Dolomite exists in only a small area of Pliocene carbonates on St. Croix. This area consists of reef, lagoonal, and platform facies that underlie and rim the shoreline of Krause Lagoon, an embayment that existed on the southern coastline of St. Croix prior to industrial development in the 1960s. Since dolomite is not found in similar facies elsewhere on St. Croix, this geographic distribution suggests that the formation of the dolomite was related to hydrologic conditions in Krause Lagoon. The dolomite is calcium-rich (57-60 mol % Ca) and exists both as euhedral rhombs 2-75 mu m in diameter and as a replacement mineral in dolomitized bioclasts. Bioclasts in the dolomitic strata often show good preservation of microstructure, particularly in large benthic forams and coralline algae. The mean 87 Sr/ 86 Sr isotopic composition of the dolomite is 0.70887 + or -0.00002 (2sigma , n = 3) which corresponds to the 87 Sr/ 86 Sr ratio of Late Miocene seawater. However, because the dolomite resides in Pliocene strata, it is difficult to invoke unmodified seawater as an agent of dolomitization. Dolomitization therefore requires a source of nonradiogenic strontium. Modern St. Croix groundwater has 87 Sr/ 86 Sr compositions between 0.7076 and 0.7085 ( n = 4), well below the ratio of both modern seawater and the dolomites. Mixing calculations show that modern St. Croix groundwater could be a significant source of nonradiogenic strontium in a dolomite formed from a two-component groundwater-seawater mix. On the basis of strontium-concentration modeling, the groundwater component responsible for the St. Croix dolomites may have ranged from 40% to 80% of the dolomitizing fluids. Stable isotopic values for the dolomite range from delta 18 O of +0.7 per thousand to +3.8 per thousand , and delta 13 C of +0.6 per thousand to +2.4 per thousand (PDB), with increasing delta 18 O and delta 13 C values from the margins to the center of the lagoon. The maximum delta 18 O values in these dolomites are too high to have formed from unaltered groundwater or seawater, even accounting for ice-volume effects. Therefore the isotopically heaviest dolomite must have precipitated from fluids enriched in 18 O, probably as a result of evaporation. Dolomitization from fluids produced from a mixture of evaporated seawater and St. Croix groundwater are consistent with the geochemistry and geologic distribution of the dolomite. Calculations show that such a scenario is possible, and may be fairly common, despite the relative complexity of the model. Documented block faulting of the Krause Lagoon area may have provided a stable hydrologic regime for a long enough time for dolomite to form, despite island uplift during the late Tertiary. Other models of dolomitization can be shown to be less likely or untenable on the basis of chemical, lithologic, or hydrologic criteria.

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