Isotope Fractionation Processes of Selected Elements

The foundations of stable isotope geochemistry were laid in 1947 by Urey s classic paper on the thermodynamic properties of isotopic substances and by Nier s development of the ratio mass spectrometer. Before discussing details of the naturally occurring variations in stable isotope ratios, it is useful to describe some generalities that are pertinent to the field of non-radiogenic isotope geochemistry as a whole. 

Isotope fractionation is pronounced when the mass differences between the isotopes of a specific element are large relative to the mass of the element. Therefore, isotope fractionations are especially large for the light elements (up to a mass number of about 40). Recent developments in analytical techniques have opened the possibility to detect small variations in elements with much higher mass numbers. The heaviest element for which natural variations have been reported is thallium with isotopes of masses 203 and 205 (Rehkamper and HalU-day 1999). 

All elements that form solid, liquid, and gaseous compounds stable over a wide temperature range are likely to have variations in isotopic composition. Generally, the heavy isotope is concentrated in the solid phase in which it is more tightly bound. Heavier isotopes tend to concentrate in molecules in which they are present in the highest oxidation state. 

Mass balance effects can cause isotope fractionations because modal proportions of substances can change during a chemical reaction. They are especially important for elements in situations where these coexist in molecules of reduced and oxidized compounds. Conservation of mass in an n component system can be described by 

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