Influence of water in Earths processes

1
Influence of water in Earths 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 -gt 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
(No Transcript)
16
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)
17
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