ABSTRACT
Minimal values of bromide content in halites of the lowermost part of the Devonian Prairie Evaporite Formation are variable, but many of these values are about half of those of the first precipitated halite from present-day sea water. This observation does not mean that the bromide/chloride ratio of the Devonian sea was necessarily different from that of the present open ocean, because the bromide/chloride ratio of the Devonian marine brines in the ocean basin could have been altered by interaction with waters from the surrounding land, similar to that observed at the present time in some coastal lakes of southern Russia.
Before any primary sylvite or carnallite crystallized, the primary brines which precipitated halite reached bromide/chloride ratios close to those of primary brines which were in the same stage of evaporation in the German Zechstein 2 basin. Both ratios are close to the equivalent values for halite crystallized from present-day sea water (0.027 weight per cent bromide).
All analyzed halites in the sylvinite zones of the Prairie Evaporite Formation have bromide values approximately half of those in the upper part of the underlying halite zone. The decreasing bromide contents in successively younger beds of the sylvinite zones indicate that the sylvinite is not a primary deposit from a marine brine, or even from a brine which had redissolved pre-existing precipitates. The sudden decrease of the bromide contents in the halites of most sylvinites is therefore explained by secondary processes. Neither the carnallites analyzed nor most of the sylvites, have primary bromide values. Only one sylvinite with primary bromide values was found.
It is concluded that primary carnallitite was precipitated and that this was later dissolved by small amounts of brines saturated with sodium chloride. As these brines became saturated with magnesium chloride secondary halite and sylvite were precipitated while the concentration of magnesium chloride in the brine increased These brines, eventually saturated with magnesium chloride, carried most of the magnesium chloride away and secondary carnallite crystallized elsewhere under favorable conditions. Red secondary carnallite, which commonly replaced halite and sylvite, is scattered through large parts of the total chloride section in many cores.
