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Geochemical Variation of Dolomite Cement Within the Cambrian Bonneterre Formation, Missouri: Evidence for Fluid Mixing
M. R. Farr
Compositionally zoned dolomite in the Cambrian Bonneterre Formation of Missouri precipitated from warm, basin-derived brine, as evidenced by its fluid inclusion homogenization and freezing temperatures and radiogenic 87Sr/86Sr composition. Preexisting host carbonates played a major role in governing parent fluid composition during precipitation of the dolomite cement. This is indicated by significant overlap in the isotopic and minor and trace element composition of the cement and host. There is a decrease in 13C and 18O with succeeding dolomite cement zones, suggesting that isotopically lighter cement "armored" the host rock, progre sively isolating the fluids from preexisting dolomite.
87Sr/86Sr values for a single cement zone (zone 4) vary from 0.7086 to 0.7201. The more radiogenic values reflect a contribution of 87Sr to the brine primarily through silicate reactions prior to entering the Bonneterre Formation. The lower values overlap with preexisting replacement dolomite and limestone within the Bonneterre, again indicating the importance of host-rock buffering.
Possible mechanisms governing the apparent control on cement composition by preexisting replacement dolomite were evaluated using computer models which simulate fluid and cement composition in: 1) a system in which a single fluid progressively interacts with host carbonate, and 2) a system in which there is mixing of fluids having undergone different degrees of fluid-rock interaction. The predicted covariation of geochemical parameters using the mixing model matches the actual data from zone 4 cement better than that offer progressive fluid-rock interaction in most cases. The contrast between the predicted mixing and fluid-rock interaction curves is greatest for cross plots involving Mn, Fe, and 13C which are effectivel buffered by host rock during progressive fluid-rock interaction. The mixing model works best for zone 4 cement data subdivided according to stratigraphic subunit and area, suggesting a great deal of spatial variation in end-member fluids.
Equilibrium modelling with the speciation-reaction modelling program PHRQPITZ suggests that dolomite supersaturation occurs when a relatively hot dolomite-saturated fluid mixes with a cooler dolomite-saturated fluid having similar PCO2 values. This is consistent with geochemical data from zone 4 cement, which indicates mixing of a relatively unbuffered fluid with fluids buffered by local host rocks.
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