Transcript EPSC 666 Mod 3 Pres

Water, Water Everywhere?
Christoph Helo
and
Aleksandra Mloszewska
Water 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?
• 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?
• Mineral phases
• Fluid phase
• Melt phase
(Ahrens, 1989)
(Ahrens, 1989)
Mantle Mineral Phases
(Ahrens, 1989)
Ohtani et al, 2004)
Water Storage in the Mantle
(Hirschmann, 2006)
The Concept of Storage Capacity
H2O storage capacity
High Pressure
 Maximum mass fraction of H2O
 Depending on:
silica-rich
decrease
• f(H2O)
fluid
mineral
• Mineral composition/assemblage
Temperature
• T, P
H2O-rich
storage capacity
increase
Partition Coefficient
mineral + fluid
 Distribution of H2O between two
phases e.g. min/fluid or min1/min2
Silicate
Hirschmann et al. (2005).
H2O
Storage: Upper Mantle
Main mineral assemblage: a-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
x
2Fe*M+ 2O o+ H2O  2FeM+ 2(OH)*o+ ½ O2
x
Hirschmann et al. (2005).
O o+ H2O  (OH)*o+ (OH)’I
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
Dpx/ol=10
Dgt/ol=2
“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
Dol/px=Dol/gt=1
“Realistic”-assumption: Dpx/ol diminishes
 0.65 wt.% H2O at 350 km
Hirschmann et al. (2005).
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: <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*]
Hirschmann et al. (2005).
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 (!!!)
The Earth’s Sponge Layer
(Hirschmann, 2006)
Water in the transition zone “observed”?
Electric conductivity in the upper and lower transition zone of the Pacific
(Wadsleyite)
Huang et al. (2005).
 Water content of transition zone: ~0.1-0.2 wt.%
(Ringwoodite)
Water in the Transition Zone: Some Implications
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
Hirschmann et al. (2005).
Water 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
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…
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 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 Mgperovskites and water storage capacity in the lower mantle. EPSL211, 189-203.