What is helium diffusion in zircon?

Answer 1

Helium diffusion in zircon is the movement of helium atoms that have been trapped within a zircon crystal matrix. That may not make sense by itself, so let's take a little trip through some background information to see if it can be made clearer. This might get a little long-winded, but it really isn't that difficult to understand when taken in moderate bites. We've said grace, so tuck in your napkin and grab your fork.

When rock crystals form, they essentially "trap" the materials of which they are made within the crystal matrix. The atoms involved are usually trapped by both chemical bonding to other atoms and the physical restrictions of the matrix itself. Imagine a room full of people holding hand and having their arms interlocked in various ways. That's the crystal matrix. But the crystal matrix can be prone to attack by different materials, substances or circumstances. Physical and chemical exposure can alter the crystal structure of the rock over time. So can thermal energy (heat), which is a type of physical assault. Some rock crystals can form and last for long periods, like millions or billions of years. Others form and rapidly deteriorate in geologic time. That said, let's move on to zircon crystals.

Zircon crystals we generally refer to are crystals of zirconium, silicon and oxygen (ZrSiO4). The focus on zircon is based on the ability of this crystal material to withstand most types of physical and chemical attack; it's a tough little bugger that can last for a long, long time. When these crystals form, a few uranium and throium atoms that are "nearby" can be incorporated into the matrix in place of the zirconium atoms that generally form the structure. These atoms of "intrusive" elements are then locked into the matrix. In general, the temperatures at which these crystals begin to form are high enough that any lead that might want to "creep in" will strongly be rejected and kept out. (Certainly helium won't be included because of the elevated temperature of formation.) The "locking out" of lead is important here because of what happens next as regards radiometric dating, which is where much of this discussion is going. That's what all the current fuss over this subject seems to be.

When we apply radiometric dating techniques to zircon crystals, we are generally looking at the decay products of uranium. Recall that uranium (whatever isotope we choose to look at) is radioactive, and it has a long, long half-life. When uranium decays, it produces a daughter product, and this daughter is also radioactive. When the daughter decays, the daughter of that decay is also radioactive. And so it goes until a stable atom of lead appears at the end of the decay chain or decay series. Also produced in the decay of uranium and its daughters is helium. Lots of it. That's because uranium and its daughters transform via alpha decay. And alpha decay, as we know, is the emission of an alpha particle, which is a helium-4 nucleus. Additionally, all that alpha decay damages the crystal matrix as alpha decay is a violent event on the atomic scale.

In review, what is key here is that uranium can be included in zircon when it forms. And when uranium decays in steps to lead, the nuclear transformations are almost all via alpha decay. In alpha decay, an alpha particle is dumped by an unstable nucleus as it transforms. And you know that an alpha particle is a pair of protons and a pair of neutrons -- a helium-4 nucleus. All that alpha decay is the source of helium in zircon crystals. In case you're wondering, electrons can quite easily "penetrate" the crystal to "convert" the alpha particle into a helium atom. Anyway, that helium is trapped in there along with all the other elements that make up the crystal matrix. That crystal matrix has also taken some damage from all that alpha decay, too. Lastly, we need to recall that helium is an inert gas. It's not chemically bonded to anything any more and is only trapped physically.

Because helium is not prone to chemical bonding with other elements or even its own kind, it is allowed some mobility within the matrix that is the zircon crystal. It is only trapped physically within the matrix. The size of the helium atom with respect to the sizes of the atoms of zirconium, silicon and oxygen will have something to do with how much the helium can move (diffuse) through the crystal. So will the damage to the crystal matrix done by all that alpha decay. (We've already mentioned that nuclear decay is a violent event on the atomic scale.) The relative sizes of the atoms involved (zirconium, silicon, oxygen, and our helium) and the spacing of those atoms within the matrix set limits on helium's mobility. We also know that the "state" of the matrix (re: damage to the matrix from nuclear decay within it) has some effect. It is critical to understand that thermal conditions affect the diffusion of helium within the crystal. We should also note that helium might diffuse into the matrix under some circumstances. Contributions due to heat are arguably the most difficult variable to understand and estimate, but they are extremely important. They vary widely -- unlike the atomic sizes and spacing of the atoms involved. Understanding the variables and being able to model them will lead to an understanding of diffusion of helium within zircon crystals.