Why are there still heavy elements in the earth's crust?

[jr1956]

When the early Earth melted, the heavy elements were drawn to the center due to gravity. Thus we have a iron core. Why shouldn't the core also have drawn in the heavy elements like uranium? The theory says the heat energy for the melt was due to the radioactive decay Why do we still have a lot of deposits of Uranium in the Earth's crust?

 

[Borek]

Separation was never perfect - we still have plenty of iron in the crust, even if the majority went down to the core.

 

[jim mcnamara]

Later comet and meteor bombardment also brought heavy elements to the outer crust. For example: the Chicxulub crater, dating ~64 million years ago, contains shocked quartz with the element iridium in it.

http://en.wikipedia.org/wiki/Chicxulub_Crater

 

[snorkack]

When the early Earth melted, the heavy elements were drawn to the center due to gravity. Thus we have a iron core.

Iron sank into core not only because it is heavy but because it is insoluble/immiscible in molten rock (and heavier than rock).

Why shouldn't the core also have drawn in the heavy elements like uranium? The theory says the heat energy for the melt was due to the radioactive decay Why do we still have a lot of deposits of Uranium in the Earth's crust?

Because, although heavy, uranium is a lithophile - it is chemically active, forms oxides and silicates and dissolves in molten rocks. So uranium and thorium stayed in rocks.

Chemically inactive heavy elements, like gold, platinum and iridium, do preferentially dissolve in iron - which is why they did mainly sink into core, and this is why they are rare/expensive near surface.

Look at water - rock salt is heavier than raw flesh or waterlogged wood. Yet flesh and wood are insoluble and sink - but once salt is dissolved in water, it absolutely will not sink out of water.

 

[jr1956]

Thank you for your answers. I had thought about the meteorite angle, but no the lithophile characteristic of Uranium. Good analogy with the salt water.