Www.ocean.cf.ac.uk/people/huw/ Dept. Earth SciencesJ. Huw Davies, Hydrofrac Model Davies, Nature,...

$: Www.ocean.cf.ac.uk/people/huw/ Dept. Earth SciencesJ. Huw Davies, Hydrofrac Model Davies, Nature, 1999.$
www.ocean.cf.ac.uk/people/huw/

Dept. Earth SciencesJ. Huw Davies,

Hydrofrac Model Davies, Nature, 1999



Temperature dependent rheologyModel with temperature dependent rheology, example here would ‘eat’ all the way to the surface.

Possible to get freezing out –see Kincaid and Sacks. We know we get magmas therefore I prefer hot end-point => need means to stop eating all the way to surface. Possibly lithosphere in wedge corner is crust (i.e. buyoancy not rheology keeps things near surface)

Starting Model

Model a little time later – not steady-state



Water reacts with mantle – Davies (1994) Davies and Stevenson (1992)

Mantle largely dry

Water enters wedge

Water reacts with largely dry peridotite

melting

Path of water in hydrated mineral

Flow of mantle

Water flow as free phase



My Favourite Model (Iwamori, EPSL,1998)

Equilibrium, 130Ma, 6cm/yr

Equilibrium, 10Ma, 6cm/yr



Iwamori (EPSL, 1998)

Disequilibrium, 130Ma, 6cm/yr



Iwamori (EPSL, 1998 – Fig Cap for Fig 5) Fig. 5. Distribution of H2O (left) and melt (right). (a) For a relatively cold slab (age 130 Myr) with a Fig. 5. Distribution of H2O (left) and melt (right). (a) For a relatively cold slab (age 130 Myr) with a

constant subduction velocity, of ~6 cm/year. A cross-sectional area of 250x250 km region with a fixed constant subduction velocity, of ~6 cm/year. A cross-sectional area of 250x250 km region with a fixed crust of 30 km thick is divided into a regular grid for numerical calculations, with a finer triangular crust of 30 km thick is divided into a regular grid for numerical calculations, with a finer triangular grid at the slab¯wedge interface and at the bottom of the slab for preventing artificial diffusion of H2O. grid at the slab¯wedge interface and at the bottom of the slab for preventing artificial diffusion of H2O. The thickness of rigid slab can be defined as 2.32 (kt)The thickness of rigid slab can be defined as 2.32 (kt)1/21/2 where k is the thermal diffusivity. If k = 10 where k is the thermal diffusivity. If k = 10-6-6 mm22 s s-1-1, then thickness = 150km. In both the oceanic side (lower left corner) and the mantle wedge side , then thickness = 150km. In both the oceanic side (lower left corner) and the mantle wedge side (upper right) of the rigid slab, the solid flow is assumed to be described by analytic corner flow solutions (upper right) of the rigid slab, the solid flow is assumed to be described by analytic corner flow solutions of incompressible fluid with constant viscosity. The dotted lines in the mantle wedge indicate the stream of incompressible fluid with constant viscosity. The dotted lines in the mantle wedge indicate the stream lines. The thermal boundary conditions are as follows: a surface temperature of 273 K; an error function lines. The thermal boundary conditions are as follows: a surface temperature of 273 K; an error function gradient for the plate age of 130 Myr and an adiabatic gradient underneath for the oceanic side gradient for the plate age of 130 Myr and an adiabatic gradient underneath for the oceanic side boundary; a linear gradient within the crust and within a thermal boundary layer of 12 km beneath the boundary; a linear gradient within the crust and within a thermal boundary layer of 12 km beneath the crust to produce the surface heat flux of 0.115 W/m2, and an adiabatic gradient underneath the arc side crust to produce the surface heat flux of 0.115 W/m2, and an adiabatic gradient underneath the arc side boundary, which gives the potential temperature ~1250°C; zero heat flux at the bottom boundary.The boundary, which gives the potential temperature ~1250°C; zero heat flux at the bottom boundary.The solid lines indicate the isothermal contours with a 200K interval. A steady geothermal structure for solid lines indicate the isothermal contours with a 200K interval. A steady geothermal structure for H2O-free subduction of the slab was assumed for an initial condition where no melt exists, then the slab H2O-free subduction of the slab was assumed for an initial condition where no melt exists, then the slab with 6 wt% H2O started to subduct. The elapsed time for this snapshot is 7.1 Myr. (b) For a hot slab with 6 wt% H2O started to subduct. The elapsed time for this snapshot is 7.1 Myr. (b) For a hot slab (age of 10 Myr) with a constant subduction velocity of ~6 cm/year. The thickness of rigid slab is 40km. (age of 10 Myr) with a constant subduction velocity of ~6 cm/year. The thickness of rigid slab is 40km. The other conditions are the same as in (a). The elapsed time for this snapshot is 4.1 Myr. (c) For a case The other conditions are the same as in (a). The elapsed time for this snapshot is 4.1 Myr. (c) For a case involving disequilibrium transportation of H2O. A small portion of the aqueous fluid (8% of the aqueous involving disequilibrium transportation of H2O. A small portion of the aqueous fluid (8% of the aqueous fluid present in each local system) is assumed to be isolated chemically in the local system. fluid present in each local system) is assumed to be isolated chemically in the local system. Consequently, once the aqueous fluid is produced, it can survive and continue to migrate even if the Consequently, once the aqueous fluid is produced, it can survive and continue to migrate even if the surrounding solid and melt are not saturated with H2O. The other conditions are the same as in (a). The surrounding solid and melt are not saturated with H2O. The other conditions are the same as in (a). The elapsed time for this snapshot is 2.9 Myr. elapsed time for this snapshot is 2.9 Myr.



Iwamori (EPSL, 1998)

Fig 6



Physical and Chemical Constraints on SZ processes Physical – (Many subduction zones – many experiments)Physical – (Many subduction zones – many experiments)

Plate velocities, Age of subducting lithosphere, thickness overriding plate crustPlate velocities, Age of subducting lithosphere, thickness overriding plate crust Shape of Benioff zone, double zone? Dip?Shape of Benioff zone, double zone? Dip? Location of magmatism – See below. Rate of magmatism, temp. of lavasLocation of magmatism – See below. Rate of magmatism, temp. of lavas Surface shape – trench depth, outer arc rise – GPS, satellite Surface shape – trench depth, outer arc rise – GPS, satellite Heat flux – Broad scale good, but at local scale there are many poorly understood Heat flux – Broad scale good, but at local scale there are many poorly understood

processesprocesses Seismology, Seismology,

Tomography – Velocity (P,S), Attenuation, low vel. zone (crust?)Tomography – Velocity (P,S), Attenuation, low vel. zone (crust?) Anisotropy – interpretation – water?Anisotropy – interpretation – water? Focal mechanisms, stress regimesFocal mechanisms, stress regimes 3D seismic – ANCORP, Shipley et al., Banks et al., 3D seismic – ANCORP, Shipley et al., Banks et al., Down- and up-dip extent of mega-thrust planeDown- and up-dip extent of mega-thrust plane

Lab measurements – rheology, anisotropy, dihedral angleLab measurements – rheology, anisotropy, dihedral angle Electrical conductivityElectrical conductivity Gravity and geoid – low density/low viscosityGravity and geoid – low density/low viscosity



Chemistry – help!

Inputs – Drilling of ocean sediments and basaltsInputs – Drilling of ocean sediments and basalts Outputs – CompositionOutputs – Composition

Major elements – differentiation, primary magmas eq. temp, degree of meltingMajor elements – differentiation, primary magmas eq. temp, degree of melting Trace elements – LILE/HFSE, B, degree of meltingTrace elements – LILE/HFSE, B, degree of melting XenolithsXenoliths Isotopes - Stable/Radiogenic – Be10, U versus Pb, Th versus BeIsotopes - Stable/Radiogenic – Be10, U versus Pb, Th versus Be Fluid/Melt Inclusions – water contentsFluid/Melt Inclusions – water contents Uranium decay chain isotopes – time scales – fastUranium decay chain isotopes – time scales – fast Volatiles - fluxesVolatiles - fluxes

Expts. – Melting expts. –Expts. – Melting expts. – Sediments, Peridotite, Basalt +/- water, composition – diamond aggregatesSediments, Peridotite, Basalt +/- water, composition – diamond aggregates Partitioning – including improved theoryPartitioning – including improved theory

Thermodynamic databases – MELTS – extend to hydrous systemsThermodynamic databases – MELTS – extend to hydrous systems Outputs from other parts of the mantle; e.g. OIB - recycled SZ plates?Outputs from other parts of the mantle; e.g. OIB - recycled SZ plates?



Location of arc relative to Benioff zone – h

mantle

slab

o.c.

Volcanic Front

h

earthquakes



h (km) – constant along arc segments, but different from segment to segment – England, Engdahl et al. (unpublished)

120

105

125110

85105

6580

105

115

135

105

105

120



Subduction Tomography – Zhao + Hasegawa (1993)

Non-unique but a 3D constraint

Resolution?

Combine with 3D reflection seismics?



Temperature of Primary Magmas Tatsumi et al., Nye and Reid, etc.Tatsumi et al., Nye and Reid, etc. Questions – Questions –

Is this how magmas form? i.e. equilibrium, batchIs this how magmas form? i.e. equilibrium, batch Do we sample and can we identify primary magmas?Do we sample and can we identify primary magmas?

Pressure/Depth

Temperature

ol+opx+cpx+l

opx+cpx+lol+cpx+l

opx+ll

ol+l

50km

1320oC

Tatsumi et al., 1983

High Alumina Basalt + 1.5% water



Chemical correlations

Stolper and Newman – Water and ChemistryStolper and Newman – Water and Chemistry Plank and Langmuir – Sediment input and Plank and Langmuir – Sediment input and

volcanic outputvolcanic output Elliott et al. - Elliott et al. - EtcEtc



Melting of sediments Claims that sediments do meltClaims that sediments do melt Experiments constrain temperature of sediment meltingExperiments constrain temperature of sediment melting Therefore constraint on temperature at sediment – wedge Therefore constraint on temperature at sediment – wedge

boundaryboundary Temperatures are generally higher than predicted by Temperatures are generally higher than predicted by

numerical thermal models (but models, while precise are numerical thermal models (but models, while precise are not very accurate – equally interpretation of presence of not very accurate – equally interpretation of presence of melt is debatable – remember difference between fluid melt is debatable – remember difference between fluid with high silica content and melt with high fluid contents with high silica content and melt with high fluid contents might be small)might be small)



Constraints – Huw Stops (You start)

h – geometryh – geometry Seismic tomographySeismic tomography Sediment meltingSediment melting Chemical correlations – including timescalesChemical correlations – including timescales Temperature of primary magmasTemperature of primary magmas

Www.ocean.cf.ac.uk/people/huw/ Dept. Earth SciencesJ. Huw Davies, Hydrofrac Model Davies, Nature,...

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