PACROFI VI - Electronic Program

Cation, Anion and 87Sr/86Sr Analyses of Inclusion Fluids Elucidate Processes of Reflux Dolomitization, Enewetak Atoll

Robert H. Goldstein and K.C. Benison

University of Kansas, Department of Geology, 120 Lindley Hall, Lawrence KS 66045

Extensively dolomitized intervals of deep-slope deposits from the Eocene section of Enewetak Atoll (Fig. 1) provide a natural laboratory for using fluid inclusions to elucidate conditions of dolomitization and for evaluating other methods commonly used in diagenetic studies. Specimens have had simple tectonic and burial histories, are geologically very young, and have never been heated. Therefore, interpretations about diagenetic conditions from fluid inclusions are unambiguous. The distribution of dolomite, its position in the paragenesis, new Sr isotope data, new stable isotope data, fluid inclusion freezing point depressions, and composition of fluid inclusion extracts strongly indicate that dolomite precipitated from deeply refluxing brines of moderate salinity. Further, fluid inclusion microthermometry combined with major-ion chemistry and 87Sr/86Sr of fluid inclusion extracts provide data that are more reliable than mineral-based geochemical techniques.

Others have already described extensive dolomitization below about 1200 meters at the base of an interval of permeable slope deposits in the F1 core. The dolomite precipitated after significant compaction and from fluids with 87Sr/86Sr higher than the Eocene host sediments. New and previous stable isotope data argue that dolomite either precipitated from cool seawater or from warm seawater that had evaporated at the surface (Berner, 1965; Gross and Tracey, 1966; Saller, 1984; Saller and Koepnick, 1990).

Much of the dolomite contains cloudy cores, rich in fluid inclusions, that are confined to dolomite growth-zone boundaries and surrounded by clear rims. This distribution indicates a relationship to crystal growth that assigns a primary origin to the fluid inclusions. Although these low-temperature inclusions provide little specific information of temperature of dolomite formation, they provide an excellent record of the chemistry of the fluid that precipitated the cloudy cores of the dolomite. Freezing point depression measurements range from 2.4 to 4.4oC (Fig. 2) and indicate fluids of dolomitization ranged from 44 to 85 ppt seawater equivalent, salinities higher than seawater but lower than the salinity of significant evaporite precipitation. Although the clear rims in the dolomite lack primary fluid inclusions, the inclusion data appear to be representative of both cloudy core and clear rim in the dolomite. Thirty point counts show ratios of clear rim/cloudy core from 2.6 to 20.3. When these are compared to new 87Sr/86Sr and stable isotope data for the same samples, no correlation appears. This suggests that cloudy cores and clear rims grew from similar fluids.

Most new 87Sr/86Sr data for the dolomite ranges from 0.70750 to 0.70873, but the 87Sr/86Sr composition of fluids extracted from the fluid inclusions ranges from 0.70957 to 0.71198 (Fig. 3). These data show that the dolomite values are intermediate in composition between that of the dolomitizing fluid and that of the Eocene host rock. This indicates that the mineral 87Sr/86Sr values result from a mixture of original rock Sr and fluid Sr, whereas the inclusions better preserve the end-member composition of the fluid that precipitated the dolomite. The 87Sr/86Sr of the fluid indicates that dolomite precipitated from a young fluid that, surprisingly, may have interacted with some unknown source of radiogenic Sr. Such a source is difficult to find in an isolated atoll such as Enewetak. Local basement rocks cannot be the source of the radiogenic values because they have extremely low values (Fig. 3). The only source of radiogenic Sr within the atoll might be windblown siliciclastic debris of continental origin. However, this material would be of such low volume compared to the depositional carbonate, that any contribution of Sr from it would likely be overwhelmed by contributions from marine carbonate and seawater-derived fluids. Therefore, the only plausible model by which the inclusion fluids could have reached such high 87Sr/86Sr would be if the windblown siliciclastics had been concentrated and isolated, perhaps in karstic conduits, and that fluids preferentially flowed through these without much reaction with the surrounding depositional carbonate.

All extracted fluid inclusions were analyzed for Na, Ca, Mg, K, Sr, Cl and SO4 to evaluate the origin and evolution of the fluid responsible for dolomitization. In general Na/K is similar to seawater indicating the brine was derived from seawater evaporation and not from dissolution of an unknown evaporite, Na/Sr is similar to seawater that had undergone some rock/water interaction, Ca/Mg is above seawater suggesting rock/water interaction of a seawater derived fluid, and Cl/SO4 shows variance attesting to removal of SO4 from pore fluids.

The only process that can explain the chemistry and distribution of the Enewetak dolomite is one in which relatively recent fluids evaporated to salinities slightly above seawater in Enewetak lagoon. The density contrast between this fluid and seawater allowed for reflux flow deep into the atoll, perhaps channeled through karstic conduits. The fluids discharged along the base of a permeable slope unit and dolomitized the basal permeable strata. This model of dolomitization from moderate salinity brines refluxing through platforms could yield dolomite reservoirs in any platform with enough restriction to develop slightly elevated salinities, and would localize dolomite in areas below lagoons and along the base of potential discharge zones near platform margins.

Finally, this study shows that application of traditional geochemical methods to studies of dolomite is likely to yield ambiguous results unless fluid inclusion studies are incorporated. Interpretation of oxygen isotopic analyses of dolomite requires assumption of the isotopic composition of the fluid or knowing the temperature of precipitation. The 87Sr/86Sr value of dolomite produces useful information, but interpretations are difficult because the mineral's composition results from mixtures of fluid-sourced Sr and original sediment sources of Sr. Incorporating fluid inclusions in a study is important because the origin of dolomitizing fluids are most easily interpreted from fluid inclusion salinity. Further, the 87Sr/86Sr of inclusion fluids is the best record of the 87Sr/86Sr of dolomitizing fluid because, unlike 87Sr/86Sr of replacement minerals like dolomite, it best preserves end-member compositions without significant contamination from the host rock.