Carbon Isotope Ratios during Metamorphism
109
liberated CO2 is in isotopic equilibrium with the carbonate before escaping, the oxygen isotopic composition of the CO2 can be estimated from
the laboratory calibration curves (O'NEIL and EpSTEIN, 1966b). We
know from these experiments that in the temperature range 500 to
700
0
C the CO2 in equilibrium with dolomite would be about 5%0 higher
in 180rO ratio than the dolomite. If the isotopic fractionations during
decarbonation are kinetically controlled, the situation may be more
complex. SHARMA and CLAYTON (1965) have measured the oxygen isotopic fractionation between CO2 and dolomite during thermal decomposition of dolomite in vacuum. They showed that even during rapid decarbonation in which no attempt is made to attain equilibrium, the kinetic
fractionations between CO2 and dolomite in the temperature range studied are apparently similar to the equilibrium fractionations.
SHIEH and TAYLOR (1969b) demonstrated that calc-silicate skarns
usually have the lowest 18 0/ 16 0 and 13C/12C ratios in comparison with
the adjacent rock units. They concluded that decarbonation reactions
are largely responsible for the lowering of the t5 18 0 and t5 13 C values. This
lowering of the 180rO ratio in the rocks of the contact aureole is in
contrast to the effects of contact metamorphic dehydration reactions,
which generally produce no change in 18 0/ 16 0 ratio. These differences
are a result of the fact that CO2-mineral oxygen isotope fractionations
are significantly larger than H20-mineral fractionations and are also
usually in the opposite direction.
Inasmuch as most marine limestones have t513C-values close to zero,
one would expect that on decarbonation the limestones would give off
CO2 with t5 13 C-values of about + 5.0%0. However, most of the CO2 sampled in geothermal areas (average about - 3 to - 5 %0, see p.63) has a
negative t5 13 C-value. This CO2 can hardly be expected to be simply derived from the decarbonation of marine limestones.
Most graphites in metamorphic rocks gave results similar to the
original organic material in the presursor sedimentary material (CRAIG,
1953; GAVELIN, 1957; LANDERGREN, 1961; HAHN-WEINHEIMER, 1960,
1966). No dependence of the 13C;nC ratios of graphites on increasing
metamorphic grade has been detected, despite the theoretical prediction
that the 13CjI2C ratios of graphites should increase because of the splitting off of isotopically light hydrocarbons. However, it might be possible
that the primary inhomogeneities are camouflaging this effect.
Graphites and carbonates frequently occur together in metamorphic
rocks. 13CjI2C data of both have been given by HAHN-WEINHEIMER
(1966) and others. Carbon fractionation factors in the system graphitecalcite have been given by BOTTINGA (1969a). From the measured isotopic difference between calcite and graphite, we may tentatively calculate "isotope equilibrium temperatures", which in almost all cases show
that isotope equilibrium cannot have been reached.
109
liberated CO2 is in isotopic equilibrium with the carbonate before escaping, the oxygen isotopic composition of the CO2 can be estimated from
the laboratory calibration curves (O'NEIL and EpSTEIN, 1966b). We
know from these experiments that in the temperature range 500 to
700
0
C the CO2 in equilibrium with dolomite would be about 5%0 higher
in 180rO ratio than the dolomite. If the isotopic fractionations during
decarbonation are kinetically controlled, the situation may be more
complex. SHARMA and CLAYTON (1965) have measured the oxygen isotopic fractionation between CO2 and dolomite during thermal decomposition of dolomite in vacuum. They showed that even during rapid decarbonation in which no attempt is made to attain equilibrium, the kinetic
fractionations between CO2 and dolomite in the temperature range studied are apparently similar to the equilibrium fractionations.
SHIEH and TAYLOR (1969b) demonstrated that calc-silicate skarns
usually have the lowest 18 0/ 16 0 and 13C/12C ratios in comparison with
the adjacent rock units. They concluded that decarbonation reactions
are largely responsible for the lowering of the t5 18 0 and t5 13 C values. This
lowering of the 180rO ratio in the rocks of the contact aureole is in
contrast to the effects of contact metamorphic dehydration reactions,
which generally produce no change in 18 0/ 16 0 ratio. These differences
are a result of the fact that CO2-mineral oxygen isotope fractionations
are significantly larger than H20-mineral fractionations and are also
usually in the opposite direction.
Inasmuch as most marine limestones have t513C-values close to zero,
one would expect that on decarbonation the limestones would give off
CO2 with t5 13 C-values of about + 5.0%0. However, most of the CO2 sampled in geothermal areas (average about - 3 to - 5 %0, see p.63) has a
negative t5 13 C-value. This CO2 can hardly be expected to be simply derived from the decarbonation of marine limestones.
Most graphites in metamorphic rocks gave results similar to the
original organic material in the presursor sedimentary material (CRAIG,
1953; GAVELIN, 1957; LANDERGREN, 1961; HAHN-WEINHEIMER, 1960,
1966). No dependence of the 13C;nC ratios of graphites on increasing
metamorphic grade has been detected, despite the theoretical prediction
that the 13CjI2C ratios of graphites should increase because of the splitting off of isotopically light hydrocarbons. However, it might be possible
that the primary inhomogeneities are camouflaging this effect.
Graphites and carbonates frequently occur together in metamorphic
rocks. 13CjI2C data of both have been given by HAHN-WEINHEIMER
(1966) and others. Carbon fractionation factors in the system graphitecalcite have been given by BOTTINGA (1969a). From the measured isotopic difference between calcite and graphite, we may tentatively calculate "isotope equilibrium temperatures", which in almost all cases show
that isotope equilibrium cannot have been reached.