What percent of the Earth's core is uranium? And how much of the heat at the core is from radioactive decay rather than other forces?
Good question! Geochemists and geophysicists agree to disagree, sometimes quite strongly. There are also disagreements within each group as well as between the two groups.
It's not just uranium. There are four isotopes whose half-lives are long enough that they can be primordial and whose half-lives are not so long that they don't produce much heat. These four isotopes are
- Uranium 235, with a half-life of 0.703 billion years,
- Potassium 40, with a half-life of 1.277 billion years,
- Uranium 238, with a half-life of 4.468 billion years, and
- Thorium 232, with a half-life of 14.056 billion years.
The consensus view amongst geochemists is that there is very little, if any, of any of these isotopes in the Earth's core. Potassium, thorium, and uranium are chemically active. They readily oxidize. In fact, they readily combine chemically with lots other elements -- but not iron. They are strongly lithophilic elements. Moreover, all three are "incompatible" elements. In a partial melt, they have a strong affinity to stay in the molten state. This means that relative to solar system abundances, all three of these elements should be strongly enhanced in the Earth's crust, slightly depleted in the Earth's mantle, and strongly depleted in the Earth's core.
Geophysicists look at the amount of heat needed to drive the Earth's magnetic field, and at the recent results from neutrino observations. From their perspective, the amount of residual heat from the Earth's formation is not near enough to drive the geomagneto. The growth of the Earth's inner core creates some heat, but not near enough to sustain the geodynamo. Geophysicists want a good amount of heat flux across the core mantle boundary to sustain the geodynamo, and to them the only viable source is radioactivity. Recent geoneutrino experiments appear to rule out uranium or thorium in the Earth's core, but not potassium 40. The neutrinos generated from the decay of potassium 40 are not detectable using current technology.
One possibility is that enough uranium is present to provide a substantial heat source -- in the deep mantle rather than the core.
Gautron et al. study the inclusion of uranium in an aluminum-doped calcium silicate perovskite, which is believed to exist in the lower mantle (the authors cite Ref. 2). With the aluminum doping, the silicate perovskite becomes compatible with uranium(IV), and thereby "all the uranium present in this region could be easily stored via its insertion in the Al-CaSiO3 perovskite." The authors suggest that thorium, which also favors the +4 oxidation state and forms a similarly large cation, may similarly be incorporated, although they directly studied only uranium.
Laurent Gautron, Steeve Greaux, Denis Andrault, Nathalie Bolfan-Casanova, Nicolas Guignot, M. Ali Bouhifd (2006). "Uranium in the Earth's lower mantle". Geophysical Research Letters 33(23), L23301. https://doi.org/10.1029/2006GL027508.
Hirose, K. (2002). "Phase transitions in the pyrolitic mantle around 670–km depth: Implications of upwellings of plumes from the lower mantle". J. Geophys. Res., 107(B4), 2078. https://doi.org/10.1029/2001JB000597.
He's correct, there is precious little uranium or thorium in the Earth's core. He's also right that an extra source of heat is required to drive the core's magnetism. Note however that, as has been known for a long time, the Earth's core is less dense than would be the case if it was pure Ni-Fe alloy. The answer is that there is a lot of sulphur down there, in fact about 10% of the moon's weight in sulphur, which most likely exists as high pressure Fe-sulphide phases. Potassium is a lithophile element and would not normally exist in the core, but potassium is soluble in Fe-sulphide. The radionuclide 40-potassium, in the sulphide, in the core, is the source of the missing heat which drives the dynamo which has created the Earth's magnetism for more vthan 3.5 billion years.