Dolomites
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carbon and oxygen isotopes in the speleothem carbonates (GALIMov and
GRINENKO, 1965; FORNACA-FINALDI et aI., 1968; HENDY and WILSON,
1968; DUPLESSY et al., 1969; and HENDY, 1971).
The isotopic composition of the calcite deposited on speleothems will
depend both on the manner in which the limestone carbonate was dissolved by the ground waters and the manner in which the precipitation
took place. Three different modes of deposition may be distinguished
(after HENDY, 1971):
"1) If isotopic equilibrium is maintained between HCOi and CO2, the calcite
precipitated will be in isotopic equilibrium with the water, and variations in 18 0/
16 0 will depend on climate alone. This will occur if the loss of C02 from solution is
slow.
2) If loss of CO2 is rapid, a kinetic fractionation will occur between HCOi and
CO2(aq), and the calcite precipitated will show a simultaneous enrichment in
l3C and 18 0.
3) If evaporation of water occurs, the calcite precipitated will be enriched in
18 0. Speleothems deposited under conditions of kinetic loss of CO2 or evaporation
of water can note be used to give paleoclimatic data."
d) Dolomites
The formation of dolomites is still a widely debated question in the
geochemistry of carbonate sediments. This applies very much to the
results of stable isotope determinations, where despite relatively large
numbers of carbon and oxygen isotope determinations, no firm conclusions appear possible (BAUSCH and HOEFS, 1972; CLAYTON et aI., 1968a
and b; DEGENS and EpSTEIN, 1964; DEUSER, 1970; EpSTEIN et aI., 1964;
FONTES et al., 1970; FRITZ and SMITH, 1970; FRITZ, 1971; GROSS and
TRACEY, 1966; HALL and FRIEDMAN, 1969; MURATA et al., 1967; NORTHROP and CLAYTON, 1966; O'NEIL and EpSTEIN, 1966b; SHEPPARD and
SCHWARCZ, 1970; TAN and HUDSON, 1971; WEBER, 1964, 1965).
At low temperatures the isotopic fractionation of oxygen and carbon
in the dolomite-water system is not very well known. Extrapolations
from high-temperature experiments give a 5.0 to 7.0%0 enrichment in 18 0
in dolomite compared to syngenetic calcite, whereas the carbon isotope
fractionation is much smaller, with about 2.5%0 enrichment of l3C in
dolomite. The problem, however, is that only very few examples with this
theoretically expected equilibrium factor have been found in nature.
Most dolomites analyzed so far are either slightly heavier or equal,
although some are even lighter in isotopic composition than coexisting
calcite (BAUSCH and HOEFS, 1972).
One way of explaining this discrepancy is to argue that the oxygen
equilibrium fractionation factor is smaller than 5 to 7%0 (FRIEDMAN and
HALL, 1963; HALL and FRIEDMAN, 1969). Another possibility is that if
99
carbon and oxygen isotopes in the speleothem carbonates (GALIMov and
GRINENKO, 1965; FORNACA-FINALDI et aI., 1968; HENDY and WILSON,
1968; DUPLESSY et al., 1969; and HENDY, 1971).
The isotopic composition of the calcite deposited on speleothems will
depend both on the manner in which the limestone carbonate was dissolved by the ground waters and the manner in which the precipitation
took place. Three different modes of deposition may be distinguished
(after HENDY, 1971):
"1) If isotopic equilibrium is maintained between HCOi and CO2, the calcite
precipitated will be in isotopic equilibrium with the water, and variations in 18 0/
16 0 will depend on climate alone. This will occur if the loss of C02 from solution is
slow.
2) If loss of CO2 is rapid, a kinetic fractionation will occur between HCOi and
CO2(aq), and the calcite precipitated will show a simultaneous enrichment in
l3C and 18 0.
3) If evaporation of water occurs, the calcite precipitated will be enriched in
18 0. Speleothems deposited under conditions of kinetic loss of CO2 or evaporation
of water can note be used to give paleoclimatic data."
d) Dolomites
The formation of dolomites is still a widely debated question in the
geochemistry of carbonate sediments. This applies very much to the
results of stable isotope determinations, where despite relatively large
numbers of carbon and oxygen isotope determinations, no firm conclusions appear possible (BAUSCH and HOEFS, 1972; CLAYTON et aI., 1968a
and b; DEGENS and EpSTEIN, 1964; DEUSER, 1970; EpSTEIN et aI., 1964;
FONTES et al., 1970; FRITZ and SMITH, 1970; FRITZ, 1971; GROSS and
TRACEY, 1966; HALL and FRIEDMAN, 1969; MURATA et al., 1967; NORTHROP and CLAYTON, 1966; O'NEIL and EpSTEIN, 1966b; SHEPPARD and
SCHWARCZ, 1970; TAN and HUDSON, 1971; WEBER, 1964, 1965).
At low temperatures the isotopic fractionation of oxygen and carbon
in the dolomite-water system is not very well known. Extrapolations
from high-temperature experiments give a 5.0 to 7.0%0 enrichment in 18 0
in dolomite compared to syngenetic calcite, whereas the carbon isotope
fractionation is much smaller, with about 2.5%0 enrichment of l3C in
dolomite. The problem, however, is that only very few examples with this
theoretically expected equilibrium factor have been found in nature.
Most dolomites analyzed so far are either slightly heavier or equal,
although some are even lighter in isotopic composition than coexisting
calcite (BAUSCH and HOEFS, 1972).
One way of explaining this discrepancy is to argue that the oxygen
equilibrium fractionation factor is smaller than 5 to 7%0 (FRIEDMAN and
HALL, 1963; HALL and FRIEDMAN, 1969). Another possibility is that if