Redox conditions in Late Permian seawater based on trace metal ratios in fluid inclusions in halite from the Polish Zechstein Basin

Abstract

Previous studies have confirmed that primary fluid inclusions in halite record and preserve information concerning the major ion chemistry of seawater. Here, we determine ratios of redox sensitive trace metals (Fe, Mn, V, Mo, U) in paleo-seawater trapped as primary fluid inclusions in halite from two boreholes (Przyborów IG3, Gorzów Wielkopolski IG1) from the Polish sector of the European Southern Permian Basin and estimate the original seawater concentrations of the redox sensitive trace metals.Our results show generally higher ratios of Fe/U, Fe/Mo, and Fe/V in fluid inclusions in halites associated with sulfate-depleted (calcium-rich) brines, compared to inclusions in halites formed in association with sulfate-rich brines in the Przyborów IG3 borehole. This behavior is consistent with the interpretation of García-Veigas et al. (2011) to reflect changes in redox conditions from oxidizing (represented by sulfate-rich brines) to reducing (represented by sulfate-depleted and calcium-rich brines) in the Zechstein epicontinental sea during the Late Permian. The correlation is more complex in halites from the Gorzów Wielkopolski IG1 borehole, likely reflecting local controls on redox state.Results suggest that ratios of Mn/U, Mn/V, and Mn/Mo in fluid inclusions may be used to refine redox conditions in the water column when these data are evaluated in combination with Fe/U, Fe/Mo, Fe/V, and Mn/Fe ratios because reduction of Mn(III, IV) to Mn(II) occurs at a higher redox potential compared to that required to reduce Fe(III) to Fe(II).The estimated original Mn and Mo concentrations in the seawater in the Late Permian Zechstein Basin are generally within the concentration range observed in modern anoxic basins while concentrations of V and Fe are generally higher and U is generally lower. Our results suggest that trace metal abundances in fluid inclusions in halite vary in response to redox changes in seawater and provide a potential redox proxy. To better understand and apply results obtained from fluid inclusions in halite as a paleoredox proxy, however, further work is required. These include the study of fluid inclusions in halite from more locations worldwide and the more detailed characterization of redox sensitive trace metal behavior during seawater evaporation.