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9.17 Geochemistry of Evaporites and Evolution of Seawater M Ba˛Bel, University of Warsaw, Warszawa, Poland BC Schreiber, University of Washington, Seattle, WA, USA

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9.17 Geochemistry of Evaporites and Evolution of Seawater M Ba˛Bel, University of Warsaw, Warszawa, Poland BC Schreiber, University of Washington, Seattle, WA, USA 9.17 Geochemistry of Evaporites and Evolution of Seawater M Ba˛bel, University of Warsaw, Warszawa, Poland BC Schreiber, University of Washington, Seattle, WA, USA ã 2014 Elsevier Ltd. All rights reserved. 9.17.1 Introduction 484 9.17.2 Definition of Evaporites 484 9.17.3 Brines and Evaporites 484 9.17.4 Environment of Evaporite Deposition 485 9.17.4.1 Evaporation 486 9.17.4.2 Freezing 486 9.17.4.3 Brine (Evaporating Waters) 487 9.17.4.4 Salinity 487 9.17.4.5 Temperature 488 9.17.4.6 Heliothermal Effect 489 9.17.4.7 pH 489 9.17.5 Seawater as a Salt Source for Evaporites 489 9.17.6 Evaporite and Saline Minerals 490 9.17.7 Model of Marginal Marine Evaporite Basin 491 9.17.7.1 Conceptual Model of the Basin 492 9.17.7.2 Quantitative Model of the Basin 495 9.17.8 Mode of Evaporite Deposition 496 9.17.9 Primary and Secondary Evaporites 498 9.17.10 Evaporation of Seawater – Experimental Approach 499 9.17.11 Crystallization Sequence before K–Mg Salt Precipitation 499 9.17.11.1 Early Salinity Rise – Calcium Carbonate Precipitation 499 9.17.11.2 Gypsum Crystallization Field 501 9.17.11.3 Halite Crystallization Field 501 9.17.12 Crystallization Sequence of K–Mg Salts 503 9.17.12.1 Natural Crystallization 503 9.17.12.2 Theoretical Crystallization Paths 504 9.17.13 Isotopic Effects in Evaporating Seawater Brines and Evaporite Salts 505 9.17.14 Usiglio Sequence – A Summary 505 9.17.15 Principles and Record of Chemical Evolution of Evaporating Seawater 505 9.17.15.1 Principle of the Chemical Divide for Seawater 505 9.17.15.2 Ja¨necke Diagrams 507 9.17.15.3 Spencer Triangle 508 9.17.16 Evaporation of Seawater – Remarks on Theoretical Approaches 509 9.17.17 Sulfate Deficiency in Ancient K–Mg Evaporites 509 9.17.17.1 Sulfate Deficiency as the Secondary Feature 511 9.17.17.2 Sulfate Deficiency as a Record of Ancient Seawater Composition 513 9.17.18 Ancient Ocean Chemistry Interpreted from Evaporites 514 9.17.18.1 Implications from Evaporite Mineralogy and from Usiglio Sequence 514 9.17.18.2 Implications of Primary Evaporite Minerals (Excluding Implications from Fluid Inclusions) 516 9.17.19 Recognition of Ancient Marine Evaporites 516 9.17.19.1 Sedimentological Criteria 517 9.17.19.2 Mineralogical Criteria 517 9.17.19.3 Geochemical Criteria 517 9.17.20 Fluid Inclusions Reveal the Composition of Ancient Brines 518 9.17.20.1 Criteria for Seawater Recognition in Halite Fluid Inclusions 519 9.17.20.2 Reconstruction of Ancient Seawater Composition from Halite Fluid Inclusions 520 9.17.21 Ancient Ocean Chemistry from Halite Fluid Inclusions – Summary and Comments 523 9.17.22 Salinity of Ancient Oceans 530 9.17.23 Evaporite Deposition through Time 531 9.17.23.1 Late Ediacaran–Phanerozoic Marine Evaporites 532 9.17.23.2 Precambrian (Pre-Ediacaran) Marine Evaporites 536 9.17.23.3 Nonmarine Evaporites in Precambrian 544 9.17.23.4 Pseudomorphs after Evaporite Minerals in Precambrian 544 Treatise on Geochemistry 2nd Edition http://dx.doi.org/10.1016/B978-0-08-095975-7.00718-X 483 484 Geochemistry of Evaporites and Evolution of Seawater 9.17.24 Significance of Evaporites in the Earth History 546 9.17.24.1 Paleogeographic Indicators 546 9.17.24.2 Seals for Hydrocarbons and More (Evaporites and Hydrocarbons) 546 9.17.24.3 Halotectonics 547 9.17.24.4 Diagenesis and Metamorphism of Evaporites 547 9.17.25 Summary 547 Acknowledgments 548 References 548 9.17.1 Introduction Braitsch, 1971; Braitsch and Garrett, 1981). Some authors have suggested other names for salt rocks precipitated by mechanisms This chapter focuses almost exclusively on marine evaporites other than evaporation (e.g., Berkey, 1922; Debenedetti, 1976; and in particular on how the chemistry of seawater is reflected Warren, 1996; Wood et al., 2005); however, these names (‘reac- in the mineralogy and facies distribution of deposits in tionites,’ ‘precipitates,’ ‘thermalites,’ ‘replacementites’, etc.,) are geologic space and time. First, the deposits formed from evap- only rarely used in the geologic literature, or are not generally oration of modern seawater are characterized together with accepted. Nowadays, the term evaporites appears to be most their distinctive crystallization paths, and then, we show how commonly used in the very broad sense (cf. Twenhofel, 1950). the mineralogy and geochemistry of evaporites have been used Nevertheless, those evaporites strongly affected by diagenesis for the interpretation of the chemical evolution of the ocean with the primary features obliterated, as those occurring in salt through time. In order to fill in the background of the main diapirs, are more commonly described as salt deposits (e.g., salt theme, we attempt to supply more detailed and up-to-date diapirs, not evaporite diapirs; cf. Hudec and Jackson, 2007). The information on the geochemistry of evaporite environments name ‘salt deposits’ also can be applied to halite or calcium and evaporite deposits important or relevant to the problem of sulfate deposits precipitated from seawater in the zones of hydro- their current geochemical studies. thermal circulation in spreading zones of the oceanic crust (Berndt and Seyfried, 1997; Hansen and Wallmann, 2003; Hov- land et al., 2006; Petersen et al., 2000; Talbot, 2008). 9.17.2 Definition of Evaporites The Latin word ‘evaporo’ means ‘to change into a vapor,’ and it 9.17.3 Brines and Evaporites is used to designate the type of rocks and salts that originate during evaporation of natural solutes on the Earth’s surface. In The common feature of all evaporites is that they are composed the nineteenth and the beginning of the twentieth century, these of salts easily soluble in water (Goldschmidt, 1937). Such deposits were simply termed ‘salt deposits’ and also, rarely, as soluble salts accumulate in natural water reservoirs and in evaporates (Goldschmidt, 1937; Grabau, 1920, p. 23). Although ocean waters in particular and are removed from these aqueous both terms, together with the term saline deposits, are in use solutions in significant quantity, only by evaporation of the today, the term evaporites (with modified spelling) introduced water. The essential feature of evaporites is that they precipitate by Berkey (1922) became the most popular and it is widely from concentrated watery solutions or brines (Sonnenfeld, accepted now. 1984, p. 1). Other inorganic chemical deposits usually contain Evaporites are difficult to define precisely. The broad defini- minerals that are only slightly soluble in water. These minerals tion was suggested by Twenhofel (1950, p. 486) who understood do not form as a result of evaporation of concentrated solu- the evaporites as a “group of sedimentary deposits whose origin is tions. The chemical behavior of such substances is commonly largely due to evaporation.” More exactly, he stated that “most relatively easy to predict and to study from solubility products evaporites result from evaporation of water of high concentra- and Eh–pH relations (Berner, 1971; Krauskopf, 1967). tion, but a few are formed by replacement, or freezing of concen- By contrast, the solubility of salts and their activity coeffi- trated waters” and added that “if subjected to heat and pressure, cients in brines vary widely and are not readily predictable as they the evaporites form new combinations” (Twenhofel, 1950, are dependent on concentrations of other ions, among other p. 487). He also included the deposits that “develop through factors (Karcz and Zak, 1987). In concentrated solutions, “the metamorphism of other evaporites” into this group (Twenhofel, water structure was shown to be completely destroyed,” and due 1950, p. 486). Evaporites are similarly defined in the current to ‘water deficiency,’ “the effects of ionic association and compe- edition of the Glossary of Geology but include “rocks with saline tition between oppositely charged ions for water molecules in minerals formed by other mechanisms, e.g., mixing of waters or their hydration shells are intensified” (Figure 1; Krumgalz, 1980, temperature change” (Neuendorf et al., 2005, p. 221). Evaporite p. 73; Kostenko, 1982). “Formation of ion pairs and triplets grains “reworked by wind or saline waters as clastic particles” are apparently is so extensive in highly saline sulfate and carbonate also considered as evaporites (Neuendorf et al., 2005, p. 221). brines that the true ionic strength may be less than half the value The latter evaporites are termed allochthonous by Hardie (1984). calculated from total molalities” (Berner, 1971, p. 48). The aver- Some authors restrict the term ‘evaporites’ for sediments age ionic strength of standard seawater is about 0.7, but salt formed exclusively by evaporation, and they use the name saline solutions with ionic strength greater than 1 may require more deposits or salt deposits for deposits formed not only by sophisticated models than those applied for seawater (Berner, evaporation but also by cooling and salting out (compare 1971). Evaporating seawater brines attain ionic strengths nearly Geochemistry of Evaporites and Evolution of Seawater 485 7 All ions in the solution Start of halite + 6 Na precipitation 2+ ) Mg 23 Cl− Start of gypsum Start of 5 2− 10 precipitation SO4 epsomite ϫ precipitation n 4 3 2 Number of ions ( 1 0 1 1.05 1.1 1.15 1.2 1.25 1.3 1.35 1.4 Density (g cm-3) 100 90 All H2O molecules Number of H2O molecules per one ion ) 80 23 10 70 ϫ n 60 Start of Start of Start of gypsum 50 halite epsomite precipitation precipitation precipitation 40 30 Number of ions ( 20 10 0 1 1.05 1.1 1.15 1.2 1.25 1.3 1.35 1.4 Density (g cm-3) Figure 1 Numbers of molecules in the evaporating Black Sea water, after data by Il’insky (1948, cited by Kostenko, 1982), recalculated by Kostenko (1982).
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