44
B. Fractionation Mechanisms of Selected Elements
one chapter. We are still unable to give a well-defined picture of the
possible, naturally occurring differences. This information is lacking for
two main reasons: 1) the theoretically expected variations in isotope
abundances should be relatively small, and 2) the reproducibility of
mass-spectrometric measurements of solid substances is in most cases
not better than the differences expected theoretically. It should be noted
that large variations in isotope ratios arise during solid source mass
spectrometry due to temperature-independent kinetic effects. The alkali
elements and the alkali earth elements in the Periodic System have several chemical features in common. For stable isotope research the relatively high volatility and mobility of alkali elements and the wide occurrence of both alkali and earth alkali elements in nearly all geological
environments are the most interesting similarities.
Variations in the isotopic composition of lithium, potassium, magnesium, and calcium might occur in nature. Differences in the isotope
abundance of lithium in meteorites may reveal important aspects of the
nucleosynthesis of lithium. Potassium appears to be mobile under the
conditions in the earth's crust. The movement of potassium seems to be
possible by magma, solutions, and diffusion. A variation of the natural
calcium and magnesium isotope abundances caused mainly by biological and kinetic effects could be expected, which might give insight into
such interesting processes as carbonate precipitation and dolomite formation.
1. Lithium
Lithium has two stable isotopes with the following abundances
(BAINBRIDGE and NIER, 1950):
6Li:
7.52% ,
7Li:
92.48% .
The significance of measuring the 7Lij6Li ratio in meteorites and comparing it with terrestrial lithium has been recognized for many years
because of the interesting nucleosynthesis of lithium.
Summarizing from the more recent papers, it may be stated that, with
the more sophisticated techniques, no systematic difference has been
found between meteoritic and terrestrial lithium (SHIMA and HONDA,
1963,1966; KRANKOWSKY and MULLER, 1964; 1967; POSCHENRIEDER et
al., 1965; DEWS, 1966; BALSIGER et al., 1968; GRADSZTAJN et aI., 1968).
BALSIGER et al. (1968) believe that lithium isotope variations, if they
occur at all, can be of only local importance and must have a sporadic
character. However, no definitely proven variations have yet been found.
B. Fractionation Mechanisms of Selected Elements
one chapter. We are still unable to give a well-defined picture of the
possible, naturally occurring differences. This information is lacking for
two main reasons: 1) the theoretically expected variations in isotope
abundances should be relatively small, and 2) the reproducibility of
mass-spectrometric measurements of solid substances is in most cases
not better than the differences expected theoretically. It should be noted
that large variations in isotope ratios arise during solid source mass
spectrometry due to temperature-independent kinetic effects. The alkali
elements and the alkali earth elements in the Periodic System have several chemical features in common. For stable isotope research the relatively high volatility and mobility of alkali elements and the wide occurrence of both alkali and earth alkali elements in nearly all geological
environments are the most interesting similarities.
Variations in the isotopic composition of lithium, potassium, magnesium, and calcium might occur in nature. Differences in the isotope
abundance of lithium in meteorites may reveal important aspects of the
nucleosynthesis of lithium. Potassium appears to be mobile under the
conditions in the earth's crust. The movement of potassium seems to be
possible by magma, solutions, and diffusion. A variation of the natural
calcium and magnesium isotope abundances caused mainly by biological and kinetic effects could be expected, which might give insight into
such interesting processes as carbonate precipitation and dolomite formation.
1. Lithium
Lithium has two stable isotopes with the following abundances
(BAINBRIDGE and NIER, 1950):
6Li:
7.52% ,
7Li:
92.48% .
The significance of measuring the 7Lij6Li ratio in meteorites and comparing it with terrestrial lithium has been recognized for many years
because of the interesting nucleosynthesis of lithium.
Summarizing from the more recent papers, it may be stated that, with
the more sophisticated techniques, no systematic difference has been
found between meteoritic and terrestrial lithium (SHIMA and HONDA,
1963,1966; KRANKOWSKY and MULLER, 1964; 1967; POSCHENRIEDER et
al., 1965; DEWS, 1966; BALSIGER et al., 1968; GRADSZTAJN et aI., 1968).
BALSIGER et al. (1968) believe that lithium isotope variations, if they
occur at all, can be of only local importance and must have a sporadic
character. However, no definitely proven variations have yet been found.