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Sr, S, and O (sub SO4) isotopes and the depositional environments of the upper Miocene evaporites, Spain

Feng H. Lu and William J. Meyers
Sr, S, and O (sub SO4) isotopes and the depositional environments of the upper Miocene evaporites, Spain
Journal of Sedimentary Research (May 2003) 73 (3): 444-450

Abstract

Upper Miocene evaporites in the Nijar Basin, southeastern Spain, contain thick, massive and coarse-crystalline selenites of the middle Messinian Yesares Formation. On the basis of their sedimentary fabrics and structures, such as euhedral terminations of crystals, well preserved algal filament inclusions, and standing-growth orientations, these selenites are interpreted to be primary, bottom-growth deposits. Most of these selenites have Sr concentrations of about 1000 ppm (range from 700 to 1800 ppm). (super 87) Sr/ (super 86) Sr values of Yesares selenites vary between 0.70890 and 0.70896 (n = 18), which are all within the known range of Messinian seawater, suggesting a marine origin. However, quantitative modeling of (super 87) Sr/ (super 86) Sr vs. Sr/Ca of fluids demonstrates that a minor amount of admixed nonmarine fluid (e.g., 20%) might have contributed to the Yesares marine brines. delta (super 34) S and delta (super 18) O (sub SO4) values of Yesares selenites range from 21 to 24% CDT and from 10 to 16% SMOW, respectively. These ranges are more variable and most values are heavier than those expected for normal Miocene marine gypsum. On the basis of quantitative modeling, these heavy delta (super 34) S and delta (super 18) O (sub SO4) values have most likely resulted from reduction and oxidation reactions of sulfur species that occurred in brines rather than from contributions of nonmarine water or from reservoir (crystallization) effects. The use of Sr, S, and O isotopes provides a possible means of distinguishing whether an ancient evaporite was deposited by marine, nonmarine, or hybrid brines. Sr isotopes, however, are insensitive to minor nonmarine contributions on the basis of quantitative modeling, and consequently marine Sr isotopes for evaporites (e.g., gypsum) do not necessarily indicate pure marine depositional environments. Compared to Sr isotopes, delta (super 34) S and delta (super 18) O (sub SO4) of marine gypsum are even more insensitive to minor nonmarine contributions because of much higher SO4 concentrations in seawater and seawater-evaporated brines than in most freshwaters. Therefore, Sr, O, and S isotopes should be used with caution to interpret depositional environments and to determine parent fluids of ancient evaporites.


ISSN: 1527-1404
EISSN: 1938-3681
Serial Title: Journal of Sedimentary Research
Serial Volume: 73
Serial Issue: 3
Title: Sr, S, and O (sub SO4) isotopes and the depositional environments of the upper Miocene evaporites, Spain
Affiliation: Chinese Academy of Sciences, Institute of Geology and Geophysics, Beijing, China
Pages: 444-450
Published: 200305
Text Language: English
Publisher: Society of Economic Paleontologists and Mineralogists, Tulsa, OK, United States
References: 43
Accession Number: 2003-047922
Categories: Isotope geochemistrySedimentary petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table, sketch map
N36°40'00" - N37°57'00", W03°10'00" - W01°34'60"
Secondary Affiliation: State University of New York at Stony Brook, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute. Reference includes data supplied by SEPM (Society for Sedimentary Geology), Tulsa, OK, United States
Update Code: 200315
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