Water, Water Water, Water Everywhere?Everywhere?

Christoph Helo Christoph Helo and and

Aleksandra MloszewskaAleksandra Mloszewska

WaterWater on Earth: Where is it?on Earth: Where is it?

• Atmosphere

• Hydrosphere

• Lithosphere: hydrothermal alteration products (micas, amphiboles, etc)

• Mantle: hydrous phase minerals, basaltic magmas

Water in the Mantle: Evidence?Water in the Mantle: Evidence?

• Erupted volcanic rocks

• Partitioning of water-bearing mineral phases under mantle conditions

• Subducted water isn’t equal to water coming out of MORs

• Mantle minerals eg. wadsleyite

• Estimates of water content

Water: How is it Stored in the Mantle?Water: How is it Stored in the Mantle?

• Mineral phases

• Fluid phase • Melt phase

(Ahrens, 1989)

(Ahrens, 1989)

(Ahrens, 1989) Ohtani et al, 2004)

Mantle Mineral PhasesMantle Mineral Phases

(Hirschmann, 2006)

Water Storage in the MantleWater Storage in the Mantle

The Concept of Storage CapacityThe Concept of Storage Capacity

fluid

High Pressure

mineral + fluid

silica-rich

H2O-rich

min

eral

Silicate H2O

Tem

pera

ture

H2O storage capacity

Maximum mass fraction of H2O

Depending on:

• T, P

• f(H2O)

• Mineral composition/assemblage

Hirschmann et al. (2005).

increase

decrease

Partition Coefficient

Distribution of H2O between two phases e.g. min/fluid or min1/min2

storage capacity

Storage: Upper MantleStorage: Upper Mantle

Main mineral assemblage: -Ol, Gt (Al2O3-rich) , Cpx, Opx

1100°C Storage capacity of olivine (Mg,Fe)2SiO4

Increasing with pressure

Maximum at about 400km of

<5000 ppm (experimental)

OH in the crystal structure

2Fe*M+ 2O

x

o+ H2O 2Fe

M+ 2(OH)*

o+ ½ O2

Ox

o+ H2O (OH)*

o+ (OH)’

I

Hirschmann et al. (2005).

Storage: Upper MantleStorage: 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

Dol/px=Dol/gt=1

Dpx/ol=10Dgt/ol=2

Hirschmann et al. (2005).

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

Main mineral assemblage: -Ol (wadsleyite), -Ol (ringwoodite), Gt, Cpx

Storage: Transition zoneStorage: Transition zone

Hirschmann et al. (2005).

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: <1 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*]

Storage: Lower Mantle (the Dessert)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 earth’s ocean mass (!!!)

(Hirschmann, 2006)

The Earth’s Sponge LayerThe Earth’s Sponge Layer

Water in the transition zone “observed”?Water in the transition zone “observed”?

Electric conductivity in the upper and lower transition zone of the Pacific

(Wadsleyite) (Ringwoodite)

Water content of transition zone: ~0.1-0.2 wt.%

Huang et al. (2005).

Water in the Transition Zone: Some Water in the Transition Zone: Some ImplicationsImplications

Hirschmann et al. (2005).

Advection through the 410 km discontinuity:

Potential partial melting,

if water content > 0.4 wt.% (model!)

Peridotite will lose all “excess” water

Further upwelling results into further

dehydration melting

Water in the Mantle: TransportWater in the Mantle: Transport

• Subduction of oceanic crust: hydrous minerals at up to 25km – 35km

• <50km 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

• 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…

ConclusionsConclusions

ReferncesRefernces

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 earth’s 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.