Metamorphic Rocks
103
CHSEN (1966) and ANDERSON (1967) have described heterogenous metamorphic rocks in which detectable isotopic disequilibrium occurs over a
distance of a few centimeters.
The possibility of using oxygen isotope fractionations as a geothermometer in metamorphic rocks was demonstrated by CLAYTON and
EpSTEIN (1958), and has since been applied by JAMES and CLAYTON
(1962), SCHWARCZ (1966), GARLICK and EpSTEIN (1967), SHEPPARD and
SCHWARCZ (1970), DEVEREUX (1968), TAYLOR and COLEMAN (1968),
CLAYTON et al. (1968c), SHIEH and TAYLOR (1969a, b), SCHWARCZ et al.
(1970), WILSON et al., (1970), and others.
Since GARLICK and EpSTEIN (1967) found that temperature as inferred from the quartz-magnetite geothermometer agreed in general with
the usual geologic estimates based on experimental studies of silicate
assemblages, the quartz-magnetite pair is generally regarded as the most
suitable for the determination of metamorphic isotopic temperatures.
However, as SCHWARCZ et al. (1970) have shown although the Ll-difference of a mineral pair general decreases with metamorphic grade there is
a significant spread within each zone, which may be governed by the
following factors:
1) The variations in the temperatures are due to compensating differences of other variables besides temperature (e.g., chemical composition,
oxygen fugacity, PH20 ).
2) The ideal isotopic geothermometer should record only the maximum temperature of metamorphism. However, a mineral pair may continue to re-equilibrate to some temperature below the maximum temperature of metamorphism.
3) Some minerals may have formed or re-equilibrated during discrete metamorphic events later than, and of a lower temperature than,
the maximum metamorphic temperature.
Assuming that the spread in Ll-differences of given mineral pairs are
due to retrograde exchange, we can take the minimum fractionations in
each zone to represent most closely the maximum temperatures of metamorphism.
Quartz-mica fractionations do not appear to be a promising geothermometer. CLAYTON (manuscript in preparation) has shown in a large
number of igneous and high-temperature metamorphic rocks, that assemblages involving micas give rarely concordant temperatures and that
quartz-muscovite fractionations usually fall in the range 2.4 to 3.4%0,
irrespective of rock type.
SCHWARCZ et al. (1970) have attempted to test a quartz and calcite
geothermometer. They find that "fractionation values decrease with increasing metamorphic grade up to the garnet zone but level otT or increase at higher grades, showing that at least at these higher grades the
103
CHSEN (1966) and ANDERSON (1967) have described heterogenous metamorphic rocks in which detectable isotopic disequilibrium occurs over a
distance of a few centimeters.
The possibility of using oxygen isotope fractionations as a geothermometer in metamorphic rocks was demonstrated by CLAYTON and
EpSTEIN (1958), and has since been applied by JAMES and CLAYTON
(1962), SCHWARCZ (1966), GARLICK and EpSTEIN (1967), SHEPPARD and
SCHWARCZ (1970), DEVEREUX (1968), TAYLOR and COLEMAN (1968),
CLAYTON et al. (1968c), SHIEH and TAYLOR (1969a, b), SCHWARCZ et al.
(1970), WILSON et al., (1970), and others.
Since GARLICK and EpSTEIN (1967) found that temperature as inferred from the quartz-magnetite geothermometer agreed in general with
the usual geologic estimates based on experimental studies of silicate
assemblages, the quartz-magnetite pair is generally regarded as the most
suitable for the determination of metamorphic isotopic temperatures.
However, as SCHWARCZ et al. (1970) have shown although the Ll-difference of a mineral pair general decreases with metamorphic grade there is
a significant spread within each zone, which may be governed by the
following factors:
1) The variations in the temperatures are due to compensating differences of other variables besides temperature (e.g., chemical composition,
oxygen fugacity, PH20 ).
2) The ideal isotopic geothermometer should record only the maximum temperature of metamorphism. However, a mineral pair may continue to re-equilibrate to some temperature below the maximum temperature of metamorphism.
3) Some minerals may have formed or re-equilibrated during discrete metamorphic events later than, and of a lower temperature than,
the maximum metamorphic temperature.
Assuming that the spread in Ll-differences of given mineral pairs are
due to retrograde exchange, we can take the minimum fractionations in
each zone to represent most closely the maximum temperatures of metamorphism.
Quartz-mica fractionations do not appear to be a promising geothermometer. CLAYTON (manuscript in preparation) has shown in a large
number of igneous and high-temperature metamorphic rocks, that assemblages involving micas give rarely concordant temperatures and that
quartz-muscovite fractionations usually fall in the range 2.4 to 3.4%0,
irrespective of rock type.
SCHWARCZ et al. (1970) have attempted to test a quartz and calcite
geothermometer. They find that "fractionation values decrease with increasing metamorphic grade up to the garnet zone but level otT or increase at higher grades, showing that at least at these higher grades the