Water, Water Everywhere? Christoph Helo and Aleksandra Mloszewska
Wateron 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)
High Pressure silica-rich Temperature fluid H2O-rich storage capacity mineral increase decrease mineral + fluid H2O Silicate The Concept of Storage Capacity H2O storage capacity • Maximummass 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).
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 <5000 ppm (experimental) OH in the crystal structure 2Fe*M+ 2Oxo+ H2O 2FeM+ 2(OH)*o+ ½ O2 Oxo+ H2O (OH)*o+ (OH)’I 1100°C Hirschmann et al. (2005).
Dpx/ol=10 Dgt/ol=2 Dol/px=Dol/gt=1 Hirschmann et al. (2005). 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
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) (Ringwoodite) Huang et al. (2005). Water content of transition zone: ~0.1-0.2 wt.%
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 Mg-perovskites and water storage capacity in the lower mantle. EPSL211, 189-203.