Transcript Earth

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?
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effects of water in the mantle
parameterized convection models including volatile
exchange (by others)
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approach to full mantle convection model including volatile
exchange and feedback
●
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show and discuss preliminary results
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next steps
Presence of even small amounts
of water significantly reduces
flow strength of rocks
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Hirth and Kohlstedt, 1996
Presence of even small amounts
of water significantly reduces
flow strength of rocks
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Solidus and liquidus position in
p,T space are a function of water
content
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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
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...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
rehydration
depth
Convection model including de- and rehydration
temperature
water content
partial melting
= dehydration
Simple initial experiments:
viscosity is function of water content
if [water] < [water]threshold then  = 1
if [water]  [water]threshold then  = 2
 1 = f *  2, f 1
● viscosity jump (factor 30) over perovskite phase transition
●
(extended Boussinesq approximation, phase transition at 670 km (g=-3MPaK1,dr /r =0.08), double-diffusive finite element model)
0
Earth: initially wet interior
t=0
t=1.3 Gyr
t=2.7 Gyr
t=4.4 Gyr
temperature
water content
viscosity
Earth: initially dry interior
t=0
t=1.3 Gyr
t=2.7 Gyr
t=4.4 Gyr
temperature
water content
viscosity
Earth
Mars; rehydration during first 500 Myr
t=0
t=1.5 Gyr
t=3.0 Gyr
t=4.5 Gyr
temperature
water content
viscosity
Earth
Mars
Venus: active surface
t=0
t=1.3 Gyr
t=2.7 Gyr
t=4.4 Gyr
temperature
water content
viscosity
Venus: stagnant lid
t=0
t=1.3 Gyr
t=2.7 Gyr
t=4.4 Gyr
temperature
water content
viscosity
Earth:
initially wet
●initially dry
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Mars
Venus:
active surface
●stagnant lid
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Earth:
initially wet
●initially dry
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Different initial situations tend
to converge over several billion
years due to balance between
degassing and regassing.
Mars
Venus:
active surface
●stagnant lid
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rapid dehydration
no rehydration
(after 500 Myr)
high viscosity
slow cooling
slow cooling
low core
heat flow
Earth:
initially wet
no magnetic
field
initially dry
hydrogen escape
●
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Mars
Venus:
active surface
●stagnant lid
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Degassing causes slow cooling or
even heating. However, this effect
would be (partly) counteracted by
temperature-dependent viscosity.
no rehydration
high viscosity
slow cooling
slow cooling
low core heat flow
no magnetic field
hydrogen escape
Earth:
initially wet
●initially dry
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Mars
Venus:
active surface
●stagnant lid
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next steps:
● temperature-dependence of viscosity
● use particle tracers for water concentration
● sensitivity tests of different parameters
● increase Rayleigh number
Peter van Thienen acknowledges the financial support provided through the European
Community's Human Potential Programme under contract RTN2-2001-00414, MAGE