112
C. Variations of Stable Isotope Ratios in Nature
A mean calculated c5D-value of -13%0 is therefore a rather meaningless number, except that it indicates that a process of preferential escape
of H relative to D must take place.
The mean calculated c5 13 e-value could be acceptable, because it coincides with the mean isotopic composition of carbonatites and diamonds.
However, it must be emphasized that the amount of carbonate bound in
sediments is very critical and also controversial.
The mean calculated c5 34 S-value of + 7.2 is certainly not correct. This
could be due to an overestimation of the amount of evaporitic sulfate by
RONovand MIGDISOV (1971), who analyzed sediments only from the
Russian platform. In this connection it must be noted that another attempt of a balance calculation by HOLSER and KAPLAN (1966) has given
a mean value for the earth's crust of 6.0 ± 2.3%0. In both calculations the
mean c5 34 S-value is too heavy compared with potential mantle sulfur.
Many questions arise in this connection. Which naturally occurring
specimens are representative for the mean c5-value of the earth's mantle?
Are these the sulfides from basaltic dikes and sills with a mean c5-value of
0.1%0 (SMITHERINGALE and JENSEN, 1963), which would be in relatively
good agreement with the mean value of meteoritic material? Is this
1.3%0, which SCHNEIDER (1970) gave for the alkali olivine basalts, showing, in his opinion, the least deviation from the original mantle value? Or
does the average value lie between 2 and 3%0, found by SIEWERS (unpublished data) for granitic rocks?
C. Variations of Stable Isotope Ratios in Nature
A mean calculated c5D-value of -13%0 is therefore a rather meaningless number, except that it indicates that a process of preferential escape
of H relative to D must take place.
The mean calculated c5 13 e-value could be acceptable, because it coincides with the mean isotopic composition of carbonatites and diamonds.
However, it must be emphasized that the amount of carbonate bound in
sediments is very critical and also controversial.
The mean calculated c5 34 S-value of + 7.2 is certainly not correct. This
could be due to an overestimation of the amount of evaporitic sulfate by
RONovand MIGDISOV (1971), who analyzed sediments only from the
Russian platform. In this connection it must be noted that another attempt of a balance calculation by HOLSER and KAPLAN (1966) has given
a mean value for the earth's crust of 6.0 ± 2.3%0. In both calculations the
mean c5 34 S-value is too heavy compared with potential mantle sulfur.
Many questions arise in this connection. Which naturally occurring
specimens are representative for the mean c5-value of the earth's mantle?
Are these the sulfides from basaltic dikes and sills with a mean c5-value of
0.1%0 (SMITHERINGALE and JENSEN, 1963), which would be in relatively
good agreement with the mean value of meteoritic material? Is this
1.3%0, which SCHNEIDER (1970) gave for the alkali olivine basalts, showing, in his opinion, the least deviation from the original mantle value? Or
does the average value lie between 2 and 3%0, found by SIEWERS (unpublished data) for granitic rocks?