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

Magnesium isotopic compositions of evaporite deposits may record information concerning brine evolution during deposition. We report Mg isotopic values (delta Mg-26(DSM3)) 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 delta Mg-26 values are relatively low and fall within a relatively narrow range (-4.12 +/- 0.03 parts per thousand to -3.81 +/- 0.07 parts per thousand). 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 a between langbeinite and model brine solution we obtained are 1.0005, 1.0004, and 1.0003 (Delta Mg-26(langb-water)approximate to 10(3)ln alpha = 0.473 parts per thousand, 0.390 parts per thousand, and 0.322 parts per thousand) at 10 degrees C, 25 degrees C, and 40 degrees 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 Mg-26 and Mg-25 isotopes. Rayleigh distillation modeling of the Salado langbeinite's relatively light Mg isotopic composition requires delta Mg-26(DSM3) values of -4 parts per thousand for the parent brine. Models favor a precipitation temperature as high as 40 degrees 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.