Significant bromine (Br) isotope composition variations are found in natural salts and brines, which are often even larger than those of co-existing chlorine isotope compositions. The cause of such large Br isotope variations remains elusive. In this study, equilibrium Br isotope fractionations among Br-bearing gaseous molecules, aqueous species and crystalline minerals are provided via the density functional theory (DFT) based first-principles calculations. These fractionation factors form the base for future Br isotope geochemistry studies. Specifically, the first-principles molecular dynamics (FPMD) method is used to evaluate the tricky solvation effects, i.e., dozens of snapshots of FPMD trajectories are selected and re-optimized to their lowest energy atomic coordinates, then the reduced partition function ratios (i.e., β factors) of aqueous Br-bearing species are obtained by computing the average values of those snapshots. In this way, the configurational effects of aqueous Br-bearing species are included. For crystalline compounds, the static first-principles periodic DFT methods are used. We find that the β factors generally increase as the oxidation states of Br increase, indicating that heavy Br isotopes tend to be enriched in substances with higher Br oxidation states. The aqueous Br-bearing species in brines have detectable but small Br isotope fractionations with each other. However, the fractionations between gaseous molecules and NaBr(aq) are significantly larger, e.g., Δ81BrCH3Br(gas)-NaBr(aq) is 0.95？±？0.041‰ at 20 °C. As for minerals, the fractionation between Br-halite(p) (pure NaBr crystal) and NaBr(aq) is close to zero, but for Br-halite(1/124) (Na125Cl124Br1) and NaBr(aq), the fractionation is 0.206？±？0.041‰. Similar results are found for the fractionations between Br-sylvite, Br-bischofite and Br-bearing solutions, indicating that Br isotope compositions are closely related to Br/Cl ratios in minerals.
By using the calculated isotope fractionation factors, we have modeled the Br isotope composition variations during salt precipitation processes. At 20 °C, the total variations of Br isotopes of salts and brines are found to be ？0.45‰ to +0.21‰ and ？0.39‰ to 0‰, respectively. They are much smaller than the observed Br isotope variations in natural samples. There must be other ways to fractionate Br isotopes to the extent that observed in nature. Based on the predicted equilibrium Br isotope fractionations among substances with different Br oxidation states, the large Br isotope variations in nature can be explained by the change of Br oxidation state. The sudden change of Br isotope compositions hence can be used as a new indicator for the change of redox conditions.
| GEOCHIMICA ET COSMOCHIMICA ACTA Volume: 297 Pages: 65-81 DOI: 10.1016/j.gca.2021.01.010 Published: MAR 15 2021 |
| Gao, Caihong; Liu, Yun |
-State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China
-International Center for Planetary Science, College of Earth Sciences, Chengdu University of Technology, Chengdu 610059, China
-CAS Center for Excellence in Comparative Planetology, Hefei 230026, China
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