The Mg isotope signature of marine Mg-evaporites

Abstract

Marine Mg-evaporites are a small oceanic sink of magnesium, precipitating only from extremely evaporated brines. The isotopic composition of Mg in seawater, d 26 Mg 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 d 26 Mg seawater , as well as for utilizing ancient evaporitic sequences as an archive for past d 26 Mg 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 (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 d 26 Mg in the brine throughout the evaporation: D carnallite-brine = +1.1‰, D epsomite-brine = +0.59‰, D bischofite-brine = +0.33‰, D kieserite-brine = À0.2‰ and D kainitebrine = À1.3‰. 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 d 26 Mg 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 Mg 2+ and with Mg-O bond length in the mineral lattice.Given these isotope fractionations, it is calculated that a volume of 0.4 Á 10 6 -0.8 Á 10 6 Km 3 of a mono-mineral assemblage of kainite or carnallite needs to precipitate in order to change seawater d 26 Mg 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 d 26 Mg 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 d 26 Mg record using basins with well-established evaporitic sequences.

Sources of Mg Enrichments in Vent Fluids From the Kermadec Arc Recorded by Li, B, and Mg Isotopes

“…Recent studies on the Mg isotope fractionation during precipitation of clay minerals and salts have shown that the magnitude and direction of isotope fractionation depends on the Mg‐O bond length and the amount of water molecules that have to be dehydrated during the formation of the minerals, and that the magnitude of isotope fractionation, consistent with equilibrium fractionation, decreases with increasing temperatures (Hindshaw et al., 2020; Li et al., 2014; Shalev et al., 2021). In addition, pH values may also have an influence on the magnitude of Mg isotope fractionation, which has been explained by differences in octahedral bond length (Shalev et al., 2021). To study the isotopic fractionation during the precipitation of caminite and thus calculate potential caminite compositions in the volcanic basement at Lower Cone, we have chosen an experimental setup that is similar to natural conditions.…”

Dynamic Climate Influence on Magnesium Isotope Variation in Saline Lacustrine Dolomite: A Case Study of the Qianjiang Formation, Jianghan Basin

“…However, authigenic clay minerals usually form in saline and alkaline lake systems. In saline and alkaline lakes, authigenic minerals, such as Mg-rich clay minerals and Mg-rich evaporites, will compete for Mg 2+ with dolomite [79,80]. In non-alkaline saline lakes, authigenic Mg minerals like Mg-rich clay and Mg-rich evaporites either precipitate to a minimal extent or do not deposit [21].…”

Enrichment Mechanism and Prospects of Deep Oil and Gas

“…and non-traditional metal isotopes (Ca, Mg, Mo, etc. ), provide novel approach in determining the effects of BSR/TSR (e.g., Cai et al, 2017), thermal histories (e.g., Mangenot et al, 2018;Cong et al, 2021), organic matter enrichment mechanisms (e.g., Zhu et al, 2022), diagenetic fluid properties (e.g., Shalev et al, 2021;Bryant et al, 2022).…”