PETROGRAPHY AND GEOCHEMISTRY OF DOLOMITES IN MONTALVA, GUÁNICA TOOLS TO ASSESS THE TIMING AND PROCESS OF DOLOMITIZATION

Dolomite is an intricate mineral that has been studied worldwide, including Puerto Rico. On the island, many authors have researched dolomites in various environments including subsurface aquifer systems and exposed isolated carbonate platforms. Recently, sugary texture-bearing rocks, suggesting dolomite, was observed within outcrops of a coastal hypersaline lagoon in Guánica. This study confirmed the presence of dolomite, measured its abundance, characterized the petrography, and examined their bulk geochemistry via stable isotopes (δ18O and δ13C), strontium trace elemental concentrations, and strontium isotopes. Ten dolomite samples were analyzed, and petrographic and geochemical results were compared to other case studies from Puerto Rico to constrain the dolomitization environment, and age of formation and to propose a dolomitization model.

Point counts suggest that dolomite is present (50% or more). Average δ18O and δ13C values relative to VPDB are 1.38‰ (± 0.1 ‰) and 0.41‰ (± 0.07) respectively. 87Sr/86Sr isotopes range from 0.708896 to 0.708946 ± 0.000010, which produce an age range from 10.21 to 6.83 Ma, (Late Miocene or Tortonian to Messinian). Sr T.E. ranges from 19.98 to 136.92 ppm. Covariant trends in δ18O and δ13C and low Sr T.E. concentrations suggest the dolomites were precipitated in mixed marine and meteoric water, with at least a 50 % seawater composition. Since isotopic signatures suggest the presence of meteoric water it is proposed that the youngest age obtained from 87Sr/86Sr ratio/age conversion should be the most accurate (6.83Ma). The youngest age of dolomitization is consistent with documented eustatic drops in sea level during the late Miocene. Mixing-zone dolomitization is proposed to be the most reasonable model based on the petrographic, geochemical, geologic history, and setting.