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Influence of water in Earth’s processes

Influence of water in Earth’s processes. EPSC666- 3rd module Cécile Noverraz. Main water-bearing movments in the mantle. Slab hydration Slab dehydration Mantle processes. Storage. Main water storages: oceanic crust and lithosphere and the transition zone (between 410 and 670 km).

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Influence of water in Earth’s processes

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  1. Influence of water in Earth’s processes EPSC666- 3rd module Cécile Noverraz

  2. Main water-bearing movments in the mantle • Slab hydration • Slab dehydration • Mantle processes

  3. Storage • Main water storages: oceanic crust and lithosphere and the transition zone (between 410 and 670 km)

  4. Slab hydration • Serpentinization at mid-ocean ridge: rifting and hydrothermal circulation • Mafic minerals (olivine, pyroxene) are metasomatized into hydrated minerals (amphibole, chlorite, serpentine) • Black smokers formation: concentration of metals at the oceanic ridge, metal leaching of the mafic crust

  5. Slab hydration • Possibility of more important hydration in the trench before subduction • Reactivation and creation of normal faults with the bend of the slab • Extensional deep fault provoke hydrothermal circulation and serpentinization

  6. Slab hydration (Hedenquist, 1994)

  7. Slab dehydration • Can occur by dehydration (destabilization of the water-bearing mineral phases) or by diffusion • depends on the stability of water-bearing minerals

  8. Mineral phases stability • Sediments: lose 2/3 of their water content before 50 km depth • Amphibolitic crust and sediments are water-depleted at ~120 km • Serpentinized lithosphere (antigorite) may be stable till 200 km • The quantity of water retained in the slab may vary with subduction speed

  9. Slab dehydration (Arcay et al., 2005)

  10. Slab dehydration 7.6 Ma (Arcay et al., 2005)

  11. Slab dehydration • Dehydration of a stagnant slab -> more effective water storage in the transition zone

  12. Slab dehydration • Change of the rheological properties of the slab (subduction pull) • Seismicity in the slab: reactivation of fractures through dehydration processes (Hirschmann, 2006)

  13. Water content influence on mantle • Augmentation of the water content in the mantle provokes partial melting Effect of H2O on peridotite solidus (Hirschmann, 2006)

  14. Water content influence on mantle • Water-influenced partial melting • Mantle hydration may favor astenospheric flow (Hirschmann, 2006)

  15. Water content influence on mantle • Temperature, strain rate and effective viscosity in the mantle wedge Dashed line: away from the hydration-dehydration zone (Arcay et al., 2005)

  16. Conclusions • Lots of model exist, but it is difficult to see how exact those are • However it is certain that water is a determining factor in mantle dynamics

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