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.
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.
These are very interesting and important points being made here.
I don't know whether the recently published (March 2016) article entitled The deep Earth may not be cooling down by D. Andrault et al. is heretical, but it certainly does address several of the core issues." is a nice pun.
The referenced article is clever; removes the need for heating by radioactive elements in the core, but also brings to mind other gravitationally driven heating processes: Io, Europa, Titan, etc, and possible influence on Pluto-Charon evolution. The youth of some features on Pluto was a surprise, I understand.
Have you a comment on these related processes?
I assumed tidal heating in the Earth-Moon system must also be consistent with the known Earth-Moon recession and length of day changes. The effects on processes such as glaciation, etc. through the Milankovitch orbital mechanisms would also seem to be involved since the geoid of the Earth presumably would be affected by processes in the core as well as ice accumulation, and orbital calculations fall apart after a few million years due to uncertainty in the initial values.
These are truly fertile areas of investigation.