18
A. Theoretical and Instrumental Background
VII. Some General Trends in Stable Isotope Geochemistry
The foundations of stable isotope geochemistry were laid in 1947 by
UREY'S paper on the thermodynamic properties of isotopic substances
and by NIER'S development of the ratio mass spectrometer.
As we shall see, the most interesting results in stable isotope geochemistry have been established with those elements from which a stable
gaseous component is known, e.g., hydrogen, carbon, oxygen, and sulfur.
There are two reasons for this: The differences in isotope composition
found in nature within these elements are very large, and gaseous compounds can be measured with better reproducibility than solid substances.
Before going into details of the naturally occurring variations of
stable isotopes, it might be useful to discuss some general trends effective
over the whole field.
1) Detectable isotope fractionations occur only when the relative
mass differences between the isotopes of a certain element are relatively
large, i.e., measurable isotope fractionations should be detectable only
within the lightest elements (in general, up to a mass number around 40).
2) All those elements that form solid, liquid, and gaseous compounds
which are stable over a wide temperature range, are likely to have variations of isotopic composition. Examples are hydrogen, carbon, nitrogen,
oxygen, and sulfur.
3) With increasing temperatures the fractionation factors decrease,
which means that the high-temperature environment, e.g., magmatic
rocks, shows smaller fractionations than the low-temperature environment, i.e., sedimentary rocks.
4) During biological reactions, e.g., during photosynthesis, bacterial
reactions, and other microbiological processes, the lighter isotope is
preferentially enriched in the reaction product relative to the starting
substances. This is especially pronounced in the cases of carbon, hydrogen, and nitrogen. When we consider biological processes such as photosynthesis and bacterial sulfate reduction, we must remember that isotopic variations during the history of the earth have probably increased.
Therefore, during late Precambrian time the occurring variations should
be smaller than at the present. Perhaps this could be proven in the case
of sulfur and carbon.
The general result of all naturally occurring fractionation processes is
that the degree of isotope fractionation increases with progression from
the deep-seated material to materials at the surface of the earth.
A. Theoretical and Instrumental Background
VII. Some General Trends in Stable Isotope Geochemistry
The foundations of stable isotope geochemistry were laid in 1947 by
UREY'S paper on the thermodynamic properties of isotopic substances
and by NIER'S development of the ratio mass spectrometer.
As we shall see, the most interesting results in stable isotope geochemistry have been established with those elements from which a stable
gaseous component is known, e.g., hydrogen, carbon, oxygen, and sulfur.
There are two reasons for this: The differences in isotope composition
found in nature within these elements are very large, and gaseous compounds can be measured with better reproducibility than solid substances.
Before going into details of the naturally occurring variations of
stable isotopes, it might be useful to discuss some general trends effective
over the whole field.
1) Detectable isotope fractionations occur only when the relative
mass differences between the isotopes of a certain element are relatively
large, i.e., measurable isotope fractionations should be detectable only
within the lightest elements (in general, up to a mass number around 40).
2) All those elements that form solid, liquid, and gaseous compounds
which are stable over a wide temperature range, are likely to have variations of isotopic composition. Examples are hydrogen, carbon, nitrogen,
oxygen, and sulfur.
3) With increasing temperatures the fractionation factors decrease,
which means that the high-temperature environment, e.g., magmatic
rocks, shows smaller fractionations than the low-temperature environment, i.e., sedimentary rocks.
4) During biological reactions, e.g., during photosynthesis, bacterial
reactions, and other microbiological processes, the lighter isotope is
preferentially enriched in the reaction product relative to the starting
substances. This is especially pronounced in the cases of carbon, hydrogen, and nitrogen. When we consider biological processes such as photosynthesis and bacterial sulfate reduction, we must remember that isotopic variations during the history of the earth have probably increased.
Therefore, during late Precambrian time the occurring variations should
be smaller than at the present. Perhaps this could be proven in the case
of sulfur and carbon.
The general result of all naturally occurring fractionation processes is
that the degree of isotope fractionation increases with progression from
the deep-seated material to materials at the surface of the earth.