Potash evaporites should house the most legible record of ancient water composition available in the geologic column because such deposits represent the end products of evaporation paths sensitive to small variations in initial major ion ratios. The means of accurately predicting these paths are now available. To reconstruct brine compositions we need to identify the most complete sequence of minerals in the deposit and to match it with an appropriate model path (this will give the basic brine type) and to identify the quantitative mineral abundances in individual sedimentation units within the deposit (these are directly related to the major ion ratios in the parent brine and allow evaluation of brine composition within close limits).

Undeniably, there are formidable practical difficulties because evaporites are readily altered on burial. If the primary mineral assemblage can be deciphered petrographically, then the procedure is straightforward. The Permian Zechstein 2 of Stassfurt, Germany, illustrates the approach. The Stassfurt potash sequence follows the equilibrium evaporation path predicted for modern seawater at 25°C. This path is very sensitive to initial brine composition; changes in Mg/K₂, Mg/SO₄, and Cl/SO₄ greater than 10 mole % will produce different paths. So, if the Stassfurt deposit is marine, then Permian seawater composition must have been similar to modern seawater. Just how similar is shown by the following brine composition reconstructed from the quantitati e mineral abundances in meter scale sections of the Stassfurt (mole ratios, modern seawater in parentheses): Na/Ca >= 49.8 (52.3), Na/K<=53.4 (47.6), Na/Cl<=0.95 (0.86), Na/SO₄<=17.3 (16.7). This implies that differences in mineralogy or elemental geochemistry (e.g., S isotopes) among Permian and post-Permian evaporites cannot be due to changes in seawater composition.

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