60
C. Variations of Stable Isotope Ratios in Nature
bien de from mafic and ultramific rocks and deep-seated batholitic rocks
is typically -60 to -90%0 (GODFREY, 1962; TAYLOR and EpSTEIN,
1966b; SHEPPARD and EpSTEIN, 1970).
However, SHIEH and TAYLOR (1969a) and TAYLOR and EpSTEIN
(1968) have shown that biotite and hornblende from shallow intrusions
commonly have lower and much more variable DjH ratios (see also
FRIEDMAN and SMITH, 1958; KOKUBU et ai., 1961). This variation seems
to be a consequence of the interaction of meteoric ground waters of
different isotopic composition with magmas or hot igneous rocks. The
DjH ratios of igneous rocks and minerals are enormously more sensitive
to such processes than are the ISOrO ratios, because igneous rocks
contain about 60 atom percent oxygen, and it thus requires exchange
with a very large amount of H20 to appreciably change their ISOrO
ratios.
Based on the fact that water samples have bD-values between - 50
and - 80%0, we cannot necessarily conclude that they must be of "primary magmatic origin". This is because waters of practically any origin
can attain such values through isotopic exchange and other fractionation steps. In all the presently analyzed, active geothermal areas CRAIG
(1963) has utilized hydrogen and oxygen isotope techniques to demonstrate the dominance of meteoric water (greater than 95%) in these
aqueous systems (see also Section C, Chapter III).
2. Sulfur Isotope Composition
SMITHERINGALE and JENSEN (1963) determined b 34 S-values of 0.1%0
for the sulfides of the normal undifferentiated basalts of tholeiitic composition. MAUGER et al. (1967) report 34Sp2S ratios of intermediate rocks,
alkali andesites through quartz latites, which showed a narrow spread
near zero.
SCHNEIDER (1970) analyzed the sulfur-isotope composition of tholeiitic, olivine alkali, and alkali-rich basalts. In tholeiitic and alkali basalts,
sulfur is present predominantly as sulfides. Alkali-rich undersaturated
basalts show both sulfide and sulfate-sulfur. The b 34 S-values of the olivine alkali basalts are concentrated closely around a mean value of
1.3%0. When the tholeiitic basalts deviate from this mean, they are enriched in 32S. Due to a higher water content of the alkali-rich basalts, an
oxidation of sulfur to sulfate may take place, connected with a slight
enrichment of 34S, if the system is open for light sulfide. SCHNEIDER
concluded that the olivine alkali basalts show the least deviation from
the original mantle value, which he gave as 1.3 ± 0.5%0.
Granites and associated sulfides show a wider variation range than
the basaltic rocks. Most of the granites have small positive b 34 S-values,
C. Variations of Stable Isotope Ratios in Nature
bien de from mafic and ultramific rocks and deep-seated batholitic rocks
is typically -60 to -90%0 (GODFREY, 1962; TAYLOR and EpSTEIN,
1966b; SHEPPARD and EpSTEIN, 1970).
However, SHIEH and TAYLOR (1969a) and TAYLOR and EpSTEIN
(1968) have shown that biotite and hornblende from shallow intrusions
commonly have lower and much more variable DjH ratios (see also
FRIEDMAN and SMITH, 1958; KOKUBU et ai., 1961). This variation seems
to be a consequence of the interaction of meteoric ground waters of
different isotopic composition with magmas or hot igneous rocks. The
DjH ratios of igneous rocks and minerals are enormously more sensitive
to such processes than are the ISOrO ratios, because igneous rocks
contain about 60 atom percent oxygen, and it thus requires exchange
with a very large amount of H20 to appreciably change their ISOrO
ratios.
Based on the fact that water samples have bD-values between - 50
and - 80%0, we cannot necessarily conclude that they must be of "primary magmatic origin". This is because waters of practically any origin
can attain such values through isotopic exchange and other fractionation steps. In all the presently analyzed, active geothermal areas CRAIG
(1963) has utilized hydrogen and oxygen isotope techniques to demonstrate the dominance of meteoric water (greater than 95%) in these
aqueous systems (see also Section C, Chapter III).
2. Sulfur Isotope Composition
SMITHERINGALE and JENSEN (1963) determined b 34 S-values of 0.1%0
for the sulfides of the normal undifferentiated basalts of tholeiitic composition. MAUGER et al. (1967) report 34Sp2S ratios of intermediate rocks,
alkali andesites through quartz latites, which showed a narrow spread
near zero.
SCHNEIDER (1970) analyzed the sulfur-isotope composition of tholeiitic, olivine alkali, and alkali-rich basalts. In tholeiitic and alkali basalts,
sulfur is present predominantly as sulfides. Alkali-rich undersaturated
basalts show both sulfide and sulfate-sulfur. The b 34 S-values of the olivine alkali basalts are concentrated closely around a mean value of
1.3%0. When the tholeiitic basalts deviate from this mean, they are enriched in 32S. Due to a higher water content of the alkali-rich basalts, an
oxidation of sulfur to sulfate may take place, connected with a slight
enrichment of 34S, if the system is open for light sulfide. SCHNEIDER
concluded that the olivine alkali basalts show the least deviation from
the original mantle value, which he gave as 1.3 ± 0.5%0.
Granites and associated sulfides show a wider variation range than
the basaltic rocks. Most of the granites have small positive b 34 S-values,