Dolomite from Reflux of Moderate Salinity Brine, Enewetak Atoll
Robert H. Goldstein
Dolomite from the Eocene of Enewetak Atoll provides a model for prediction of dolomite reservoirs. Others have noted dolomite below about 1200 meters at the base of permeable slope strata, and that dolomite postdates compaction, formed from fluids with 87Sr/86Sr higher than the host strata, and that dolomite stable isotope values argue for precipitation from cool seawater or warm evaporated seawater.
Dolomite contains cloudy cores, rich in primary fluid inclusions. Fluid inclusion ice melting ranges from -2.4 to -4.4°C (higher salinity than seawater; 44 to 85 ppt). Ratios of clear rim/cloudy core compared to new 87Sr/86Sr and stable isotope data yield no correlation indicative of differences between clear rims and cloudy cores. Dolomite 87Sr/86Sr are 0.70750 to 0.70873, but fluid inclusion 87Sr/86Sr are 0.70957 to 0.71198, indicating inclusions best preserve end-member compositions for the dolomitizing fluid. Thus, dolomite precipitated from a young fluid that, surprisingly, may have interacted with some unknown source of radiogenic Sr. For fluid inclusions, Na/K is similar to seawater indicating components were derived from seawater evaporation and not from dissolution of an evaporite, Na/Sr and Ca/Mg are similar to seawater modified by rock/water interaction, and Cl/SO4 suggests removal of SO4 from pore fluids.
The only viable explanation for the Enewetak dolomite is that young fluids evaporated to salinities slightly above seawater in Enewetak lagoon. The density contrast allowed for reflux deep into the atoll, discharging through permeable slope strata. This model could predict distributions of dolomite in any platform with slight restriction and appropriate climate.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California