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Are there any estimates of the amount of Uranium there is in Earth's Crust?

From what I know, it's supposed that there are large amounts of Uranium in Earth's Core, the decay of which is responsible for the core maintaining its high temperature. Mining the core is hardly imaginable, while the resources in Crust are more accessible available for mining: so how much is in the Crust, and at what levels of concentration? (which affects the viability of accessing it).

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  • $\begingroup$ Are you talking about accessible uranium or uranium in total? If you are talking about accessible uranium then the concentrations in the crust are not that important but rather how easily the element in concentrated. Take for example the rare earth elements, which are not actually rare. They are just dispersed. On the other hand, gold, which is less abundant readily forms concentrated deposits. $\endgroup$ – Gimelist Nov 4 '14 at 10:21
  • $\begingroup$ The core is comprised of nickel-iron alloy. In a very general sense, heat is generated due to gravity compressing the nickel/iron. As such, nuclear decay isn't thought to be the main source of heat from within the earth. Uranium decay does generate heat in the crust, along with the decay of both Potassium and Thorium which can be harnessed through enhanced geothermal wells, although these three elements are abundant, they are not responsible for the majority of the earths internal heat. $\endgroup$ – norman_h Dec 11 '16 at 6:52
  • $\begingroup$ Note that the radioactive decay heat production in Earth's interior happens mainly in the mantle, the core is unlikely to contain much radioactive material. $\endgroup$ – Camilo Rada Feb 11 at 17:08
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According to wikipedia, there are around 5.5 million tonnes of uranium in ore deposits that are commercially viable at current prices, and perhaps 35 million tonnes that are potentially viable if prices increase.

Also according to wikipedia, the Earth's crust (to 25 km depth) contains an estimated 10^14 tonnes (100 trillion tonnes), while the oceans may contain 10^10 tonnes (10 billion tonnes). This presumably includes the ore figures stated above.

The previous link states that "The decay of uranium, thorium, and potassium-40 in the Earth's mantle is thought to be the main source of heat", however no estimate is given for quantities. Allègre, Lewin and Dupré (1988) state that "The concentration of U in the primordial mantle (bulk Earth) has been determined to be ~21±1 ppb". Back of the envelope calculations would then give us that the mantle is 84% of the Earth by volume (probably slightly less than that by mass?), and the mass of the earth is 6*10^21 tonnes, which would give us, very approximately, 0.84 * (6*10^21) * (2.1*10^-8) ~= 10^14 tonnes, or roughly the same as is in the Earth's crust.

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  • $\begingroup$ I'm not a geologist, but it seems odd to me that the crust would contain as much of such a heavy element as the mantle, considering the latter is 84 times the mass. Might be good if someone feels like double checking these figures. I converted everything to tonnes for consistency, it's possible I miscalculated somewhere. $\endgroup$ – naught101 Apr 16 '14 at 10:28
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    $\begingroup$ The reason why the crust has more U than the mantle is because the element is a lithophile element. On a global scale this results in a crust-phile behavior. $\endgroup$ – tobias47n9e Apr 16 '14 at 10:35
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    $\begingroup$ An element that likes being a rock? Cool! So I guess my calculation probably isn't too far off the mark... $\endgroup$ – naught101 Apr 16 '14 at 10:39
  • $\begingroup$ The nomenclature is a little confusing because the prefix "lith" usually refers to rock and both mantle and crust are made from rock. I think the nomenclature comes from iron smelting where the iron element (siderophile) separate from the rock (lithophile) which roughly corresponds to the Earth's mantle+crust and the Earth's core $\endgroup$ – tobias47n9e Apr 16 '14 at 10:44
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    $\begingroup$ Hope this helps (word limit in comments makes it difficult): The Astenosphere behaves like a liquid but is at all times solid. But from a petrological standpoint the crust (mostly feldspar, quartz) is very different from the mantle (including mantle-lithosphere) which is mostly composed of olivine, pyroxene, hornblende and an Aluminium-mineral. Many elements prefer either the mantle or the crust because of their relation to those minerals. The partitioning of the elements is as far as I know only indirectly influenced by the astenosphere convecting. $\endgroup$ – tobias47n9e Apr 16 '14 at 11:05
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The question "how much" is a little unprecise for geochemical questions. Rather you should think about:

-How much of the element is there per Rock-Volume (=Concentration)?

The crust has a higher U-concentration than the mantle because U is drawn into the minerals that compose the crust. Or you could also say that the minerals that compose the mantle don't like to incorporate U. The same is true for the solid iron core and the iron melt that composes the outer core.

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  • $\begingroup$ For geochemical purposes concentrations are more meaningful than absolute amounts, I suspect that's a comment on the geochemistry tag? Because then it's probably more suited as a comment to the question than as part of your answer. $\endgroup$ – hugovdberg Apr 17 '14 at 12:28

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