Walter E. Dean, 1987. "Section 5 Trace and Minor Elements in Evaporites", Marine Evaporites, Walter E. Dean, B. Charlotte Schreiber, Walter E. Dean, Gerald M. Friedman, Robert J. Hite, Roy D. Nurmi, Omer B. Raup, B. Charlotte Schreiber, Douglas J. Shearman
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Evaporite minerals can be some of the purest chemical compounds, in terms of lack of trace contaminants, manufactured by any geologic process. For example, some natural gypsum deposits contain lower trace-element concentrations than reagent grade calcium sulfate. There are relatively few published analyses of evaporite minerals, and most of these are whole rock analyses rather than analyses of single mineral phases. Fortunately, most evaporite rocks contain one dominant mineral so that comparison of analyses of a particular evaporite lithology (e.g. halite rocks from several localities) is usually more meaningful than a comparison of analyses of other sedimentary rock types that have more complex and variable mineral compositions (e.g. sandstone from several localities). Most published chemical analyses of evaporites consist of only one analysis of a particular lithology from a given formation or subunit within a formation; there are few systematic studies available where the investigators have tried to determine the compositional variability of a particular evaporite lithology within one formation. The study of bromine geochemistry is an outstanding example of the application of systematic chemical analyses applied to evaporite units in order to determine compositional variability and to try and solve geologic problems involving correlation, paleosalinity, and diagenesis (see discussion by Raup and Hite later in these notes).
In this section, we will examine the ways in which trace and minor elements are incorporated into evaporite minerals, summarize some published and unpublished chemical analyses of the more common evaporite minerals (gypsum, anhydrite, and halite), and discuss several other examples in
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Evaporites were classified on the basis of their environmental relationships, particularly with respect to the under- and over-lying sedimentary sequences. The scope of knowledge that went into establishing this classification was limited to deposits developed in cratonic (continental crust) areas of the world. The advent of the concept of sea-floor spreading, together with new data collected by the Deep Sea Drilling Project and extensive submarine seismic surveys, both on the continental margins and in the deep-sea, enables us to classify evaporitic sediments on the basis of tectonic settings as well as sediment affinities. The various divisions are in a sense artificial; the one classification readily overlaps with the other, and each of the groupings may grade through time and space.