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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
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
● Presence of even small amounts of water significantly reduces flow strength of rocks
Hirth and Kohlstedt, 1996
● 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
● 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
● 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
● 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
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
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
parameterized convection model by Franck and Bounama (1995)
conclusions:● rapid outgassing (100 Myr) in early Earth● less efficient outgassing for Venus
dept
h
temperature water content
partial melting = dehydration
rehydration
Convection model including de- and rehydration
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)
temperature water content viscosity
t=0
t=1.3 Gyr
t=2.7 Gyr
t=4.4 Gyr
Earth: initially wet interior
temperature water content viscosity
t=0
t=1.3 Gyr
t=2.7 Gyr
t=4.4 Gyr
Earth: initially dry interior
Earth
temperature water content viscosity
t=0
t=1.5 Gyr
t=3.0 Gyr
t=4.5 Gyr
Mars; rehydration during first 500 Myr
Earth
Mars
temperature water content viscosity
t=0
t=1.3 Gyr
t=2.7 Gyr
t=4.4 Gyr
Venus: active surface
temperature water content viscosity
t=0
t=1.3 Gyr
t=2.7 Gyr
t=4.4 Gyr
Venus: stagnant lid
Earth:●initially wet●initially dry
Mars
Venus:●active surface●stagnant lid
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.
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
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
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