subduction of plates are important elements in the plume tectonics paradigm.

Since the beginning of the plate tectonics paradigm chemical and thermal processes have been studied extensively. Many papers and text books include comprehensive reviews of these studies (e.g., Bebout et al., 1996). Previous studies have discussed water transport by the crustal layer of slab and mantle wedge peridotite (e.g., Schmidt & Poli, 1998; Okamoto & Maruyama, 1999; Maruyama & Liou, 2005).

Sources & Further reading

1. Soichi Omori & Tetsuya Komabayashi in Yuen, D.A., Maruyama, S, Karato, Shun-ichiro & Windley, B., (Eds), 2007, Superplumes: Beyond Plate Tectonics, Springer.