110
C. Variations of Stable Isotope Ratios in Nature
5. Sulfur Isotope Distribution in Metamorphic Rocks
Sulfur isotope data from metamorphic rocks (ore minerals from metamorphic ore deposits are excluded and not discussed here) are very
scarce. SPEELMANN and SCHW ARCZ (1966) have argued that with increasing metamorphic grade, a homogenization of sulfur isotope distribution
takes place. However, unpublished results of SIEWERS have shown that
this process seems to be more complex.
6. Variations of the 41K/39K Ratio in Contact Aureoles
The use of the 41KrK ratio to detect and interpret movement of
potassium in contact zones has been reported by SCHREINER and VERBEEK (1965), VERBEEK and SCHREINER (1967), and SCHREINER and WELKE
(1971). The results are consistent with a diffusive transfer of potassium
from the "hot" pluton into the "cold" sediments that would result in a
preferential enrichment of the lighter isotope 39K in the sediments.
X. Balance Calculations
Balance calculations of the average isotope ratio of the earth's crust
are very problematical, because the numbers used are very rough estimations with a high degree of uncertainty. Balance computations of the
following type are based on the assumption that all elements now found
in the hydrosphere, in sediments, and in metamorphic and igneous crustal rocks must be derived from the earth's mantle.
As can be seen in Table 11, the mean calculated t5-values for hydrogen, carbon, and sulfur for the continental crust do not agree with experimental data, except for carbon. The discrepancies in the mean t5-values
for hydrogen and sulfur cannot be explained in the same way. If we
regard the oceans as the condensation product of water degassed from
the earth during an early stage, then the mean t5D-value of the oceans
should represent the mean t5D-value of the crust and the mantle. However, this is not the case.
In the upper zones of the atmosphere a photo-dissociation of water
vapor into H2 and O2 takes place. Hydrogen, as the lightest gas, can
continually escape from the gravity field of the earth. This process very
probably proceeds parallel with an isotope fractionation causing a preferentialloss of H and a retention of D. The magnitude of this effect is
rather difficult to assess. However, one can say that there has probably
been a progressive enrichment of deuterium in the ocean with time, due
to the preferential escape ofH relative to D.
C. Variations of Stable Isotope Ratios in Nature
5. Sulfur Isotope Distribution in Metamorphic Rocks
Sulfur isotope data from metamorphic rocks (ore minerals from metamorphic ore deposits are excluded and not discussed here) are very
scarce. SPEELMANN and SCHW ARCZ (1966) have argued that with increasing metamorphic grade, a homogenization of sulfur isotope distribution
takes place. However, unpublished results of SIEWERS have shown that
this process seems to be more complex.
6. Variations of the 41K/39K Ratio in Contact Aureoles
The use of the 41KrK ratio to detect and interpret movement of
potassium in contact zones has been reported by SCHREINER and VERBEEK (1965), VERBEEK and SCHREINER (1967), and SCHREINER and WELKE
(1971). The results are consistent with a diffusive transfer of potassium
from the "hot" pluton into the "cold" sediments that would result in a
preferential enrichment of the lighter isotope 39K in the sediments.
X. Balance Calculations
Balance calculations of the average isotope ratio of the earth's crust
are very problematical, because the numbers used are very rough estimations with a high degree of uncertainty. Balance computations of the
following type are based on the assumption that all elements now found
in the hydrosphere, in sediments, and in metamorphic and igneous crustal rocks must be derived from the earth's mantle.
As can be seen in Table 11, the mean calculated t5-values for hydrogen, carbon, and sulfur for the continental crust do not agree with experimental data, except for carbon. The discrepancies in the mean t5-values
for hydrogen and sulfur cannot be explained in the same way. If we
regard the oceans as the condensation product of water degassed from
the earth during an early stage, then the mean t5D-value of the oceans
should represent the mean t5D-value of the crust and the mantle. However, this is not the case.
In the upper zones of the atmosphere a photo-dissociation of water
vapor into H2 and O2 takes place. Hydrogen, as the lightest gas, can
continually escape from the gravity field of the earth. This process very
probably proceeds parallel with an isotope fractionation causing a preferentialloss of H and a retention of D. The magnitude of this effect is
rather difficult to assess. However, one can say that there has probably
been a progressive enrichment of deuterium in the ocean with time, due
to the preferential escape ofH relative to D.