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Others form and rapidly deteriorate in geologic time. Zircon crystals we generally refer to are crystals of zirconium, silicon and oxygen (Zr Si O4).
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.
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.
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.
Also produced in the decay of uranium and its daughters is helium. That's because uranium and its daughters transform via alpha decay.
It is critical to understand that thermal conditions affect the diffusion of helium within the crystal.
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.
Imagine a room full of people holding hand and having their arms interlocked in various ways. 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.
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.