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| Titel |
Magnesium isotopic fractionation between Mg salts and brine in the course of evaporation of marine derived brines |
| VerfasserIn |
Netta Shalev, Boaz Lazar, Ludwik Halicz, Gavrieli Ittai |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250090758
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| Publikation (Nr.) |
 EGU/EGU2014-5013.pdf |
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| Zusammenfassung |
| The Mg isotopic compositions (δ26Mg) of seawater-derived-brines and Mg-salts that
precipitate from such brines during the course of evaporation were measured in laboratory
experiments, in Mg-salts from the geological record and in the Dead Sea brine system. Mg
evaporites are one of the sink fluxes in the global Mg geochemical cycle and play an
important role in the evolution of hypersaline water bodies during the Phanerozoic including
the modern Dead Sea and its predecessors, from the Pliocene Sedom Lagoon to the Late
Pleistocene brine lakes1,2.
The advanced evaporative evolution of marine derived brines includes precipitation of a
series of Mg minerals3: epsomite (MgSO4-
7H2O), hexahydrite (MgSO4-
6H2O), kieserite
(MgSO4-
H2O), kainite (MgSO4KCl-
3H2O), carnallite (KMgCl3-
6H2O), bischofite
(MgCl2-
6H2O) and in some cases polyhalite (K2MgCa2(SO4)4-
2H2O). To the best of our
knowledge, just two Mg isotopic fractionation factors between Mg salts and brines
(Δ26Mgsalt-brine) were determined up to date: the equilibrium fractionation between
epsomite and MgSO4 solution was found to be about 0.6o4 and the fractionation between
carnallite and Dead Sea brine was found to be 0.6o,5.
Here we provide Mg isotope fractionation factors based on δ26Mg measurement in brines
and precipitating Mg-salts during the evaporation path of seawater. The sequence of Mg salts
precipitated in our evaporation experiments was as follows: Mg-sulfate salts started
to precipitate at Li scale degree of evaporation (DELi) of >50. The next salts to
precipitate were Mg-K-sulfate salts at DELi -90, followed by Mg-K-chloride salts at
DELi >150 and by Mg-chloride salt at DELi=195 (the end of the experiment). Our
isotopic measurements show that Mg isotopes fractionate significantly and in different
directions depending on the Mg mineral phase. The Δ26Mgsalt-brine for carnallite
was greater than 0.6oand the Δ26Mgsalt-brine for kainite was about -1.2o.
These results were corroborated by the δ26Mg values measured on Mg minerals
from the geological record on which we measured a value of 1.72ofor Permian
carnallite from Klodawa, Poland and -2.02ofor Messinian kainite from Sicily,
Italy.
The opposite fractionations during the precipitation of different Mg mineral
phases in the course of evaporation of seawater reveal a rather complex evolution
of the δ26Mg value in marine derived brines. The present day seawater δ26Mg is
-0.83oand it increased to -0.6owhen brine evaporation reached the kianite facies and
decreased down to -1.0owhen brine evaporation reached the bischofite facies.
These new data may provide the evaporite signal for models reconstructing the
evolution of the Dead Sea and the secular variations in the marine δ26Mg record in
general.
References:
Mackenzie, F. and Andersson, A., 2013. Geochemical Perspectives 2, 1–227.
Katz, A. and Starinsky, A., 2009. Aquatic Geochemistry, 15, 159–194.
Eugster, H.P. et al., 1980. Geochimica et Cosmochimica Acta,44, 1335-1347.
Li, W. et al., 2011. Geochimica et Cosmochimica Acta, 75, 1814–1828.
Gavrieli, I. et al., 2009. Geochimica et Cosmochimica Acta Supplement, 73,
A419. |
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