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 Mg in seawater, δ26Mgseawater, has recently been shown to be an effective tool for reconstructing the budget modern and past oceans. However, estimations isotope fractionation between their solution required full quantification effect evaporitic on as well utilizing ancient sequences archive δ26Mgseawater. Here, we estimate marine-derived brine along course seawater evaporation, up degree evaporation >200. sequence Mg-salts included epsomite (MgSO4·7H2O), kainite (KMgClSO4·3H2O), carnallite (KMgCl3·6H2O), kieserite (MgSO4·H2O) bischofite (MgCl2·6H2O). following values, either negative or positive, were calculated difference salt its brine, evolution δ26Mg throughout evaporation: Δcarnallite-brine = +1.1‰, Δepsomite-brine +0.59‰, Δbischofite-brine +0.33‰, Δkieserite-brine −0.2‰ Δkainite-brine −1.3‰. Magnesium compositions determined minerals different ages geological record corroborate these results. Due precipitation multi-mineral assemblages having values opposing signs, value changes slightly (<0.5‰) path, despite considerable removal (>50%). fractionations correlate with number water molecules coordinated Mg2+ Mg-O bond length mineral lattice. Given fractionations, it is that volume 0.4 · 106–0.8 106 Km3 mono-mineral assemblage needs precipitate order change by 0.1‰. This huge far larger than known date global record. Therefore, concluded impact formation δ26Mgseawater insignificant since Proterozoic. results this study suggest preserved basins may used to: 1) quantify geochemical processes fractionate Mg-isotopes within basins, such dolomitization; 2) complete secular variations curve marine using well-established sequences.