Carbon Isotope Composition
61
while some gave abnormally low or high 34S/32S ratios (SHIMA et al.,
1963; BOTH et aI., 1969; SIEWERS, unpublished data).
The idea of using sulfur isotope studies to distinguish between "primary" differentiated granites and "secondary" partially melted metamorphic rocks is as old as the very first sulfur isotope measurements by
THODE et al. (1949). SIEWERS (unpublished manuscript) demonstrated
that this distinction cannot be done.
3. Carbon Isotope Composition
Carbonatites are magmatic carbonates (mostly calcite and dolomite)
associated with alkaline rocks. It is generally agreed that they crystallized from deep-seated carbonate magmas, which is in accordance with
the isotope data. The majority of carbonatites fall in the range of 6 to
10%0 for the 180rO ratio and -5.0 to -8.0%0 for the 13Cl2c ratio
(BAERTSCHI, 1957; CONWAY and TAYLOR, 1969; DEINES, 1970; KUKHARENKO and DONTSOVA, 1964; TAYLOR et al., 1967). Those samples falling
outside this range may have been affected be weathering or by hydrothermal alteration.
Carbonates, often present in trace amounts in igneous rocks, show
(j180-values that are quite heavy and (j13C-values that are quite light and
very variable, indicating a groundwater origin or a re-equilibration under low temperatures (O'NEIL et al., 1970; HOEFS, 1972).
HOEFS (1965) has shown that carbon occurs in at least two compounds in igneous rocks, in an oxidized form (carbonates and CO2 in
fluid inclusions) and in a reduced form (maybe elemental carbon and
"organic" compounds). The reduced carbon has a strange, very light
isotopic composition (-19.0 to - 28.0%0, CRAIG, 1953; HOEFS, 1972).
Theoretically, these (j-values can be explained by two very different mechanisms: a secondary, maybe also groundwater, origin and a primary
origin of anorganically produced organic compounds. Because of the
striking similarities with the carbon isotopic composition in meteorites
- mainly carbonaceous chondrites - and in lunar rocks, HOEFS (1972)
has favored the second possibility.
Since diamonds need high pressures for their formation, their (jvalues ( - 3.0 to - 8.0%0), like those of carbonatites, may be regarded as
the primary carbon isotope ratio on earth (CRAIG, 1953; WICKMAN,
1956; VINOGRADOV et al., 1965). (See also Section C, X.)
BOTTINGA (1969b) has calculated the carbon isotope fractionation
for graphite, diamond, and CO2 • In accordance with the theoretical
calculations, the few measurements on diamond and graphite in contact
with each other always show a slight enrichment in 13C in diamond with
respect to graphite (VINOGRADOV et al., 1966; VINOGRADOV et aI., 1967;
61
while some gave abnormally low or high 34S/32S ratios (SHIMA et al.,
1963; BOTH et aI., 1969; SIEWERS, unpublished data).
The idea of using sulfur isotope studies to distinguish between "primary" differentiated granites and "secondary" partially melted metamorphic rocks is as old as the very first sulfur isotope measurements by
THODE et al. (1949). SIEWERS (unpublished manuscript) demonstrated
that this distinction cannot be done.
3. Carbon Isotope Composition
Carbonatites are magmatic carbonates (mostly calcite and dolomite)
associated with alkaline rocks. It is generally agreed that they crystallized from deep-seated carbonate magmas, which is in accordance with
the isotope data. The majority of carbonatites fall in the range of 6 to
10%0 for the 180rO ratio and -5.0 to -8.0%0 for the 13Cl2c ratio
(BAERTSCHI, 1957; CONWAY and TAYLOR, 1969; DEINES, 1970; KUKHARENKO and DONTSOVA, 1964; TAYLOR et al., 1967). Those samples falling
outside this range may have been affected be weathering or by hydrothermal alteration.
Carbonates, often present in trace amounts in igneous rocks, show
(j180-values that are quite heavy and (j13C-values that are quite light and
very variable, indicating a groundwater origin or a re-equilibration under low temperatures (O'NEIL et al., 1970; HOEFS, 1972).
HOEFS (1965) has shown that carbon occurs in at least two compounds in igneous rocks, in an oxidized form (carbonates and CO2 in
fluid inclusions) and in a reduced form (maybe elemental carbon and
"organic" compounds). The reduced carbon has a strange, very light
isotopic composition (-19.0 to - 28.0%0, CRAIG, 1953; HOEFS, 1972).
Theoretically, these (j-values can be explained by two very different mechanisms: a secondary, maybe also groundwater, origin and a primary
origin of anorganically produced organic compounds. Because of the
striking similarities with the carbon isotopic composition in meteorites
- mainly carbonaceous chondrites - and in lunar rocks, HOEFS (1972)
has favored the second possibility.
Since diamonds need high pressures for their formation, their (jvalues ( - 3.0 to - 8.0%0), like those of carbonatites, may be regarded as
the primary carbon isotope ratio on earth (CRAIG, 1953; WICKMAN,
1956; VINOGRADOV et al., 1965). (See also Section C, X.)
BOTTINGA (1969b) has calculated the carbon isotope fractionation
for graphite, diamond, and CO2 • In accordance with the theoretical
calculations, the few measurements on diamond and graphite in contact
with each other always show a slight enrichment in 13C in diamond with
respect to graphite (VINOGRADOV et al., 1966; VINOGRADOV et aI., 1967;