Abstract

The Searles Lake evaporites (late Quaternary) consist of saline and mud layers permeated by brines. Two mineral pairs (gaylussite-pirssonite, mirabilite-thenardite) are used as indicators of relative aH2o throughout the stratigraphic column. Variations are attributed mostly to changes in brine salinity and partly to temperature.

These aH2o-sensitive minerals locally coexist with trona, nahcolite, burkeite, northupite, tychite, hanksite, and aphthitalite, which are sensitive to both aH2o and aco. Such assemblages permit construction of schematic isothermal aH2o-aco, diagrams. Field boundary sequences are derived from both theoretical considerations and observed assemblages; their slopes are determined by the stoichiometry of possible reactions. Predicted assemblages invariably agree with observed assemblages.

By means of these diagrams, present-day lateral and stratigraphic variations in relative aH2o and aco2 in the deposit are reconstructed. They show that aH2o and aco2 vary independently. Many of the present activities reflect depositional conditions; some indicate post-depositional events.

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