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, δ²⁶Mg_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 δ²⁶Mg_seawater, as well as for utilizing ancient evaporitic sequences as an archive for past δ²⁶Mg_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 (MgSO₄·7H₂O), kainite (KMgClSO₄·3H₂O), carnallite (KMgCl₃·6H₂O), kieserite (MgSO₄·H₂O) and bischofite (MgCl₂·6H₂O). 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 δ²⁶Mg in the brine throughout the evaporation: Δ_{carnallite-brine} = +1.1‰, Δ_{epsomite-brine} = +0.59‰, Δ_{bischofite-brine} = +0.33‰, Δ_{kieserite-brine} = −0.2‰ and Δ_{kainite-brine} = −1.3‰. 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 δ²⁶Mg 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 · 10⁶–0.8 · 10⁶ Km³ of a mono-mineral assemblage of kainite or carnallite needs to precipitate in order to change seawater δ²⁶Mg 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 Mg-evaporites formation on δ²⁶Mg_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 δ²⁶Mg record using basins with well-established evaporitic sequences.