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Revisiting Dolomitization in the Isla de Mona Reef Complex: Marine, Mixing, Reflux, or Biogenic?


Predicting the origin of dolomite is critical to understanding porosity and permeability in conventional oil and gas reservoirs. Studies evaluating the mechanism for dolomitization can be applied to these reservoirs and used for predicting porosity distribution. This study provides a test of the mixing-zone model for dolomitization in an area where geochemical data suggest a mixing-zone origin, yet alternative explanations are also viable. The purpose of this study is to determine the origin of dolomite in Miocene and Pliocene strata on Isla de Mona, a small island off the western coast of Puerto Rico. The island is composed of two formations, the upper Lirio Limestone, and the lower, partially dolomitized Isla Mona Dolomite. The dolomitization on Isla de Mona was originally hypothesized to be caused by a mixing-zone based on stable isotope trends (González et al. 1997), but the mixing-zone model has recently been challenged (Machel, 2004). Given the geologic setting of Isla de Mona, four dolomitization models are considered plausible and consistent with the stable isotopes—marine, mixing, reflux, or biogenic. For models other than mixing, the isotopic data would have to be explained by precipitation of dolomite from seawater or evaporated seawater followed by later alteration of the dolomite, or by sampling a physical mixture of two dolomites of different origin. This study evaluates each of these models based on integrated observations of: (1) petrographic character of dolomite textures; (2) distribution of dolomite in relation to sequence boundaries; (3) C and O stable isotopes; (4) fluid inclusions; and (5) cement stratigraphy. Preliminary CL work shows two dolomite phases, a non-luminescent sparry dolomite and a luminescent phase associated with red algal grains and the outer rims of dolomite rhombohedra. Fluid inclusion data (Tm ice = −2.4 to −1.9 °C) on the nonluminescent dolomite yield a range of salinities between 35 and 45 ppt. These data indicate seawater to evaporated seawater and suggest that mesohaline reflux is applicable. With the current data set, the origin of the Isla de Mona dolomite is interpreted to result from marine and mesohaline waters, not a mixing zone. The isotope trends are either interpreted to result from alteration of the dolomite in meteoric water, a physical mixture between the sampled dolomite and later dolomite, or a physical mixture between the earlier dolomite and a later mixing-zone dolomite.