2.2.1 Gamma-Ray Spectrometry: The Method
The geochemical composition of magmatic rocks in the sense of element
spectrum and total element concentration, is first of all dependent on the
magma from which they were derived, for all magmatic rocks contain radioactive elements which show a regular rate of decay over time. During this decay,
characteristic energies in the frequency band of gamma rays are emitted, which
can be measured. However, only three elements show a sufficient concentration
in the natural environment for the released gamma-rays to be determined by
routine measurements in the field; these being Potassium (K), Uranium (U) and
Thorium (Th) (IAEA 2003). For K, the decay of 40 K to 40Ar is measured, and
since 40 K represents a fixed amount of total K, the measurement can be used to
estimate this. The case is different for U and Th, which do not show a single
radioactive decay, but a whole decay chain. Since neither 238U nor 232Th show
gamma-ray emissions during the first decay step, the decay of daughter products
214Bi and 208Tl is measured in order to estimate their concentrations, using an
e (for equivalent) in front of the element (eU and eTh respectively). The problem
arising from this methodological approach is that mobile phases occurring in the
decay chain and leaving the system can lead to erroneous values. In the case of
Th, the decay of 208Tl to 208Pb is measured. In the Thorium decay chain as
intermediate product 220Rn occurs, which is a gas and is easily lost from the
soil system. This is the reason why Beckett (2007) stated that soil units defined
by eU or eTh alone are inherently erroneous. We do not agree with this
conclusion, since it is not the total element concentration of U or Th that is of
interest for the correlation with soil information, but the radiation produced as a
result of the soil forming process. Therefore, eU and eTh, usually expressed as
ppm, represent here only a formalism (and this formalism should probably be
changed to measured radiation intensities) used in order to compare results on a
global scale, but are not meant to represent the real concentrations of U or Th in
a sample.
Another important aspect is the occurrence of the radioactive elements in
specific minerals, since this has consequences for the weathering behaviour
(which equates to mobility) and appearance in different grain size fractions.
K dominantly occurs in mass minerals like feldspars and micas, and in specific
environments also in salt minerals. In soils developed from granite, feldspars and
micas can appear in the coarse grain fraction; however, under acidic conditions they
are decomposed and transformed to clay minerals. While feldspar is usually
transformed to kaolinite, with K released from the crystalline structure and potentially leached, the micas transform to illite, in which K is an inherent component of
the crystalline lattice. Illite can then be further transformed into clay mineral types
with lower K concentrations (i.e., vermiculite and smectite), or be subject to grainsize selective transportation processes. As a consequence, in most cases and in the
long-term, K tends to be lost from the soil system.
36
K. Stahr et al.
The geochemical composition of magmatic rocks in the sense of element
spectrum and total element concentration, is first of all dependent on the
magma from which they were derived, for all magmatic rocks contain radioactive elements which show a regular rate of decay over time. During this decay,
characteristic energies in the frequency band of gamma rays are emitted, which
can be measured. However, only three elements show a sufficient concentration
in the natural environment for the released gamma-rays to be determined by
routine measurements in the field; these being Potassium (K), Uranium (U) and
Thorium (Th) (IAEA 2003). For K, the decay of 40 K to 40Ar is measured, and
since 40 K represents a fixed amount of total K, the measurement can be used to
estimate this. The case is different for U and Th, which do not show a single
radioactive decay, but a whole decay chain. Since neither 238U nor 232Th show
gamma-ray emissions during the first decay step, the decay of daughter products
214Bi and 208Tl is measured in order to estimate their concentrations, using an
e (for equivalent) in front of the element (eU and eTh respectively). The problem
arising from this methodological approach is that mobile phases occurring in the
decay chain and leaving the system can lead to erroneous values. In the case of
Th, the decay of 208Tl to 208Pb is measured. In the Thorium decay chain as
intermediate product 220Rn occurs, which is a gas and is easily lost from the
soil system. This is the reason why Beckett (2007) stated that soil units defined
by eU or eTh alone are inherently erroneous. We do not agree with this
conclusion, since it is not the total element concentration of U or Th that is of
interest for the correlation with soil information, but the radiation produced as a
result of the soil forming process. Therefore, eU and eTh, usually expressed as
ppm, represent here only a formalism (and this formalism should probably be
changed to measured radiation intensities) used in order to compare results on a
global scale, but are not meant to represent the real concentrations of U or Th in
a sample.
Another important aspect is the occurrence of the radioactive elements in
specific minerals, since this has consequences for the weathering behaviour
(which equates to mobility) and appearance in different grain size fractions.
K dominantly occurs in mass minerals like feldspars and micas, and in specific
environments also in salt minerals. In soils developed from granite, feldspars and
micas can appear in the coarse grain fraction; however, under acidic conditions they
are decomposed and transformed to clay minerals. While feldspar is usually
transformed to kaolinite, with K released from the crystalline structure and potentially leached, the micas transform to illite, in which K is an inherent component of
the crystalline lattice. Illite can then be further transformed into clay mineral types
with lower K concentrations (i.e., vermiculite and smectite), or be subject to grainsize selective transportation processes. As a consequence, in most cases and in the
long-term, K tends to be lost from the soil system.
36
K. Stahr et al.