Stable isotope compositions of fluid inclusions in biogenic carbonates

Abstract

Measurements have been made of hydrogen, carbon, and oxygen isotope compositions of inclusion waters and CO 2 extracted from eleven species of modern marine and freshwater skeletal carbonates. The samples were collected in environments of highly contrasting temperatures and isotopic compositions of ambient waters. Inclusion waters were extracted in vacuum by thermal decrepitation of samples that were previously treated with H 2O 2 to remove organic matter. Water extractions were quantitative above 200°C for aragonite species and above 350–400°C for calcite species. Amounts of water liberated ranged from 0.6% to 2.2% and were generally very reproducible within a species but varied strongly from one species to another. Except for red algae and corals, the δ 18O values of the shells are in accord with crystallization of carbonate at or near equilibrium with marine water of δ 18O near 0‰. The inclusion waters, however, are not in oxygen isotope equilibrium with ambient water and have high δ 18O values of +6 to +18‰. These high δ 18O values do not result from partial exchange between water and either the host carbonate or small amounts of CO 2 released during decrepitation. δD values of inclusion waters range from −80 to −10‰ and are sensitive to the presence of small amounts of organic matter. The data for each species define a distinct field in δD-δ 18O space that is controlled by a vital fractionation effect. Stable isotope compositions of inclusion waters can be explained by metabolic reactions that incorporate relatively 18O-rich O 2 that is dissolved in the water and used by the organism in respiration. Thus, inclusion waters in shells probably represent remnants of metabolic fluids produced by the mantle epithelium. The stable isotope compositions of such waters most likely result from varying metabolic rates that are specific to each species, as well as to formation temperature and the isotopic composition of ambient waters. Inclusion fluids in biogenic carbonates constitute an isotopic reservoir that has heretofore been unrecognized. The δ 18O and δD values of the inclusion waters are very different from those of meteoric, magmatic, and seawaters. Because these trapped fluids are released by the shells during heating, they could play a role in burial diagenesis. Burial of significant amounts of biogenic carbonates could liberate enough water to control diagenetic or metamorphic reactions in some cases. In the absence of other types of fluids, the participation of inclusion waters in such reactions should be easily recognized by their distinctive isotopic compositions.