ight-ring radiohalos is not a mystery for geologists. They must have formed from uranium-238 decay. The mystery is the source of the one-ring, two-ring, and three-ring radiohalos that are found in the same rock specimens (Figures 2–4).
Geologists find four types of radiohalos in granite. One was caused by the decay of uranium, and the others came from the decay of another radioactive element, polonium.
The source of eight-ring radiohalos is still there, but nothing is usually visible at the center of other types of radiohalos.
We can see the source of the eight-ring radiohalos because it is still there—a zircon crystal that still hosts uranium. But nothing is usually visible at the center of the other types of radiohalos. The source has vanished!
Fortunately, it is still possible to determine the source of the rings. By carefully measuring the distance from the center of the radiohalo to each ring, we can identify which type of nucleus formed each ring (Figure 5).1 In each case, the “smoking gun” was a variation of the radioactive element polonium.2 Polonium-218, polonium-214, and polonium-210 generate exactly the right amount of energy to produce the three-ring, two-ring, and one-ring radiohalos.
The problem is that polonium is never found alone in rocks. It is a rare, unstable element that appears quickly during the decay of uranium and then decays into stable elements, such as lead. The only possible source of the polonium was the decay of uranium. But we do not find a uranium source at the center of the one-, two-, and threering radiohalos! The crucial clue is the appearance of polonium during the uranium-238 decay chain. As the uranium atom’s nucleus becomes progressively smaller, three variations of polonium appear briefly in sequence at the end of the chain.
FIGURE 5—Composite schematic drawing of the four types of radiohalos: (a) a uranium-238 radiohalo, (b) a polonium-218 radiohalo, (c) a polonium-214 radiohalo, and (d) a polonium-210 radiohalo. Different types of atoms (nuclides) cause each ring in the radiohalos, and the particles that they send out have different energies (MeV).
We can be certain that no other radioactive element appeared at the center of the polonium radiohalos. If any other elements had been there, such as uranium or radon, they would have formed other rings as they decayed.
So where did the polonium atoms come from?
The best possible answer is that the polonium traveled from a nearby source where uranium atoms were decaying. Is there such a nearby uranium source? Absolutely! The same flakes that host the polonium radiohalos usually contain uranium radiohalos, usually less than a fraction of an inch from polonium radiohalos (Figure 6).
The source of uranium radiohalos still survives in granite rocks: tiny zircon crystals. But geologists are mystified because the source of polonium radiohalos is missing. The answer lies only a tiny way up the biotite sheet . . . the zircons!
Biotite flakes often have numerous polonium radiohalos, in addition to uranium radiohalos.
FIGURE 7—Diagram of a cross-section through a biotite flake where radiohalos are found. The uranium-238 in the zircon crystal generated the uranium-238 radiohalo. Water flowing past the crystal carried decaying atoms along the same sheet to a nearby location, where a polonium-210 radiohalo developed. Nothing remains at the center of this radiohalo, however, because it dissolved quickly.
Yet under normal conditions, like those we see in the earth today, that migration would be impossible. To understand the magnitude of the problem, you need to understand how many polonium atoms had to migrate and how quickly they had to travel.
Scientists have estimated that each radiohalo’s discoloration initially develops after 100 million alpha-particles have been emitted from its center. It does not become dark until 500 million particles, and it does not become very dark until one billion alpha particles have been ejected.3 This means that each polonium radiohalo needs at least 500 million polonium atoms. And many polonium radiohalos often appe