on the role of water in diverging planetary geodynamics some preliminary results peter van thienen...
TRANSCRIPT
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On the Role of Water in Diverging Planetary Geodynamics
some preliminary results
Peter van Thienen and Philippe Lognonné
Département de Géophysique Spatiale et Planétaire,Institut de Physique du Globe de Paris
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What am I going to talk about?
● effects of water in the mantle
● parameterized convection models including volatile exchange (by others)
● approach to full mantle convection model including volatile exchange and feedback
● show and discuss preliminary results
● next steps
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● Presence of even small amounts of water significantly reduces flow strength of rocks
Hirth and Kohlstedt, 1996
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● Presence of even small amounts of water significantly reduces flow strength of rocks
● Solidus and liquidus position in p,T space are a function of water content
Katz et al., 2003
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● Presence of even small amounts of water significantly reduces flow strength of rocks
● Solidus and liquidus position in p,T space are a function of water content
● water behaves as incompatible element during partial melting -> extracted from matrix in melting zones
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● Presence of even small amounts of water significantly reduces flow strength of rocks
● Solidus and liquidus position in p,T space are a function of water content
● water behaves as incompatible element during partial melting -> extracted from matrix in melting zones
● Mantle rehydration by subducting slabs
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● Presence of even small amounts of water significantly reduces flow strength of rocks
● Solidus and liquidus position in p,T space are a function of water content
● water behaves as incompatible element during partial melting -> extracted from matrix in melting zones
● Mantle rehydration by subducting slabs
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So water exchange of solid planet may be very important for:● geodynamical behaviour● chemical differentiation● evolution of the system
...but also crucial for generation and evolution of hydrosphere/ atmosphere:● early wet phase of martian surface● life
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parameterized convection model by McGovern and Schubert (1989)
Conclusions:● important effects on thermal evolution● rapid equilibration● degassing compensated by temperature increase● regassing compensated by temperature decrease● volatiles reinforce regulation of Urey ratio
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parameterized convection model by Franck and Bounama (1995)
conclusions:● rapid outgassing (100 Myr) in early Earth● less efficient outgassing for Venus
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dept
h
temperature water content
partial melting = dehydration
rehydration
Convection model including de- and rehydration
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Simple initial experiments:
● viscosity is function of water contentif [water] < [water]
threshold then =1
if [water] [water]threshold then =2
1f 2, f 1● viscosity jump (factor 30) over perovskite phase transition
(extended Boussinesq approximation, phase transition at 670 km (=-3MPaK-1,d/0=0.08), double-diffusive finite element model)
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temperature water content viscosity
t=0
t=1.3 Gyr
t=2.7 Gyr
t=4.4 Gyr
Earth: initially wet interior
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temperature water content viscosity
t=0
t=1.3 Gyr
t=2.7 Gyr
t=4.4 Gyr
Earth: initially dry interior
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Earth
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temperature water content viscosity
t=0
t=1.5 Gyr
t=3.0 Gyr
t=4.5 Gyr
Mars; rehydration during first 500 Myr
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Earth
Mars
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temperature water content viscosity
t=0
t=1.3 Gyr
t=2.7 Gyr
t=4.4 Gyr
Venus: active surface
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temperature water content viscosity
t=0
t=1.3 Gyr
t=2.7 Gyr
t=4.4 Gyr
Venus: stagnant lid
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Earth:●initially wet●initially dry
Mars
Venus:●active surface●stagnant lid
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Earth:●initially wet●initially dry
Mars
Venus:●active surface●stagnant lid
Different initial situations tend to converge over several billion years due to balance between degassing and regassing.
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Earth:●initially wet●initially dry
Mars
Venus:●active surface●stagnant lid
no rehydration(after 500 Myr)high viscosityslow cooling
slow coolinglow core heat flowno magnetic fieldhydrogen escape
rapid dehydration
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Earth:●initially wet●initially dry
Mars
Venus:●active surface●stagnant lid
Degassing causes slow cooling oreven heating. However, this effectwould be (partly) counteracted by temperature-dependent viscosity.
no rehydrationhigh viscosityslow cooling
slow coolinglow core heat flowno magnetic fieldhydrogen escape
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Peter van Thienen acknowledges the financial support provided through the European Community's Human Potential Programme under contract RTN2-2001-00414, MAGE
next steps:● temperature-dependence of viscosity● use particle tracers for water concentration● sensitivity tests of different parameters● increase Rayleigh number