The Mg isotope signature of marine Mg-evaporites

Marine Mg-evaporites are a small oceanic sink of magnesium, precipitating only from extremely evaporated brines. The isotopic composition of Mg in seawater, delta Mg-26(seawater), 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 and their precipitating solution are required for full quantification of the isotope effect of the evaporitic sink on delta Mg-26(seawater), as well as for utilizing ancient evaporitic sequences as an archive for past delta Mg-26(seawater). 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 (MgSO4 center dot 7H(2)O), kainite (KMgClSO4 center dot 3H(2)O), carnallite (KMgCl3 center dot 6H(2)O), kieserite (MgSO4 center dot H2O) and bischofite (MgCl2 center dot 6H(2)O). 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 delta Mg-26 in the brine throughout the evaporation: Delta(carnallite-brine) = +1.1 parts per thousand, Delta(epsomite-brine) = +0.59 parts per thousand, Delta(bischofite-brine) = +0.33 parts per thousand, Delta(kieserite-brine) =-0.2 parts per thousand and Delta(kainite-brine) =-1.3 parts per thousand. Magnesium isotopic compositions determined on minerals from different ages in the geological record corroborate well these results. Due to precipitation of multi-mineral assemblages having isotope fractionation values of opposing signs, the delta Mg-26 value of the brine changes only slightly (<0.5 parts per thousand) 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 . 10(6)-0.8 . 10(6) Km(3) of a mono-mineral assemblage of kainite or carnallite needs to precipitate in order to change seawater delta Mg-26 by only 0.1 parts per thousand. 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 Mg-evaporites formation on delta Mg-26(seawater) 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) quantify geochemical processes that fractionate Mg-isotopes within these basins, such as dolomitization; and 2) complete the secular variations curve of the marine delta Mg-26 record using basins with well-established evaporitic sequences. (C) 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/).

Automated summary

The study estimates the magnesium isotope fractionation between magnesium evaporites and modern marine-derived brine, showing that the impact of evaporite formation on seawater magnesium isotope has been insignificant since the Proterozoic. The results suggest that the magnesium isotopic composition of evaporites preserved in the geological record may be used to understand geochemical processes within evaporitic basins and complete the secular variations curve of the marine magnesium isotope record.