Tracing natural processes using isotopic abundances is probably the most successful aspect of modern geochemistry. The methods relying on isotopic data are analytically intensive since they depend on complex separation procedures and expensive equipment, but overall they are conceptually simple and robust. The natural variability of the abundances of isotopes in nature results from different processes:

1. Under a variety of thermodynamic and kinetic conditions, isotopes distribute themselves unevenly among co-existing phases (minerals, liquids, gases). These effects are in general very subtle as are the differences between the isotopes that create them, which explains why they escaped detection until the 1950s.

2. Radioactivity removes the parent isotope from an element and adds the decay product to another: 87Rb becomes 87Sr. We will see in the next chapter that the rate of removal of the parent isotope is identical anywhere and at any time in the universe, so this process only affects the relative abundances of the radiogenic isotopes.

3. Cosmic rays are particles produced outside the Solar System and their origin is still not quite understood. The energy of some of these particles, mostly protons and α particles, occasionally exceeds the nuclear binding energy. In the upper atmosphere, some nuclei, such as nitrogen and oxygen, get chipped by the collision, a process known as “spallation.” Most particles reaching the ground are actually secondary and can be accounted for by spallation.

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