Magnesium
45
2. Potassium
Potassium has three naturally occurring isotopes with the following
abundances (NIER, 1950):
39K:
93.08% ,
4OK:
0.0119% ,
41K:
6.91 % .
4°K is radioactive and decays dually, producing stable 40Ca and stable
4°Ar.
The conclusion that the 39Kj41K ratio is constant in nature, which has
been generally accepted for a long time, may need to be revised, since
several indications do not support this assumption.
Rocks originating from great depths may have lower 39Kj41K ratios
than rocks of crustal origin (LETOLLE, 1962). A similar trend on micas
from genetically different rocks has been observed by LEBEDEV et al.
(1966).
SCHREINER and VERBEEK (1965) described different 39Kj41K ratios for
granites and sediments. The results would be consistent with a diffusive
transfer of potassium from the granite into the sediment, which would
result in a preferential enrichment of the lighter isotope in the sediments.
A similar relationship in the contact zone between a granite and an
amphibolite has been demonstrated by VERBEEK and SCHREINER (1967),
and SCHREINER and WELKE (1971).
LETOLLE (1963) argued that in the precipitation of potassium-containing evaporites a preferential enrichment of the heavier isotope occurs
relative to ocean water.
3. Magnesium
Natural magnesium is composed of three isotopes (WHITE et at.,
1956):
24Mg:
25Mg:
26Mg:
78.8% ,
10.15% ,
11.06% .
The search for naturally occurring variations of the magnesium isotopes
does not seem to be hopeless, if we note the wide occurrence of magnesium, for instance in carbonates, silicates, seawater, evaporites, and biological samples. However, all samples analyzed so far lie in the range of
reproducibility of the mass-spectrometric measurement (SHIMA, 1964;
BUCHS et ai., 1965; CATANZARO and MURPHY, 1966; TAKEMATSU et at.,
1967).
The discrepancy between the above results and those of D AUGHTR Y et
at. (1962), who found differences up to 5%, is possibly due to instrumental errors by DAUGHTRY.
45
2. Potassium
Potassium has three naturally occurring isotopes with the following
abundances (NIER, 1950):
39K:
93.08% ,
4OK:
0.0119% ,
41K:
6.91 % .
4°K is radioactive and decays dually, producing stable 40Ca and stable
4°Ar.
The conclusion that the 39Kj41K ratio is constant in nature, which has
been generally accepted for a long time, may need to be revised, since
several indications do not support this assumption.
Rocks originating from great depths may have lower 39Kj41K ratios
than rocks of crustal origin (LETOLLE, 1962). A similar trend on micas
from genetically different rocks has been observed by LEBEDEV et al.
(1966).
SCHREINER and VERBEEK (1965) described different 39Kj41K ratios for
granites and sediments. The results would be consistent with a diffusive
transfer of potassium from the granite into the sediment, which would
result in a preferential enrichment of the lighter isotope in the sediments.
A similar relationship in the contact zone between a granite and an
amphibolite has been demonstrated by VERBEEK and SCHREINER (1967),
and SCHREINER and WELKE (1971).
LETOLLE (1963) argued that in the precipitation of potassium-containing evaporites a preferential enrichment of the heavier isotope occurs
relative to ocean water.
3. Magnesium
Natural magnesium is composed of three isotopes (WHITE et at.,
1956):
24Mg:
25Mg:
26Mg:
78.8% ,
10.15% ,
11.06% .
The search for naturally occurring variations of the magnesium isotopes
does not seem to be hopeless, if we note the wide occurrence of magnesium, for instance in carbonates, silicates, seawater, evaporites, and biological samples. However, all samples analyzed so far lie in the range of
reproducibility of the mass-spectrometric measurement (SHIMA, 1964;
BUCHS et ai., 1965; CATANZARO and MURPHY, 1966; TAKEMATSU et at.,
1967).
The discrepancy between the above results and those of D AUGHTR Y et
at. (1962), who found differences up to 5%, is possibly due to instrumental errors by DAUGHTRY.