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Abstract:

Growth of chevron halites during evaporation of water in saline pans, tidal flats, and in laboratory experiments has been cited as evidence that chevron halite in ancient evaporite deposits also formed from evaporating brines at the earth’s surface. Permian halites in the Palo Duro Basin (Texas) contain fluid inclusions that have stable isotopic compositions that may be comparable to evaporated seawater. In contrast, waters from chevron halite fluid inclusions in potash ore of the Devonian Prairie Evaporite Formation (Saskatchewan) have δD values near –100 and δ13O values near –12‰. which are substantially different from values expected from evaporated seawater These values are comparable to basinal brines that probably affected evaporites in the subsurface.

The thermometric properties of the fluid inclusions in chevron halites from the central Western Canada Sedimentary Basin indicate that the fluids are Na-K-Ca-Mg-Cl brines, whereas inclusions from the eastern portion of the basin have identical isotopic compositions but thermometric properties of Na-K-Cl brines. These Na-K-Cl brines do not represent the expected composition for simple evaporation of seawater to potash facies.

Chemical and isotopic compositions from fluid inclusions in chevron halites from the Prairie Evaporite Formation indicate that, as well as forming at the surface, chevron halite may form in the subsurface or, less likely, recrystallize without destruction of its delicate texture. Laboratory experiments show that brines will precipitate chevron halites simply by cooling in closed systems. Thus, fluids trapped in evaporites may represent warm basinal waters that dissolved the evaporite and precipitated halite during cooling. Therefore, from microthermometric and stable isotope compositions of fluid inclusion waters in chevron halites, the assumption that chevron texture in halite is unique to surface processes in both non-marine and marine evaporite deposits is not valid in all instances.

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