OXYGEN AND CARBON ISOTOPE STUDIES OF CONTACT METAMORPHISM OF CARBONATE ROCKS

O18/O16 and C12/C13 ratios have been determined for carbonate rocks and coexisting minerals from two contact metamorphic aureoles at Birch Creek, California and Marble Canyon, Texas. The pelitic metasediments and granitic intrusions of the Birth Creek locality were also analyzed for their O18/16 and D/H ratios. Oxygen and carbon isotope fractionations in coexisting dolomite and calcite are interrelated but show no obvious correlation with sample distance from the intrusive contact. Small-scale oxygen isotopic exchange effects between rock units are observed within a few feet of the intrusive-country rock contacts and the marble-schist contacts at Birch Creek. Oxygen isotopic temperatures obtained from quartz-biotite fractionations in the biotite schists of Birch Creek show a systematic decrease with increasing distance away from the intrusive contact; the isotopic contact temperature is calculated to be 535-45°C. Low O18/16 and C13/12ratios of the contact metamorphic marbles generally correlate well with presence of calc-silicate minerals, indicating that the CO2 liberated during metamorphic decarbonation reactions is enriched in both O18and C12 relative to the carbonates. Material balance calculations indicate that the liberated CO2 is about 5 per mille richer in O18 and about 6 per mille richer in C13 than coexisting calcite. © 1969 Oxford University Press.