23

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?

Steve Farkus
  • 341
  • 1
  • 2
  • 5

3 Answers3

31

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.

David Hammen
  • 23,587
  • 1
  • 60
  • 102
  • 1
    Your response is confusing in that you write about the lack of uranium and the reasons it's lacking and then bring up that a lot more heat is needed to support the geomagneto effect. It sounds like the geophysicists are on the right track; maybe the uranium is deep in the mantle if not the core. – Steve Farkus May 04 '15 at 03:33
  • 6
    Why is it confusing? We don't really know, unless someone comes up with new evidence or a better interpretation of the existing one. The two competing theories presented are based on different interpretations of different data. – Sergiu Paraschiv May 04 '15 at 09:08
  • 1
    I think the last paragraph is what he means. On the one hand, there is comparatively very little of the listed radioactive elements in the earths core, more in the mantle, the highest percentage near the surface, on the other hand, there's enough of of these radioactive elements to heat up the earth's core. It's a curious combination of points, not necessarily contradicting, but I see what Steve is asking. I also liked David's answer - wanted to point that out and not sound critical. – userLTK May 04 '15 at 09:17
  • 3
    Sometimes, scientists agree to disagree. That's a good thing: If we knew everything, science would be dead. A couple of related areas where scientists agree to disagree are the heat flow across the core-mantle boundary (estimates range from less than 4 TW to over 17 TW) and the age of the Earth's solid inner core (estimates range from not possibly less than 1.2 billion years old and most likely over 2.2 billion years old, to not possibly more than 1 billion years old and most likely less than 700 million years old). Competing theories, or better, competing hypotheses. – David Hammen May 04 '15 at 09:27
  • Maybe you should clarify how geochemists arrive at the said conclusion. – stali May 04 '15 at 12:37
  • @stali -- Late response, but the penultimate paragraph does explain how geochemists arrived at their conclusion. Chemistry explains why even though gold is significantly more prevalent in the universe than is uranium (by a factor of about 20:1), gold is much less prevalent in the Earth's crust than is uranium (by a factor of about 1:600). Uranium is a strongly lithophilic and is an incompatible element. Anthropomorphizing, uranium very much "wants" to be in the crust rather than the mantle, and in the mantle rather than the core. – David Hammen Jun 21 '16 at 10:28
  • Just a quick spanner, you seem to just use 4 isotopes. But there are other long lived primodials on earth e.g. Pt 196 Bismuth is wholly radioactive. Now Platinum and Bismuth are easily reduced to the metal and so should therefore be able to get to the core. I have not thought this through thoroughly but its a thought. Maybe there is food for a study – Owen Sep 23 '17 at 11:00
  • 1
    @Owen - Those are the only four primordial nuclides that count with regard to being a significant heat source for the Earth. Shorter lived unstable nuclides are not primordial, while longer-lived ones have such long half lives that they do not represent a significant heat source. Platinum 190 and Bismuth 209 have half lives of $6.5\times10^{11}$ and $19\times10^{18}$ years, respectively, far too long to make them anything close to significant heat sources. – David Hammen Sep 24 '17 at 14:35
  • @Owen "Now Platinum and Bismuth are easily reduced to the metal and so should therefore be able to get to the core." Bismuth is actually chalcophilic, so it will more likely be found in sulfide minerals than in the core. – Oscar Lanzi Jan 23 '23 at 16:50
  • There may be much more potassium in the core than previously thought. In short, potassium can combine with iron at high enough pressure. See my answer here – Spencer Feb 11 '23 at 18:59
  • @Spencer The amount of potassium in the core has long been in doubt. Existing neutrino detection experiments are not capable of detecting the low energy neutrinos emitted by potassium-40 decay, and whether potassium remains as completely incompatible as it is near the surface has been debated. Scientists do know that there must be some lighter elements in the core as the density is too low for a pure iron-nickel mix. The prime suspect is sulfur, but perhaps it's potassium. Perhaps. This is once again a place where scientists agree to disagree. – David Hammen Feb 12 '23 at 02:49
3

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.[1] 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.

References

  1. 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.

  2. 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.

Oscar Lanzi
  • 3,786
  • 8
  • 19
2

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.

Gordon Stanger
  • 14,238
  • 23
  • 44
  • 4
    You state this as if it's fact. Citation needed. As far as I know, scientists do not know whether there's a significant amount of potassium-40 in the Earth's core. The inverse beta reaction used in current neutrino detectors cannot detect the neutrinos emitted by potassium-40 decay. The peak energy in that decay is less than the minimum energy needed to trigger the inverse beta reaction. – David Hammen Sep 18 '15 at 15:06
  • 1
    There is a scientific consensus that the outer core must contain some light element (or light elements) in addition to iron, nickel, and trace amounts of heavier elements. From what I've read, geologists and geophysicists agree to disagree on which light element is most responsible for the reduced density of the outer core compared to that of an iron/nickel. Some say it's oxygen, others silicon, yet others, sulfur. Or perhaps carbon. As far as I can tell, there is no scientific consensus on this, either. – David Hammen Sep 18 '15 at 15:10
  • 1
    True, it's a deduction rather than a fact, but the longevity of radioactive heat in the core has to be explained somehow, and the potassium solution is much more plausible than any of the three alternatives (thorium and two isotopes of uranium). – Gordon Stanger Sep 25 '15 at 23:32
  • What longevity of radioactive heat in the core are you writing about? Depending on which author one reads, this varies from none to barely measurable to a whole lot. As of this date, there is no instrumentation that can detect potassium 40 decay from the center of the Earth. As of this date, whether some extra heat source is needed to maintain the Earth's magnetic field, beyond residual heat from the Earth's formation and the continued formation of the inner core, is subject to debate. Estimates vary widely in the scientific literature. – David Hammen Sep 25 '15 at 23:40
  • 1
    @DavidHammen as far as we know, potassium, uranium and thorium are some of the most lithophile elements there are. If you want to suggest they reside in the core, it's up to you to propose a mechanism of why they would be there in the first place. – Gimelist Sep 29 '15 at 00:30
  • 1
    @Michael - I am not saying that K, U, and Th are present in substantial amounts in the core. I said exactly the opposite! However, explaining the amount of heat needed to explain the geodynamo without radioactivity has been problematic. 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. – David Hammen Jun 21 '16 at 09:53
  • @DavidHammen actually in some of my own experiments, I ended up having quite a lot of K in liquid Fe. Funny how things turn out to be. :) – Gimelist Jun 21 '16 at 10:04
  • 1
    @Michael -- The standard geophysics POV is that chemistry is rather different at the high temperatures and pressures deep inside the Earth, and this may allow supposedly strongly lithophile elements such as K, U, and Th to be present in the core. Substantial amounts of uranium and thorium in the core are apparently ruled out by neutrino detectors, but potassium is not. We do not yet have the technology to detect the low energy neutrinos emitted by the decay of potassium-40. – David Hammen Jun 21 '16 at 10:13
  • @DavidHammen those were high P-T experiments. Not as extreme as the core, but still high. – Gimelist Jun 21 '16 at 10:29
  • @DavidHammen The paper I linked to suggested 100-1000 ppm of potassium would be consistent with their experiments. – Spencer Feb 11 '23 at 19:14
  • @GordonStanger You didn't link to a paper in this answer, or in any comment on this answer. – David Hammen Feb 11 '23 at 19:57
  • @DavidHammen I think you meant to @ me instead of Gordon? I linked to this paper in my answer to a different question, which I linked to in my comment to your answer here. – Spencer Feb 11 '23 at 23:10
  • I don't know how reliable neutrino detection is for finding out how much radioactive materials are in the core, given how elusive neutrinos are. However low the concentration of uranium in the core may be, it's still true the total could be large, and it could be much more concentrated at the center, due to uranium's much greater atomic weight compared to iron. The possible nuclear chain reactions due to uranium at the center could include those that nuclear physicists are familiar with, and other reactions they aren't familiar with, due to the high pressure. – Richard Peterson Feb 23 '23 at 03:50