62
C. Variations of Stable Isotope Ratios in Nature
and HOERING, 1961). We may, therefore, tentatively propose that diamond may form from graphite; although there are other petrological
arguments, which do not favor this assumption.
III. Volcanic Gases and Geothermal Waters
Most silicate magmas solidify over a large temperature range. The
residual material consists of a fluid made up predominantly of water
containing substances with particularly stable molecules that are gaseous at magmatic temperatures and compounds that are very soluble in
water. A fluid of this type may be produced in small amounts by the
crystallization of basaltic magma and in larger amounts by the crystallization of a granitic magma.
It might be expected that the residual fluid from a crystallizing
magma should show a concentration of substances of low melting
points, the so-called volatiles, which were originally dissolved in the
silicate melt. The general nature of these volatiles can be inferred from
the composition of volcanic gases. Gases collected directly from volcanic
vents or from cooling lava flows, and small quantities of trapped gas that
may be obtained by heating lava specimens, all have approximately the
same composition. In all instances, H20 is the main component, CO2
being the next most abundant. Sulfur may occur as S02, as elemental
sulfur or as H2S. From thermodynamic considerations it appears that in
a mixture of water-sulfur-carbon in proportions found actually in nature, sulfur will be present as S02 at high temperatures and as H2S at
low temperatures.
One ofthe principal conclusions drawn from stable isotope studies of
fluids in geothermal systems is that most hot spring waters (maybe 95%
and more) are not primary magmatic waters but meteoric waters derived
from local precipitation (CRAIG et ai., 1956; CRAIG, 1966; SHEPPARD et
ai., 1969). Most hot spring waters have deuterium contents similar to
those of local precipitations, but are usually enriched in 18 0 by isotopic
exchange with the country rock at elevated temperatures. This so-called
oxygen-isotopic shift is demonstrated in Fig. 26.
However, in areas where the DjH ratios of the local meteoric waters
are similar to "magmatic" water values, stable-isotope techniques cannot
distinguish conclusively between the meteoric-hydrothermal and the
magmatic-hydrothermal solutions.
The knowledge of the isotopically competitive processes involving
the different compounds of carbon and sulfur found in volcanoes and
fumaroles is of great importance in understanding carbon and sulfur
isotope geochemistry, since it is by volcanism that much of the carbon
and sulfur is injected into the earth's crust.
C. Variations of Stable Isotope Ratios in Nature
and HOERING, 1961). We may, therefore, tentatively propose that diamond may form from graphite; although there are other petrological
arguments, which do not favor this assumption.
III. Volcanic Gases and Geothermal Waters
Most silicate magmas solidify over a large temperature range. The
residual material consists of a fluid made up predominantly of water
containing substances with particularly stable molecules that are gaseous at magmatic temperatures and compounds that are very soluble in
water. A fluid of this type may be produced in small amounts by the
crystallization of basaltic magma and in larger amounts by the crystallization of a granitic magma.
It might be expected that the residual fluid from a crystallizing
magma should show a concentration of substances of low melting
points, the so-called volatiles, which were originally dissolved in the
silicate melt. The general nature of these volatiles can be inferred from
the composition of volcanic gases. Gases collected directly from volcanic
vents or from cooling lava flows, and small quantities of trapped gas that
may be obtained by heating lava specimens, all have approximately the
same composition. In all instances, H20 is the main component, CO2
being the next most abundant. Sulfur may occur as S02, as elemental
sulfur or as H2S. From thermodynamic considerations it appears that in
a mixture of water-sulfur-carbon in proportions found actually in nature, sulfur will be present as S02 at high temperatures and as H2S at
low temperatures.
One ofthe principal conclusions drawn from stable isotope studies of
fluids in geothermal systems is that most hot spring waters (maybe 95%
and more) are not primary magmatic waters but meteoric waters derived
from local precipitation (CRAIG et ai., 1956; CRAIG, 1966; SHEPPARD et
ai., 1969). Most hot spring waters have deuterium contents similar to
those of local precipitations, but are usually enriched in 18 0 by isotopic
exchange with the country rock at elevated temperatures. This so-called
oxygen-isotopic shift is demonstrated in Fig. 26.
However, in areas where the DjH ratios of the local meteoric waters
are similar to "magmatic" water values, stable-isotope techniques cannot
distinguish conclusively between the meteoric-hydrothermal and the
magmatic-hydrothermal solutions.
The knowledge of the isotopically competitive processes involving
the different compounds of carbon and sulfur found in volcanoes and
fumaroles is of great importance in understanding carbon and sulfur
isotope geochemistry, since it is by volcanism that much of the carbon
and sulfur is injected into the earth's crust.