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Title: 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 (δ 26MgDSM3) measured from an evaporite deposit of langbeinite (K 2Mg 2(SO 4) 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 δ 26Mg 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 Mg 2+ 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 (Δ 26Mg langb-water≈10 3lnα = 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 preferentiallymore » incorporates the heavier 26Mg and 25Mg isotopes. Rayleigh distillation modeling of the Salado langbeinite's relatively light Mg isotopic composition requires δ 26Mg DSM3 values of -4‰ for the parent brine. Models favor a precipitation temperature as high as 40°C under equilibrium conditions. Finally, 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.« less

Authors:
 [1];  [2];  [3];  [4];  [5]; ORCiD logo [1]; ORCiD logo [1]
  1. Univ. of Hong Kong (China). Dept. of Earth Sciences
  2. Chinese Academy of Sciences (CAS), Guiyang (China). State Key Lab.of Ore Deposit Geochemistry, Inst. of Geochemistry
  3. Univ. of California, Davis, CA (United States). Dept. of Earth and Planetary Sciences
  4. Univ. of California, Davis, CA (United States). Dept. of Earth and Planetary Sciences; Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
  5. Berkeley Geochronology Center, Berkeley, CA (United States); Univ. of California, Berkeley, CA (United States). Dept. of Earth and Planetary Science
Publication Date:
Research Org.:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org.:
USDOE National Nuclear Security Administration (NNSA)
OSTI Identifier:
1466959
Report Number(s):
LLNL-JRNL-752506
Journal ID: ISSN 1525-2027; 938874
Grant/Contract Number:  
AC52-07NA27344
Resource Type:
Accepted Manuscript
Journal Name:
Geochemistry, Geophysics, Geosystems
Additional Journal Information:
Journal Volume: 19; Journal Issue: 8; Journal ID: ISSN 1525-2027
Publisher:
American Geophysical Union
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; parent brine; evaporite; langbeinite; Mg isotopes; fractionation factor

Citation Formats

Feng, Chongqin, Gao, Caihong, Yin, Qing-Zhu, Jacobsen, Benjamin, Renne, Paul R., Wang, Jun, and Chang, Su-Chin. Tracking Physicochemical Conditions of Evaporite Deposition by Stable Magnesium Isotopes: A Case Study of Late Permian Langbeinites. United States: N. p., 2018. Web. doi:10.1029/2017GC007361.
Feng, Chongqin, Gao, Caihong, Yin, Qing-Zhu, Jacobsen, Benjamin, Renne, Paul R., Wang, Jun, & Chang, Su-Chin. Tracking Physicochemical Conditions of Evaporite Deposition by Stable Magnesium Isotopes: A Case Study of Late Permian Langbeinites. United States. doi:10.1029/2017GC007361.
Feng, Chongqin, Gao, Caihong, Yin, Qing-Zhu, Jacobsen, Benjamin, Renne, Paul R., Wang, Jun, and Chang, Su-Chin. Fri . "Tracking Physicochemical Conditions of Evaporite Deposition by Stable Magnesium Isotopes: A Case Study of Late Permian Langbeinites". United States. doi:10.1029/2017GC007361. https://www.osti.gov/servlets/purl/1466959.
@article{osti_1466959,
title = {Tracking Physicochemical Conditions of Evaporite Deposition by Stable Magnesium Isotopes: A Case Study of Late Permian Langbeinites},
author = {Feng, Chongqin and Gao, Caihong and Yin, Qing-Zhu and Jacobsen, Benjamin and Renne, Paul R. and Wang, Jun and Chang, Su-Chin},
abstractNote = {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. Finally, 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.},
doi = {10.1029/2017GC007361},
journal = {Geochemistry, Geophysics, Geosystems},
number = 8,
volume = 19,
place = {United States},
year = {2018},
month = {5}
}

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