Water, Water Everywhere

1
Water, Water Everywhere?

• Christoph Helo
• and
• Aleksandra Mloszewska

2
Water on Earth Where is it?

• Atmosphere
• Hydrosphere
• Lithosphere hydrothermal alteration products
  (micas, amphiboles, etc)
• Mantle hydrous phase minerals, basaltic magmas

3
Water in the Mantle Evidence?

• Erupted volcanic rocks
• Partitioning of water-bearing mineral phases
  under mantle conditions
• Subducted water isnt equal to water coming out
  of MORs
• Mantle minerals eg. wadsleyite
• Estimates of water content

4
Water How is it Stored in the Mantle?

• Mineral phases
• Fluid phase
• Melt phase

(Ahrens, 1989)
(Ahrens, 1989)
5
Mantle Mineral Phases
(Ahrens, 1989)
Ohtani et al, 2004)
6
Water Storage in the Mantle
(Hirschmann, 2006)
7
The Concept of Storage Capacity

• H2O storage capacity
• Maximum mass fraction of H2O
• ? Depending on
• T, P
• f(H2O)
• Mineral composition/assemblage

Partition Coefficient ? Distribution of H2O
between two phases e.g. min/fluid or min1/min2
Hirschmann et al. (2005).
8
Storage Upper Mantle
Main mineral assemblage a-Ol, Gt (Al2O3-rich) ,
Cpx, Opx

• Storage capacity of olivine (Mg,Fe)2SiO4
• Increasing with pressure
• Maximum at about 400km of
• lt5000 ppm (experimental)
• OH in the crystal structure
• 2FeM 2Oxo H2O ? 2FeM 2(OH)o ½ O2
• Oxo H2O ? (OH)o (OH)I

1100C
Hirschmann et al. (2005).
9
Storage Upper Mantle

• Storage capacity of Opx, Cpx and Gt
• Partiton coefficients for high P hardly
  constrained
• Low P data Dol/px 10, and Dol/gt 2
• H2O analysis at high P similar storage capacity
  for olivine and enstatite
• significant higher capacities for Al-Opx

Storage capacity for the upper mantle
Minimum-assumption Dpx/ol Dgt/ol 1 ?
0.4wt. H2O at 410 km Maximum-assumption
Dpx/ol 10, Dgt/ol 2 ? 1.2 wt. H2O at 390
km Realistic-assumption Dpx/ol diminishes ?
0.65 wt. H2O at 350 km
10
Storage Transition zone
Main mineral assemblage b-Ol (wadsleyite), g-Ol
(ringwoodite), Gt, Cpx

• Storage capacity of wadsleyite (Mg,Fe)2SiO4
• Pure wadsleyite capacity highly dependent on
  temperature
• Fe-wadsleyite higher capacity (1-3 wt)
• no T dependence
• Ringwoodite lt1 wt
• ? At the top of transition zone
• H2O storage capacity of 0.9-1.5 wt.
• OH in the crystal structure (point defects)
• 1.) O1- or O2-Side as (OH)o
• 2.) M2-Side as (2H)xM
• 3.) Free proton as H

Hirschmann et al. (2005).
11
Storage Lower Mantle (the Dessert)

• Perovskite between 0 1800 ppm H2O meassured,
  highly depending on the composition (Al, Fe, Ca)
  and analysis
• Ferropericlase 20 2000 ppm H2O
• Stishovite 2 - 72 ppm H2O
• Magnesiwüstite 2000 ppm H2O
• Large uncertainties in the actual water content
  due to analytical difficulties, e.g. inclusions
  of superhydrous phases
• Depening on the model water storage capacities
  vary between
• 3 to three times the earths ocean mass (!!!)

12
The Earths Sponge Layer
(Hirschmann, 2006)
13
Water in the transition zone observed?
Electric conductivity in the upper and lower
transition zone of the Pacific
(Wadsleyite)
(Ringwoodite)
Huang et al. (2005).
? Water content of transition zone 0.1-0.2 wt.

14
Water in the Transition Zone Some Implications

• Advection through the 410 km discontinuity
• Potential partial melting,
• if water content gt 0.4 wt. (model!)
• Peridotite will lose all excess water
• Further upwelling results into further
• dehydration melting

Hirschmann et al. (2005).
15
Water in the Mantle Transport

• Subduction of oceanic crust hydrous minerals at
  up to 25km 35km
• lt50km most water released due to P-T conditions
• At 400km eclogite transforms into garnetite
• Water that is left is held in more stable
  minerals and transported into transition zone

16
Conclusions

• Little constrains, many speculations
• Lower mantle dry (dessert )
• Transition zone wet? (sponge?)
• Upper mantle in between
• Phase B minerals (e.g. wadsleyite, ringwoodite)
  important potential water-bearing phases
• A wet transition zone might have significant
  implications for mantle convection, melt
  generation

17
Refernces
Bercovici, D., and Karato, S.-i., 2003.
Whole-manrle convection and the transition zone
water filter. Nature 425, 39-43. Bolfan-Casanova,
N., Keppler, H., Rubie, D.C., Water partitioning
between nominally anhydrous minerals in the
MgO-SiO2-H2O system up to 24 GPa. Implications
for the disribution of water in the earths
mantle Hirschmann, M.M., Aubaud, C., Wihters,
A.C., 2005. Storage capacity of H2Oin nominally
anhydrous minerals in the upper mantle. EPSL 236,
167-181. Hirschmann, M.M., 2006. Water,Melting,
and the Deep Earth H2O Cycle. Annu Rev Earth
Planet Sci 34, 629-653. Huang, X., Xu, Y.,
Karato, S.-i., 2005. Water content in thr
transition zone from conductivity of wadsleyite
and ringwoodite. Nature 434, 746-749. Litasov K.,
Ohtani, E., Langenhosrt, F., Yurimoto, H.,
Tomoaki, K., Kondo, T., 2003. Water solubility in
Mg-perovskites and water storage capacity in the
lower mantle. EPSL211, 189-203.