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How does melting a material reset its radioactive clock?

How is it that melting an asteroid can "reset" its atomic clock?  The recent news article on a meteorite that originated on the moon was able to list the major impact events it experienced while on the moon, and to date them.  It stated that this is possible because each event melted part of it, and thus the atomic clock of that part was reset.  So how does melting a stone cause decayed radioactive elements to return to their original form?  When we melt lead it doesn't revert to uranium.  The same question applies to dating the solar system.  Shouldn't the radioactive elements have been decaying for 8 billion, 12 billion years; ever since they were blasted into existence in ancient supernovae?  

In the solid state, everything is pretty well locked into place, so that relatively few particles can escape. In the liquid or gaseous state, however, particles can move around and boil off. Because of radioactive decay, the material starts out with a larger amount of daughter isotope than is chemically favorable, so some of it will escape to bring the material to equilibrium. Think of air bubbles frozen in an ice cube--the air is prevented from escaping by the solid ice, but will escape when the cube melts because the energy of the system is lower without the bubbles. (Further separation could occur during solidification, due to different crystallizations of the parent and daughter isotopes.) This means that the information about how much decay took place prior to melting is lost.

Therefore we can only figure out the time since melting if we can determine the amount of daughter isotope present at solidification. One way to do this is to find another isotope which doesn't participate in the radioactive decay (call it isotope B, for boring), find a non-radioactive rock (or one that was recently liquid), and measure the ratio of the daughter isotope to isotope B. This ratio will be the same for any material in equilibrium, so it was the ratio in the sample when it solidified. You would then measure the amount of isotope B in your meteorite (or whatever), and multiply it by the equilibrium ratio, yielding the amount of daughter isotope at solidification. If you subtract this from the total amount of daughter isotope that you measure in the sample, you have the amount of daughter isotope due to radioactive decay since solidification, which tells you the time since solidification.

September 2004, Sara Slater (more by Sara Slater) (Like this Answer)

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