34
B. Fractionation Mechanisms of Selected Elements
V20 5 and O 2. In the case ofV20 5 , pure sulfides are mixed with V20 5 and
sealed in a quartz tube, which is heated up to 1000° C. At this temperature, V20 5 releases sufficient oxygen to oxidize the sulfide to S02. To
avoid the formation of S03, which would result in an isotope fractionation between S02 and S03, the quartz tubes are rapidly cooled to room
temperatures.
For the extraction of sulfates and total sulfur a suitable solvent and
reduction reagent is needed. THODE et al. (1961) used a reducing agent
which was a mixture of HCI, H 3P02 and HI. The H 2S that forms may be
precipitated as CdS.
Sulfides disseminated in rock samples are disintegrated with
hydrochloric acid in the presence of aluminum according to the method
described by RICKE (1964). The resulting H 2S may be precipitated as
CdS. For pyrite, however, another preparation technique is necessary,
because pyrite is attacked too slowly with HCl. Pyrite may be oxidized
with nitric acid and bromine, and the resulting sulfate is then treated as
above.
3. Fractionation Mechanisms
There are two types of reactions producing sulfur isotope variations:
1) a kinetic effect, during the bacterial reduction of sulfate to "light"
H 2S, which gives by far the largest fractionations in the sulfur cycle
2) various chemical exchange reactions, e.g., between sulfate and sulfides on the one hand, and between the sulfides themselves on the other,
where there is a definite order of concentrating 34S.
1) There are certain species of bacteria, of which the best known is
Desulphovibrio desulphuricans, living under anaerobic conditions,
which reduce sulfate into "light" H 2S. THODE et al. (1951), JONES and
STARKEY (1957), FEELY and KULP (1957), HARRISON and THODE (1957a,
b), KAPLAN et al. (1960), NAKAI and JENSEN (1960 and 1964), KAPLAN
and RITTENBERG (1964), and KEMP and THODE (1968) showed that fractionations of up to nearly 5% could be achieved in the laboratory under
a variety of conditions.
In the experiments cited above such parameters as temperature, hydrogen donor type and concentration, sulfate concentration, and bacterial population density were varied.
KEMP and THODE (1968) argued that the following steps are involved
in the reduction process:
a) intake of sulfate,
b) organic complexing of sulfate,
c) reduction of sulfate to sulfite,
d) reduction of organically bound sulfite,
e) production of hydrogen sulfide.
B. Fractionation Mechanisms of Selected Elements
V20 5 and O 2. In the case ofV20 5 , pure sulfides are mixed with V20 5 and
sealed in a quartz tube, which is heated up to 1000° C. At this temperature, V20 5 releases sufficient oxygen to oxidize the sulfide to S02. To
avoid the formation of S03, which would result in an isotope fractionation between S02 and S03, the quartz tubes are rapidly cooled to room
temperatures.
For the extraction of sulfates and total sulfur a suitable solvent and
reduction reagent is needed. THODE et al. (1961) used a reducing agent
which was a mixture of HCI, H 3P02 and HI. The H 2S that forms may be
precipitated as CdS.
Sulfides disseminated in rock samples are disintegrated with
hydrochloric acid in the presence of aluminum according to the method
described by RICKE (1964). The resulting H 2S may be precipitated as
CdS. For pyrite, however, another preparation technique is necessary,
because pyrite is attacked too slowly with HCl. Pyrite may be oxidized
with nitric acid and bromine, and the resulting sulfate is then treated as
above.
3. Fractionation Mechanisms
There are two types of reactions producing sulfur isotope variations:
1) a kinetic effect, during the bacterial reduction of sulfate to "light"
H 2S, which gives by far the largest fractionations in the sulfur cycle
2) various chemical exchange reactions, e.g., between sulfate and sulfides on the one hand, and between the sulfides themselves on the other,
where there is a definite order of concentrating 34S.
1) There are certain species of bacteria, of which the best known is
Desulphovibrio desulphuricans, living under anaerobic conditions,
which reduce sulfate into "light" H 2S. THODE et al. (1951), JONES and
STARKEY (1957), FEELY and KULP (1957), HARRISON and THODE (1957a,
b), KAPLAN et al. (1960), NAKAI and JENSEN (1960 and 1964), KAPLAN
and RITTENBERG (1964), and KEMP and THODE (1968) showed that fractionations of up to nearly 5% could be achieved in the laboratory under
a variety of conditions.
In the experiments cited above such parameters as temperature, hydrogen donor type and concentration, sulfate concentration, and bacterial population density were varied.
KEMP and THODE (1968) argued that the following steps are involved
in the reduction process:
a) intake of sulfate,
b) organic complexing of sulfate,
c) reduction of sulfate to sulfite,
d) reduction of organically bound sulfite,
e) production of hydrogen sulfide.