The lithosphere: a zone refinery?

[Loren Booda]

Do planets that undergo a gradual cooling inward of a liquid core in essence perform zone refining, like that used to purify silicon for semiconductors? Starting with a solidifying crust, the crystallization of molten rock selectively freezes out first the materials with both the lowest melting points and least density, pushing elements like tungsten and uranium toward the center. How closely would the geological compositions of the Earth and other terrestrial planets (especially once solidified) bear out this pattern? At what level would you most likely find crystalline matter?

 

[Andre]

Well, an early hypothesis is to assume that Venus is still in a young phase compared to Earth just after forming. The basic problems with that is that Venus has lost all but traces of it's water and has no mechanism left to burry the carbon in the soil.

If Earth would have started the same way, how would it have prevented the same to happen? So if we forget about Venus, it actually appears that the the compression heat at the surface did not exceed the Venus levels, allowing the water to settle instead of splitting into it's basic components, the hydrogen dissipating into space.

There is also a rock that supports that version:

http://www.sciencedaily.com/releases/2001/01/010111073459.htm

 

[Loren Booda]

Thanks for the education, Andre. That find seems most revolutionary - 2 billion more years for Terran evolution! At what temperature does water dissociate into hydrogen and oxygen?

 

[matthyaouw]

Do planets that undergo a gradual cooling inward of a liquid core in essence perform zone refining, like that used to purify silicon for semiconductors? Starting with a solidifying crust, the crystallization of molten rock selectively freezes out first the materials with both the lowest melting points and least density, pushing elements like tungsten and uranium toward the center.

Earth's inner core is actually solidifying from the middle outwards- Pressure makes the inner core solid, despite it being hotter than the liquid outer core, but as temperature falls, the pressure is able to solidify more and more of the inner core.

The stratification of the layers of the Earth was indeed due to sinking of denser minerals during the Earth's fully molten stage (http://csmres.jmu.edu/geollab/Fichter/PlateTect/heathistory.html) and some further stratification still occurs now in that lighter minerals such as quartz rise up due to their lower density as crust melts at subdction zones.

How closely would the geological compositions of the Earth and other terrestrial planets (especially once solidified) bear out this pattern? At what level would you most likely find crystalline matter?

On earth, you would indeed find layering, with a core thoght to consist mostly of iron, followed by a mantle, thought to be made of alkaline material similar to that which is found deep in ocean crust, which is represented at the surface by ophiolites (http://volcano.und.edu/vwdocs/vw_hyperexchange/ophiolites.html) .
Next comes the crust, which is more of a mixture, consisting of alkaline material directly accreated from the mantle in oceanic plates, and the lower edges of continental plates, through to acidic (granitic) material comprising most of the continents.
As for crystalline material- if Earth were to fully cool, crystaline material would span the crust and the whole mantle, with crystals becoming increasingly coarse towards the middle. The core I'm not too sure about- being slowly cooling liquid, I'd imagine it would form crystals, but I'm not sure. There is some data suggesting the inner core is in fact one large crystal http://www.psc.edu/science/Cohen_Stix/cohen_stix.html

It's hard to apply this to other planets, as we have little to analyse but a very few rocks from some, and aerial imagary.

 

[Loren Booda]

Thanks for a great thumbnail explanation, matt. I always thought we were riding on a predominantly liquid center; now it seems we're bound to be a crystal planet. Do you know how close the uranium at our core is to critical mass?

 

[matthyaouw]

I don't have a clue about the critical mass, sorry.
I should probably clarify- The mantle is thought to be solid, but flowing by plastic deformation. It's quite a commn misconception that it is composed of flowing magma.

 

[Bystander]

?How close to critical mass?" Uranium is a lithophile. The oxide is more soluble in the crustal rocks, than in the iron core. Short answer? There ain't no critical mass, there ain't no chance of a critical mass forming.

 

[Loren Booda]

How has the coriolis effect influenced the structure of the lithosphere?

 

[matthyaouw]

The only effect that I can think of is the slight bulging of the Earth at the equator due to centrifugal force. I'd imagine that coriolis force is insignificant compared to the tectonic forces acting on the lithosphere, so effects would be minimal if they exist at all. I wonder if the coriolis force can affect the movements of plastic flow (i.e. the mantle), or whether it is just fluids.