Stable Isotopes as Tracers of Fluid/Rock Interactions During Massive Platform Dolomitization, Little Bahama Bank

Volker C. Vahrenkamp, Peter K. Swart

Little Bahama Bank, an isolated carbonate platform in the northern Bahamas (70 km to 200 km in size), exhibits massive subsurface dolomitization of late Tertiary carbonates. Strontium isotope ratios suggest two bank-wide episodes of dolomitization, a Miocene period for Miocene rocks and a short Pleistocene episode for Pliocene carbonates. In order to constrain the nature and evolution of dolomitizing waters, an extensive survey of carbon and oxygen isotopes was conducted on four cores (up to 100 m long) from a transect across the bank.

The oxygen isotope values of the dolomites range from +2 to +4 ^pmil (PDB) but when corrected for the effects of major element nonstoichiometry are remarkably constant (+4.1 ^pmil) throughout the entire platform. In contrast the carbon isotopic compositions are extremely variable, with the margin dolomites exhibiting heavier values than those from the platform interior. This variability is present in both the Miocene (+3.2 to +1.5 ^pmil) and the Pliocene sections (+2.4 to +1.9 ^pmil).

Time constraints and the stable isotopic signatures suggest that oxygen isotopes were dominated bank wide by an overwhelming and compositionally constant isotope pool (water dominated) linked to a powerful flow system, while carbon signatures became controlled by the (calcitic) precursor rock composition (-0.8 ^pmil) toward the bank interior. For the carbon reservoir this indicates fluid evolution from water dominated at the bank margin to rock dominated in the platform interior. Carbon isotope differences (0.8 ^pmil) between the two dolomite generations at the bank margin consequently reflect carbon fluctuations of the dolomitizing fluid pool.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.