Ancient Basins and Stratigraphic Evolution

Warren, John K.

doi:10.1007/978-3-319-13512-0_5

Cited by 1 publication

(1 citation statement)

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“…Uniform sulfur isotopic composition observed in langbeinite samples (10.05 6 0.27&, 10.11&, and 10.78 6 0.39& for mean d 34 S; Renne et al, 2001) support this mechanism. Rapid growth typically occurs in cold, arid environments where sublimation can occur or in relatively warm climates (Warren, 1999(Warren, , 2010). Langbeinite's relatively high precipitation temperature (Braitsch, 1964(Braitsch, , 1971 raises the possibility that the Salado langbeinite developed in a sabkha, salt pan, or salina, which often form in warm climates with high evaporation rates.…”

Section: Interpreting Mg Isotope Systematics From the Salado Langbeinitementioning

confidence: 99%

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

Feng

Gao

Yin

et al. 2018

Geochem Geophys Geosyst

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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.

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