Water and Mantle Melting

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Published on December 30, 2007

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Water and Mantle Melting:  Water and Mantle Melting How water influences the melting function - depression of solidus - dF/dT, dF/dP etc. Effects on composition of melts Importance to mantle rheology and dynamics Water storage in anhydrous minerals :  Water storage in anhydrous minerals Water-saturated solidus:  Water-saturated solidus Water-saturated solidus:  Water-saturated solidus Water-saturated solidus:  Water-saturated solidus Depth to the under-saturated solidus: Approach I. Regression of experimental data. :  Depth to the under-saturated solidus: Approach I. Regression of experimental data. Depth to the under-saturated solidus: Approach I. Regression of experimental data.:  Depth to the under-saturated solidus: Approach I. Regression of experimental data. Depth to the under-saturated solidus: Approach I. Regression of experimental data. II. :  Depth to the under-saturated solidus: Approach I. Regression of experimental data. II. =XH2OBulk/[DH2O(1-F)+F] Depth to the under-saturated solidus: Approach I. Regression of experimental data.:  Depth to the under-saturated solidus: Approach I. Regression of experimental data. Depth to the under-saturated solidus: Approach II. Theory-based & constrained by water-saturated & dry solidi. :  Depth to the under-saturated solidus: Approach II. Theory-based & constrained by water-saturated & dry solidi. Depth to the under-saturated solidus: Approach II. Theory-based & constrained by water-saturated & dry solidi:  Depth to the under-saturated solidus: Approach II. Theory-based & constrained by water-saturated & dry solidi water-saturated solidus aH2O=1 dry solidus aH2O=0 Map contours of aH2O between dry & saturated solidi. This gives Tsolidus(aH2O,P) Next we need to know aH2O.... Depth to the under-saturated solidus: Approach II. Theory-based & constrained by water-saturated & dry solidi:  Depth to the under-saturated solidus: Approach II. Theory-based & constrained by water-saturated & dry solidi 2) Compute fraction saturation XH2O(P)=Col0/ Colsat(P) Olivine water content: Col0 of MORB source Colsat(P) (there’s something wrong with the data &/or analysis used for this part) Depth to the under-saturated solidus: Approach II. Theory based & constrained by data for water-saturated & dry solidi:  Depth to the under-saturated solidus: Approach II. Theory based & constrained by data for water-saturated & dry solidi 2) Compute fraction saturation XH2O(P)=Col0/ Colsat(P) 3) aH2O(P)=lXH2O(P), with activity coefficient l = 1.0 Depth to the under-saturated solidus: Approach II. :  Depth to the under-saturated solidus: Approach II. water-saturated solidus aH2O=1 dry solidus aH2O=0 Map aH2O (in the solid) onto P-T phase diagram, Tsolidus(aH2O,P) Compute fraction saturation XH2O(P)=Col0/ Colsat(P) aH2O(P)=lXH2O(P), with activity coefficient l = 1.0 Combine (1) and (3) to get Tsolidus(aH2O (P) ,P) Tsolidus(P) Depth to the under-saturated solidus: Approach III. adapted from Hirth and Kohlstedt:  Depth to the under-saturated solidus: Approach III. adapted from Hirth and Kohlstedt Depth to the under-saturated solidus: Approach III. adapted from Hirth and Kohlstedt:  Depth to the under-saturated solidus: Approach III. adapted from Hirth and Kohlstedt pMELTS can deal with H2O in anhydrous minerals only hydrous minerals, melt, or vapor therefore... -Compute aH2O in olivine like HK, -Solves equilibrium mineralogy -Checks if pMELTS predicts melt or vapor phase Melting between the hydrous and dry solidus dF/dT, dF/dP:  Melting between the hydrous and dry solidus dF/dT, dF/dP I. Katz et al.’s parameterization Melting between the hydrous and dry solidus dF/dT, dF/dP:  Melting between the hydrous and dry solidus dF/dT, dF/dP I. Katz et al.’s parameterization Melting between the hydrous and dry solidus dF/dT, dF/dP:  Melting between the hydrous and dry solidus dF/dT, dF/dP II. Hirth and Kohlstedt water-saturated solidus aH2O=1 dry solidus aH2O=0 Batch melting equation gives F=[Col0/Col-1]D0/(1-D0) Col0=810 ppm at z=115 km Col=360 ppm at z=92 km dF/dz~1%/20 km 92 115 Melting between the hydrous and dry solidus dF/dT, dF/dP:  Melting between the hydrous and dry solidus dF/dT, dF/dP III. Azimow et al. Pressure- & concentration- dependent water-mineral D’s Compute water contents & activities in minerals after HK Equilibrium mineralogy & melt fraction Water and Mantle Melting:  Water and Mantle Melting How water influences the melting function - depression of solidus - dF/dT, dF/dP etc. Effects on composition of melts Importance to mantle rheology and dynamics Importance of Hydrous Melting on Magma Composition:  Importance of Hydrous Melting on Magma Composition Importance of Hydrous Melting on Magma Composition:  Importance of Hydrous Melting on Magma Composition Ito and Mahoney [2006, 2006] Slide24:  Azimow et al. [2004] Data from Mid-Atlantic Ridge near Azores hotspot Light lines- computed primary magma Heavy lines- corrected for low-P fractionation Importance of Hydrous Melting on Mantle Dynamics:  Importance of Hydrous Melting on Mantle Dynamics Hirth & Kohlstaedt [1996]: olivine with 810 ppm H2O is ~102 less viscous than dry olivine. Importance of Hydrous Melting on Mantle Dynamics:  Importance of Hydrous Melting on Mantle Dynamics Hirth & Kohlstaedt [1996]: olivine with 810 ppm H2O is ~102 less viscous than dry olivine. Importance of Hydrous Melting on Mantle Dynamics:  Importance of Hydrous Melting on Mantle Dynamics Importance of Hydrous Melting on Mantle Dynamics:  Importance of Hydrous Melting on Mantle Dynamics Controls the flattening of seafloor subsidence and limits lithospheric thickening on old seafloor? Ritzwoller et al., [EPSL, 226, 2004] Importance of Hydrous Melting on Mantle Dynamics:  Importance of Hydrous Melting on Mantle Dynamics Limits buoyant upwelling and melt production at hotspots Water and Mantle Melting:  Water and Mantle Melting How water influences the melting function - depression of solidus - dF/dT, dF/dP etc. Effects on composition of melts Importance to mantle rheology and dynamics

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