Dolomite stoichiometric variability resulting from changing aquifer conditions, Barbados, West Indies

Abstract

Electron microprobe backscatter imaging (BSI) and wavelength-dispersive spectrometric analysis (WDS) of late Pleistocene dolomite cements from southeastern Barbados indicate variability in dolomite stoichiometry occurs on a micron scale. Replacement dolomitization and formation of dolomite cements occurred in a dilute mixing zone between normal marine and meteoric phreatic waters during the initial stages of a sea-level fall, following the oxygen isotope stage 7.3 glacio-eustatic highstand. The sea-level fall resulted in a progressive change to more meteoric conditions through time. BSI documents replacement dolomitization, dolomite cementation, alternating dolomite and low-Mg calcite (LMC) cementation, and finally LMC cementation. Dolomite cements show continuous, micron-scale, growth-perpendicular zonations in BSI response (similar to cathodoluminescent banding), indicating variation in dolomite stoichiometry. WDS measurements indicate average dolomite matrix compositions of 41 mol% MgCO 3 and stoichiometrically zoned dolomite cements average 42 mol% MgCO 3. Dolomite cement zonation likely represents variations in dolomite saturation state as a function of variable mixing proportions. In all samples studied, the latest “generation” of dolomite shows marked increases in MgCO 3 content. Latest dolomites are essentially stoichiometric, averaging 49.5 mol% MgCO 3. These stoichiometric dolomites are commonly directly followed by blocky LMC meteoric phreatic cement (1–4 mol% MgCO 3), indicating they represent precipitation from the most dilute, near-meteoric waters of the mixing zone.