STABLE-ISOTOPE COMPOSITIONS OF FLUID INCLUSIONS IN BIOGENIC CARBONATES

Measurements have been made of hydrogen and oxygen isotope composition s of inclusion waters extracted from eleven species of modern marine a nd freshwater skeletal carbonates. Except for red algae and corals, th e delta(18)O values of the shells are in accord with crystallization o f carbonate at or near equilibrium with marine water of delta(18)O nea r 0%parts per thousand. The inclusion waters, however, are not in oxyg en isotope equilibrium with ambient water and have high delta(18)O val ues of + 6 to + 18 %parts per thousand. delta D values of inclusion wa ters range from -80 to -10 %parts per thousand and are sensitive to th e presence of small amounts of organic matter. The data for each speci es define a distinct Field in delta D-O-18 space that is controlled by a ''vital fractionation effect''. Stable isotope compositions of incl usion waters can be explained by two kind of in situ isotope fractiona tions (1) incorporation of O-18-rich O-2 through respiration and more likely (2) CO2 based calcification which involves preferentially (H2O) -O-16 through kinetic or equilibrium reactions with CO2 to form CO32.. Thus, inclusion waters in shells probably represent remnants of metab olic fluids produced by the mantle epithelium and modified during the shell building. Inclusion fluids in biogenic carbonates constitute a s ignificant isotopic reservoir that has heretofore been unrecognized. T he delta(18)O and delta D values of the inclusion waters are very diff erent from those of meteoric, magmatic and seawaters. Because these tr apped fluids are released by the shells during heating, they could pla y an important role in burial diagenesis or metamorphism.