Non-marine brine evolution

Nonmarine brines by definition do not draw on the near isochemical reservoir of an ocean. A more diverse, less predictable, suite of evaporite minerals precipitates during evaporation of such continental waters. In nonmarine settings, rivers and groundwaters are the source of most of the ions that are ultimately deposited as evaporite salts in a groundwater sump. In a closed hydrological system the composition of nonmarine brines depends on the lithologies that are leached in the drainage basin surrounding a salt lake (Eugster and Hardie, 1978). Flow through limestone aquifers in hydrologically-closed basins produces inflow watesrs rich in Ca and HCO3, dolomite dissolution generates Mg, igneous and metamorphic matrices yield silica-rich Ca–Na–HCO3 waters. Pyritic shales and other sulphide-rich sediments will contribute sulphate ions, whereas basic and ultrabasic rocks tend to produce alkaline Mg–HCO3 waters.

 

In all, Eugster and Hardie (1978) distinguished five major water types in what they termed “hydrologically closed” continental evaporite basins:

 1. Ca–Mg–Na–(K)–Cl

 2. Na–(Ca)–SO4–Cl

 3. Mg–Na–(Ca)–SO4–Cl

 4. Na–CO3–Cl

 5. Na–CO3–SO4–Cl waters

 

As any one of these waters concentrates within a particular evaporite sump, it deposits a characteristic suite of evaporite minerals. First precipitates are the alkaline earth carbonates: low-magnesian calcite, high-magnesian calcite, aragonite, and dolomite. The mineralogy of this initial precipitate depends on the Mg/Ca ratio of the parent brine. The subsequent evaporation pathway of a basin brine is determined by the proportions of calcium, magnesium, and bicarbonate ions in the brackish inflow waters. It sets up geochemical divides early in the evolution of a lake brine, these divides define the subsequent geochemical and mineralogical evolution of a lake basin.

 

If the lake waters are enriched in HCO3 compared to Mg and Ca (i.e. HCO3 >> Ca+Mg), then the brine follows path I . Ca and Mg are depleted during the initial precipitation of alkaline earth carbonates, leaving excess HCO3 in the brine. As HCO3 is the next most abundant ion in these waters it combines with Na in the next stage of concentration. Sodium carbonate minerals, such as trona, natron and nahcolite, precipitate. Little or no gypsum can form from pathway-I brines as Ca is completely used up during the preceding alkaline earth carbonate stage. In contrast, during the evaporation of modern seawater, all the HCO3 is depleted in the initial alkaline earth precipitates (mostly as aragonite and Mg-calcite). The excess Ca then combines with SO4 to form gypsum. It is chemically impossible for sodium carbonate to form by evaporation of modern seawater. The assumption that the proportions of calcium to bicarbonate in seawater have not changed much in the last 600-800 m.y. implies that trona salts or their pseudomorphs indicate nonmarine settings throughout the Phanerozoic and much of the Neoproterozoic.

 

If initial inflow waters have (Ca + Mg) >> HCO3 then, after the initial evaporitic carbonate precipitates, the brines become enriched in the alkaline earths but depleted in HCO3 and CO3. If the relative volume of HCO3 is low, little carbonate can precipitate with further concentration. Brine evolution follows path II, whereby excess alkaline earths (Ca, Mg) left after depletion of carbonate combine with sulphate ions to precipitate large volumes of sulphates (gypsum and/or epsomite). Path II precipitates a suite of continental salts and bitterns similar to that derived from modern seawater. If the ratio of (Ca+Mg)/HCO3 is near unity (path III) carbonate precipitation can be extensive and voluminous. As Ca is progressively removed there is a progressive increase in the Mg/Ca ratio of the residual brine until large volumes of high-Mg calcites, dolomites and even magnesites precipitate.

 

 

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