Influence of water in Influence of water in Earth’s processesEarth’s processes

EPSC666- 3rd moduleEPSC666- 3rd module

Cécile NoverrazCécile Noverraz

Main water-bearing movments Main water-bearing movments in the mantlein the mantle

Slab hydrationSlab hydration Slab dehydrationSlab dehydration Mantle processesMantle processes

Storage Storage

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

Slab hydrationSlab hydration

Serpentinization at mid-ocean ridge: Serpentinization at mid-ocean ridge: rifting and hydrothermal circulationrifting and hydrothermal circulation

Mafic minerals (olivine, pyroxene) are Mafic minerals (olivine, pyroxene) are metasomatized into hydrated minerals metasomatized into hydrated minerals (amphibole, chlorite, serpentine) (amphibole, chlorite, serpentine)

Black smokers formation: concentration Black smokers formation: concentration of metals at the oceanic ridge, metal of metals at the oceanic ridge, metal leaching of the mafic crustleaching of the mafic crust

Slab hydrationSlab hydration

Possibility of more important Possibility of more important hydration in the trench before hydration in the trench before subductionsubduction

Reactivation and creation of normal Reactivation and creation of normal faults with the bend of the slabfaults with the bend of the slab

Extensional deep fault provoke Extensional deep fault provoke hydrothermal circulation and hydrothermal circulation and serpentinizationserpentinization

Slab hydrationSlab hydration

(Hedenquist, 1994)

Slab dehydrationSlab dehydration Can occur by Can occur by

dehydration dehydration (destabilization (destabilization of the water-of the water-bearing mineral bearing mineral phases) or by phases) or by diffusiondiffusion

depends on the depends on the stability of stability of water-bearing water-bearing mineralsminerals

Mineral phases stabilityMineral phases stability

Sediments: lose 2/3 of their water Sediments: lose 2/3 of their water content before 50 km depthcontent before 50 km depth

Amphibolitic crust and sediments are Amphibolitic crust and sediments are water-depleted at ~120 kmwater-depleted at ~120 km

Serpentinized lithosphere (antigorite) Serpentinized lithosphere (antigorite) may be stable till 200 kmmay be stable till 200 km

The quantity of water retained in the The quantity of water retained in the slab may vary with subduction speedslab may vary with subduction speed

Slab dehydration Slab dehydration

(Arcay et al., 2005)

Slab dehydrationSlab dehydration

(Arcay et al., 2005)

7.6 Ma

Slab dehydrationSlab dehydration

Dehydration of a stagnant slab -> more Dehydration of a stagnant slab -> more effective water storage in the transition effective water storage in the transition zonezone

Slab dehydrationSlab dehydration Change of the rheological properties of the Change of the rheological properties of the

slab (subduction pull)slab (subduction pull) Seismicity in the slab: reactivation of Seismicity in the slab: reactivation of

fractures through dehydration processesfractures through dehydration processes

(Hirschmann, 2006)

Water content influence on Water content influence on mantlemantle

Augmentation Augmentation of the water of the water content in the content in the mantle mantle provokes provokes partial meltingpartial melting

Effect of H2O on peridotite solidus (Hirschmann, 2006)

Water content influence on Water content influence on mantlemantle

Water-influenced partial meltingWater-influenced partial melting Mantle hydration may favor astenospheric Mantle hydration may favor astenospheric

flowflow

(Hirschmann, 2006)

Water content influence on Water content influence on mantlemantle

Temperature, strain rate and effective viscosity in Temperature, strain rate and effective viscosity in the mantle wedgethe mantle wedge

Dashed line: away from the hydration-dehydration zone (Arcay et al., 2005)

ConclusionsConclusions

Lots of model exist, but it is difficult Lots of model exist, but it is difficult to see how exact those areto see how exact those are

However it is certain that water is a However it is certain that water is a determining factor in mantle determining factor in mantle dynamicsdynamics