Geoscience Reference
In-Depth Information
3
Fractionationofstableisotopes
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 over-
all 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 gen-
eral 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:
87
Rb becomes
87
Sr. 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
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 “spalla-
tion.” Most particles reaching the ground are actually secondary and can be accounted
for by spallation.
4. For all we should be concerned about at this point, the effect of nucleosynthetic pro-
cesses stopped with the collapse of the solar nebula. We will see that some rare objects
found in meteorites and known as refractory inclusions still keep the memory of a
pre- or early-solar isotopic variability. Turbulence in the solar nebula and mixing upon
planetary accretion and differentiation have for all practical purposes erased - with
the remarkable exception of oxygen for which mass-independent fractionation among
the Earth, the Moon, Mars, and the different groups of meteorites calls for different
reservoirs - most traces of these heterogeneities in planetary bodies.
α
Thermodynamic modes of isotopic fractionation are collectively referred to as mass-
dependent processes and are the topic of this chapter. Kinetic fractionation will be returned
to in
Section 5.4
.
The logic underlying thermodynamic fractionation can be applied to
isotopes with the added advantage that the behavior of two isotopes of the same element
