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