The geochemistry and petrogenesis of carnallite and its relationship to the diagenesis of the devonian prairie formation
Koehler, Geoffrey David
The Prairie Formation in Saskatchewan represents the largest of several evaporite cycles preserved in the rocks of the Paleozoic Elk Point Basin. Solutions of 1-5 molal NaCl, KCl, CaCl2, and MgCl2, and synthetic brines were analyzed for hydrogen and oxygen isotopic compositions by vacuum distillation-microequilibration techniques. No effect on the hydrogen isotopic composition was observed for any of the solutions within the precision of ±5 per mil determined from analyses of distilled water. Oxygen isotopic compositions of solutions measured by vacuum distillation-microequilibration indicate that oxygen isotopes are not fractionated during analysis of NaCl, KCl, and CaCl2 solutions but analyses of MgCl2 solutions show depletions in 18O relative to pure water that can be related to the concentration of Mg2+ in the solution. The greatest depletion in 18O occurs in the 4 molal MgCl2 solutions which have δ18O values 6 per mil lower than that measured for pure water. Hydration water in carnallite (KMgCl3·6H2O) is depleted in D relative to brine from which carnallite precipitates. The temperature dependence of this fractionation can be expressed by: 10001 n α carnallite-water = -18.4(106T-2) + 162 In contrast, oxygen isotopes do not fractionate between carnallite and brine over this range in temperature. Exchange experiments indicate that the rate of isotope exchange between carnallite and brine is significant, with t1/2 approx 2 yrs. These results indicate that the stable isotopic composition of hydration water of camallite can be used to determine the isotopic compositions, temperatures, and sources of fluid from which they originally formed or recrystallized. The stable isotopic compositions carnallites from the Esterhazy Member of the Prairie Formation indicate that this mineral has been diagenetically altered by basinal fluids. Most of the carnallite is the result of relatively recent fluids at low temperature (25-50°C) and may be the result of processes involving low fluid/rock ratios and the reaction of sylvite (KCl) with Mg-rich basinal brines. These Mg-rich fluids may be related to de-watering of primary carnallite during early burial of the potash deposit, and may represent dolomitizing agents in the Elk Point Basin. Hematite in the Prairie Formation records three magnetizations: A, B, and C. The A magnetization corresponds to the Tertiary field, the B magnetization to the Cretaceous field. Paleomagnetic, petrographic, radiometric ages, and isotope data indicate that evaporites in the Elk Point Basin have been significantly affected by major fluid events that occurred during the Late Paleozoic, Cretaceous, and Early Tertiary. These fluids are most probably related to brines that have their origins within the basin, mobilized by major tectonic events that have affected North America.