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The Mg Isotope Signature of Marine Mg-Evaporites Research Collection Journal Article The Mg isotope signature of marine Mg-evaporites Author(s): Shalev, Netta; Lazar, Boaz; Halicz, Ludwik; Gavrieli, Ittai Publication Date: 2021-05-15 Permanent Link: https://doi.org/10.3929/ethz-b-000473596 Originally published in: Geochimica et Cosmochimica Acta 301, http://doi.org/10.1016/j.gca.2021.02.032 Rights / License: Creative Commons Attribution 4.0 International This page was generated automatically upon download from the ETH Zurich Research Collection. For more information please consult the Terms of use. ETH Library Available online at www.sciencedirect.com ScienceDirect Geochimica et Cosmochimica Acta 301 (2021) 30–47 www.elsevier.com/locate/gca The Mg isotope signature of marine Mg-evaporites Netta Shalev a,b,c,⇑, Boaz Lazar a, Ludwik Halicz b,d, Ittai Gavrieli b a Institute of Earth Science, The Hebrew University of Jerusalem, Edmond J. Safra Campus, 91904 Jerusalem, Israel b Geological Survey of Israel, 32 Y. Leibowitz St., 9692100 Jerusalem, Israel c Institute of Geochemistry and Petrology, Department of Earth Sciences, ETH Zu¨rich, Clausiusstrasse 25, 8092 Zu¨rich, Switzerland d Faculty of Chemistry, Biological and Chemical Research Centre, University of Warsaw, Zwirki_ i Wigury 101, 02-089 Warsaw, Poland Received 18 March 2020; accepted in revised form 19 February 2021; Available online 27 February 2021 Abstract Marine Mg-evaporites are a small oceanic sink of magnesium, precipitating only from extremely evaporated brines. The 26 isotopic composition of Mg in seawater, d Mgseawater, has recently been shown to be an effective tool for reconstructing the Mg budget of the modern and past oceans. However, estimations of the Mg isotope fractionation between the Mg-evaporites 26 and their precipitating solution are required for full quantification of the isotope effect of the evaporitic sink on d Mgseawater, 26 as well as for utilizing ancient evaporitic sequences as an archive for past d Mgseawater. Here, we estimate the Mg isotope fractionation between Mg-evaporites and modern marine-derived brine along the course of seawater evaporation, up to degree evaporation of >200. The sequence of Mg-salts included epsomite (MgSO4Á7H2O), kainite (KMgClSO4Á3H2O), carnal- lite (KMgCl3Á6H2O), kieserite (MgSO4ÁH2O) and bischofite (MgCl2Á6H2O). The following isotope fractionation values, either negative or positive, were calculated from the isotope difference between the salt and its precipitating brine, and from the evolution of d26Mg in the brine throughout the evaporation: Dcarnallite-brine = +1.1‰, Depsomite-brine = +0.59‰, Dbischofite-brine = +0.33‰, Dkieserite-brine = À0.2‰ and Dkainite- brine = À1.3‰. Magnesium isotopic compositions determined on minerals from different ages in the geological record corrob- orate well these results. Due to precipitation of multi-mineral assemblages having isotope fractionation values of opposing signs, the d26Mg value of the brine changes only slightly (<0.5‰) throughout the evaporation path, despite the considerable Mg removal (>50%). The isotope fractionations are shown to correlate with the number of water molecules coordinated to the Mg2+ and with Mg-O bond length in the mineral lattice. Given these isotope fractionations, it is calculated that a volume of 0.4 Á 106–0.8 Á 106 Km3 of a mono-mineral assemblage of kainite or carnallite needs to precipitate in order to change seawater d26Mg by only 0.1‰. This huge volume is by far larger than the volume of these minerals known to date in the global geological record. Therefore, it is concluded that the impact of 26 Mg-evaporites formation on d Mgseawater has been insignificant since the Proterozoic. The results of this study suggest that the Mg isotopic composition of Mg-evaporites preserved in the geological record of evaporitic basins may be used to: 1) quan- tify geochemical processes that fractionate Mg-isotopes within these basins, such as dolomitization; and 2) complete the sec- ular variations curve of the marine d26Mg record using basins with well-established evaporitic sequences. Ó 2021 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/ licenses/by/4.0/). Keywords: Magnesium isotopes; Magnesium salts; Marine evaporites; Seawater evaporation; Isotope fractionation; d26Mg; Chemical evolution of seawater Abbreviation: DE, Degree of evaporation ⇑ Corresponding author at: Institute of Geochemistry and Petrology, Department of Earth Sciences, ETH Zu¨rich, Clausiusstrasse 25, 8092 Zu¨rich, Switzerland. E-mail address: [email protected] (N. Shalev). https://doi.org/10.1016/j.gca.2021.02.032 0016-7037/Ó 2021 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). N. Shalev et al. / Geochimica et Cosmochimica Acta 301 (2021) 30–47 31 1. INTRODUCTION Table 1 The chemical formulas of some Mg-salts minerals. Magnesium is the third most abundant cation in the Mineral Symbol Chemical composition ocean and, due to its long residence time (13 Myr; Bischofite Bi MgCl ∙6H O Berner and Berner, 1996) relative to the mixing time of 2 2 Bloedite Bl Na2Mg(SO4)2∙4H2O the oceans, it is well-mixed. The concentration and isotopic Carnallite Car KMgCl3∙6H2O composition of Mg in seawater are determined by the ocea- Epsomite Ep MgSO4∙7H2O nic Mg budget, which is controlled by Mg supply from riv- Hexahydrite Hx MgSO4∙6H2O ers, and Mg removal, mainly by precipitation of carbonate Kainite Kai KMgClSO4∙3H2O ∙ minerals and hydrothermal reactions with the volcanic Kieserite Ki MgSO4 H2O ∙ oceanic crust and, to a lesser extent, by reverse weathering Leonite Le K2Mg(SO4)2 4H20 Á and precipitation of Mg-evaporites (e.g., Elderfield and Polyhalite Poly K2Ca2Mg(SO4)4 2H2O Schultz, 1996; Holland, 2005; Arvidson et al., 2006). Thus, Sylvite Syl KCl understanding and quantifying the Mg budget of the mod- ern and ancient oceans are important to our understanding of how fundamental Earth processes, such as weathering, the degree of enclosure of the basin, the precipitating volcanism and sedimentation, have changed globally brine’s temperature, whether or not continuous reaction throughout the geological past, and how these processes with the already precipitated salts is maintained, and if are linked to Earth’s carbon cycle and long-term climate additional reactions within the evaporitic basin take place change (e.g., Holland, 2005; Elderfield, 2010). The isotopic (e.g., Eugster et al., 1980; Harvie et al., 1980; McCaffrey 26 et al., 1987; Shalev et al., 2018b). For example, when the composition of dissolved Mg in seawater, d Mgseawater, has recently been shown to be a reliable proxy for the recon- precipitating salts are continuously separated from the structions of the Mg budget of the modern and past oceans brine (i.e., fractional precipitation) during the course of ° (e.g., Tipper et al., 2006; Pogge Von Strandmann et al., modern seawater evaporation at 25 C, the precipitating 2014; Higgins and Schrag, 2015; Li et al., 2015; minerals include epsomite, kainite, carnallite, kieserite and Gothmann et al., 2017; Shalev et al., 2019; Xia et al., bischofite (e.g., Eugster et al., 1980; Shalev et al., 2018b; 2020). However, a full understanding of the isotopic com- Table 1). But, when the evolving solution is allowed to react positions of the oceanic Mg inputs and outputs and reliable with previously precipitated salts over the course of the 26 evaporation, the precipitating minerals include polyhalite, record of d Mgseawater in the past are needed in order to reconstruct the oceanic Mg-isotope budget. The scarcity epsomite, hexahydrite, carnallite, kieserite and bischofite of Mg isotope data from Mg-evaporites has thus far pre- (e.g., Eugster et al., 1980; Table 1). vented estimations of their potential effect, as a Mg-sink, To enable both the isotope characterization of the evap- d26 oritic Mg sink and the use of ancient Mg-evaporites as on the Mgseawater value, as well as their use as an archive 26 26 archives of past d Mgseawater, it is required to determine for past d Mgseawater. Magnesium-evaporites precipitate from extremely evap- the Mg isotope fractionation between the Mg-evaporites orated seawater, conditions that are typically reached only and their precipitating solution. Li et al. (2011) experimen- ‰ in fully or nearly enclosed basins. Such evaporites are found tally determined that epsomite (Table 1) is ca. +0.6 in large volumes in giant evaporitic basins (e.g., the Per- ‘heavier’ than its precipitating artificial Mg-SO4 solution. mian Zechstein basins in northern Europe or the Messinian However, as detailed above, epsomite is only one mineral basins around the Mediterranean; e.g., Warren, 2010), out of the five Mg-minerals that precipitate along the inferring that the evaporitic output flux of Mg from the course of evaporation of modern seawater (fractional path; ocean was not constant through time, and was higher in e.g., Eugster et al., 1980; Shalev et al., 2018b). Using quan- periods during which these giant evaporitic basins existed tum chemical density functional theory, Feng et al. (2018) (e.g., Arvidson et al., 2006). Magnesium-potassium evapor- calculated the equilibrium isotope fractionation between ite minerals in the geological record are important archives langbeinite, K2Mg2(SO4)3, and its precipitating solution, ‰ ° d26 for ancient brines and can be used to estimate past seawater to be +0.4 at 25 C. Based on this value and the Mg ‰ compositions and climate (e.g., Holland et al., 1986; of three Permian langbeinite samples (-3.9 ) they sug- d26 Hardie, 1991; Horita et al., 2002; Warren, 2010). As chem- gested that the Mg value of the Permian parent brine 26 À ‰ ical deposits, these evaporites are direct recorders of the was extremely Mg-depleted, ca. 4 . However, the chemistry of ancient marine-derived brines (e.g., Babel mechanism of Mg isotope fractionation during mineral pre- and Schreiber, 2014), whereby variations in ocean chem- cipitation is still enigmatic.
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