20
B. Fractionation Mechanisms of Selected Elements
2) Hydrogen is nearly omnipresent in the forms H20, OH, Hrgas,
and CH4 , even at great depths in the earth's mantle. Therefore, it is
conceivable that hydrogen plays a major role, directly or indirectly, in
almost all naturally occurring geological processes.
3) All major isotope fractionation processes are effective in the hydrogen cycle.
In Fig. 7 natural variation ranges of hydrogen isotope composition of
some geologically important reservoirs are given.
1. Preparation Technique and Mass-Spectrometric Measurement
The determination of the D/H ratio is usually carried out on H2 gas. Water is
converted to hydrogen by being passed over hot metallic uranium at about 750
0
C
e.g., as described by CRAIG (1961 a) and GODFREY (1962). Most of the hydrogen
generated from hydroxyl-bearing minerals is liberated in the form of water, but
some is liberated as molecular hydrogen (SA YIN and EpSTEIN, 1970a). The resulting
H2 gas is converted in many laboratories to water by reaction with copper oxide.
The water is then treated as described above.
Due to the great relative mass difference between Hand D, the separation
angle between the two isotopes is so large that a special type of mass-spectrometer
with an additional tube joined to the usual tube has been developed (e.g., FRIEDMAN,1953).
A difficulty in measuring D/H isotope ratios is that, along with the Ht and
HD+ formation in the ion source, Hj is produced as a by-product through the
reaction:
Ht +H-+Ht.
The resolution between HD + and H j in most mass-spectrometers for isotope
abundance measurements is not large enough to remove Ht; therefore, a Hj
correction has to be made. The Ht concentration is minimized by working at low
gas pressures, by addition of a repeller electrode to the source, and by use of as high
an accelerating voltage as possible (FRIEDMAN, 1953). The analytical error for
hydrogen isotope data is usually given as ± 1 to ±2%o.
2. Standard
The standard now used world-wide is SMOW (Standard Mean
Ocean Water) (CRAIG, 1961 b). D/H ratios, however, occur in the literature for at least six different "working tap water" standards.
Because there are large variations in b-values, the deviations from the
standard can be given in the literature either in % or in %0. Like all other
stable isotope data, all bD-values are given in the following in %0.
3. Fractionation Mechanisms
a) Vapor Pressure and Freezing-Point Differences
The most effective processes that produce hydrogen isotope variations are those due to vapor pressure differences, and to a much smaller
degree, those due to differences in freezing points. Because the vapor
pressure and freezing point of HDO are slightly lower than those of
B. Fractionation Mechanisms of Selected Elements
2) Hydrogen is nearly omnipresent in the forms H20, OH, Hrgas,
and CH4 , even at great depths in the earth's mantle. Therefore, it is
conceivable that hydrogen plays a major role, directly or indirectly, in
almost all naturally occurring geological processes.
3) All major isotope fractionation processes are effective in the hydrogen cycle.
In Fig. 7 natural variation ranges of hydrogen isotope composition of
some geologically important reservoirs are given.
1. Preparation Technique and Mass-Spectrometric Measurement
The determination of the D/H ratio is usually carried out on H2 gas. Water is
converted to hydrogen by being passed over hot metallic uranium at about 750
0
C
e.g., as described by CRAIG (1961 a) and GODFREY (1962). Most of the hydrogen
generated from hydroxyl-bearing minerals is liberated in the form of water, but
some is liberated as molecular hydrogen (SA YIN and EpSTEIN, 1970a). The resulting
H2 gas is converted in many laboratories to water by reaction with copper oxide.
The water is then treated as described above.
Due to the great relative mass difference between Hand D, the separation
angle between the two isotopes is so large that a special type of mass-spectrometer
with an additional tube joined to the usual tube has been developed (e.g., FRIEDMAN,1953).
A difficulty in measuring D/H isotope ratios is that, along with the Ht and
HD+ formation in the ion source, Hj is produced as a by-product through the
reaction:
Ht +H-+Ht.
The resolution between HD + and H j in most mass-spectrometers for isotope
abundance measurements is not large enough to remove Ht; therefore, a Hj
correction has to be made. The Ht concentration is minimized by working at low
gas pressures, by addition of a repeller electrode to the source, and by use of as high
an accelerating voltage as possible (FRIEDMAN, 1953). The analytical error for
hydrogen isotope data is usually given as ± 1 to ±2%o.
2. Standard
The standard now used world-wide is SMOW (Standard Mean
Ocean Water) (CRAIG, 1961 b). D/H ratios, however, occur in the literature for at least six different "working tap water" standards.
Because there are large variations in b-values, the deviations from the
standard can be given in the literature either in % or in %0. Like all other
stable isotope data, all bD-values are given in the following in %0.
3. Fractionation Mechanisms
a) Vapor Pressure and Freezing-Point Differences
The most effective processes that produce hydrogen isotope variations are those due to vapor pressure differences, and to a much smaller
degree, those due to differences in freezing points. Because the vapor
pressure and freezing point of HDO are slightly lower than those of