Non Life Sciences > Geology, Palaeontology & Archaeology
Are radioactive elements in the Earth's mantle evenly distributed?
evan_au:
Since the earth appears to be cooling from a once-molten state, we can generally assume that the elements are roughly distributed by density, with the densest ones at the core, and lighter ones towards the outside.
Since the naturally radioactive elements with a half life > 109 years tend to have a high density, we can assume that there is more of these heavy minerals like Uranium, Radium and Thorium closer to the center of the earth, inside the iron core. One exception is Potassium, which is radioactive, but fairly light, and found in silicate minerals.
Radioactive minerals do occur in scattered deposits on the surface of the Earth, from igneous rocks and meteorites. These are often sorted into ore beds because of the melting temperatures of their minerals, their chemical properties, or because of sorting on the basis of density (like mineral sands).
Bill S:
--- Quote --- we can assume that there is more of these heavy minerals like Uranium, Radium and Thorium closer to the center of the earth, inside the iron core.
--- End quote ---
Is there any evidence for the presence of radioactive elements in the core?
evan_au:
Since we have not really drilled any deeper than the crust, there is no direct evidence at this time.
Indirect evidence comes from models of earth's cooling rate, which would be slowed by the ongoing decay of radioactive elements in the core and the insulation of the crust.
Here are the results of some neutrino observations from within the earth. It suggests that half of the earth's heat comes from uranium & thorium decay. It mentions the current theory that only iron-soluble elements would be in the core - but with the small sample size (111 neutrinos so far), it will take a while for this new technique to narrow down the geographical distribution within the earth. http://physicsworld.com/cws/article/news/2011/jul/19/radioactive-decay-accounts-for-half-of-earths-heat
I guess we could go looking for radioactive materials within differentiated meteorites - to see whether radioactive minerals are enhanced or depleted in iron meteorites vs stony meteorites.
Bill S:
--- Quote ---The most popular model of radioactive heating is based on the bulk silicate Earth (BSE) model, which assumes that radioactive materials, such as uranium and thorium, are found in the Earth's lithosphere and mantle – but not in its iron core.
--- End quote ---
Thanks for the link, Evan_au. I knew I had got the idea from somewhere, but I'd forgotten where it came from.
evan_au:
I just found this: The Goldschmidt classification scheme suggests that Uranium & Thorium easily react with oxygen, and will stay in the crust (lithophiles), while others are soluble in iron and will remain in the core (siderophiles).
http://en.wikipedia.org/wiki/Goldschmidt_classification
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