Tracking Physicochemical Conditions of Evaporite Deposition by Stable Magnesium Isotopes: A Case Study of Late Permian Langbeinites

Abstract

Magnesium isotopic compositions of evaporite deposits may record information concerning brine evolution during deposition. We report Mg isotopic values (δ26MgDSM3) measured from an evaporite deposit of langbeinite (K2Mg2(SO4)3) found in the Permian Salado Formation. We used these data to model Mg isotope fractionation between langbeinite and its parent brine. In addition, both measured and theoretical results are used to estimate precipitation temperature and interpret depositional environment. The Salado langbeinite δ26Mg values are relatively low and fall within a relatively narrow range (–4.12 ± 0.03‰ to −3.81 ± 0.07‰). Equilibrium fractionation factors between langbeinite and aqueous Mg2+ solutions were calculated using quantum chemical density functional theory. All computations were performed at the B3LYP/6‐31 + G(d,p) level. Solvation effects were addressed using a solvent model (“water‐droplet” approach) and mineral structures were investigated using volume variable cluster models (VVCM). The equilibrium Mg isotopic fractionation factors α between langbeinite and model brine solution we obtained are 1.0005, 1.0004, and 1.0003 (Δ26Mglangb‐water≈103lnα = 0.473‰, 0.390‰, and 0.322‰) at 10°C, 25°C, and 40°C, respectively. These relatively large equilibrium fractionation factors indicate significant Mg isotope fractionation between langbeinite and its parent brine during precipitation, as langbeinite preferentially incorporates the heavier 26Mg and 25Mg isotopes. Rayleigh distillation modeling of the Salado langbeinite's relatively light Mg isotopic composition requires δ26MgDSM3 values of −4‰ for the parent brine. Models favor a precipitation temperature as high as 40°C under equilibrium conditions. Potential disequilibrium precipitation conditions suggested by Mg isotopic data also imply rapid deposition in a hot, arid sedimentary environment prevailing in the southwestern U.S. during the Late Permian.

Machine Learning Can Assign Geologic Basin to Produced Water Samples Using Major Ion Geochemistry

“…Hydrocarbons produced from the Michigan Basin are generally sourced from the Antrim or the Utica Shale, while basinal brines are thought to be derived from the evaporation of Paleozoic seawater, dissolution of evaporites (which could easily influence Mg concentrations in a developing brine, e.g., Feng et al (2018)), and general water-rock interactions (Wilson & Long, 1993;Hanor & McIntosh, 2006;Swezey, 2002;McIntosh et al, 2011). The Green River basin produces hydrocarbons from mostly Cretaceous formations (Toner et al, 2018) and was thought to have developed under a playa-lake model, which may drive the Mg distinction observed here, as calcite precipitation during playa-lake formation drives Mg/Ca ratios in the developing brine (Eugster & Surdam, 1973).…”

The Mg isotope signature of marine Mg-evaporites

“…However, as detailed above, epsomite is only one mineral out of the five Mg-minerals that precipitate along the course of evaporation of modern seawater (fractional path; e.g., Eugster et al, 1980;Shalev et al, 2018b). Using quantum chemical density functional theory, Feng et al (2018) calculated the equilibrium isotope fractionation between langbeinite, K 2 Mg 2 (SO 4 ) 3 , and its precipitating solution, to be +0.4‰ at 25°C. Based on this value and the d 26 Mg of three Permian langbeinite samples (-3.9‰) they suggested that the d 26 Mg value of the Permian parent brine was extremely 26 Mg-depleted, ca.…”