STABLE-ISOTOPE COMPOSITIONS OF FLUID INCLUSIONS IN BIOGENIC CARBONATES

Measurements have been made of hydrogen, carbon, and oxygen isotope co mpositions of inclusion waters and CO2 extracted from eleven species o f modem marine and freshwater skeletal carbonates. The samples were co llected in environments of highly contrasting temperatures and isotopi c compositions of ambient waters. Inclusion waters were extracted in v acuum by thermal decrepitation of samples that were previously treated with H2O2 to remove organic matter. Water extractions were quantitati ve above 200-degrees-C for aragonite species and above 350-400-degrees -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 deltaO-18 values of the shells are in accord with crystallizatio n of carbonate at or near equilibrium with marine water of deltaO-18 n ear 0 parts per thousand. The inclusion waters, however, are not in ox ygen isotope equilibrium with ambient water and have high deltaO-18 va lues of +6 to +18 parts per thousand. These high deltaO-18 values do n ot result from partial exchange between water and either the host carb onate or small amounts of CO2 released during decrepitation. deltaD va lues of inclusion waters range from -80 to -10 parts per thousand and are sensitive to the presence of small amounts of organic matter. The data for each species define a distinct field in deltaD-deltaO-18 spac e that is controlled by a vital fractionation effect. Stable isotope c ompositions of inclusion waters can be explained by metabolic reaction s that incorporate relatively O-18-rich O2 that is dissolved in the wa ter and used by the organism in respiration. Thus, inclusion waters in shells probably represent remnants of metabolic fluids produced by th e mantle epithelium. The stable isotope compositions of such waters mo st likely result from varying metabolic rates that are specific to eac h species, as well as to formation temperature and the isotopic compos ition of ambient waters. Inclusion fluids in biogenic carbonates const itute an isotopic reservoir that has heretofore been unrecognized. The deltaO-18 and deltaD values of the inclusion waters are very differen t from those of meteoric, magmatic, and seawaters. Because these trapp ed 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 metamo rphic reactions in some cases. In the absence of other types of fluids , the participation of inclusion waters in such reactions should be ea sily recognized by their distinctive isotopic compositions.