Stable strontium isotope fractionation in abiotic and microbially mediated barite in modern continental settings

Barite (BaSO4) which incorporates Sr in its crystal structure (~10,000 ppm Sr; Averyt et al., 2003) precipitates at only a few subaerial springs worldwide via either microbial processes (e.g., Senko et al., 2004) or abiotic processes (e.g., Bonny and Jones, 2008b). Significant mass dependent strontium (Sr) isotopic fractionation has been identified recently in many types of natural samples with a potential use as a paleo-environmental proxy related to temperature, presence of microbes, source of Sr and secondary mineral precipitation (e.g. Krabbenhoft et al., 2010; Bohm et al., 2012). Understanding the controls on variations in stable Sr-isotopes between natural samples and within sample types may provide important information on biogeochemical cycling and processes involving Sr. Both synthetic and natural barite samples were analyzed using field and laboratory techniques. Stable Sr isotopic fractionation was examined in abiotically precipitated barite at given conditions (e.g., temperature, saturation index, Sr/Ba ratio in solution) in the absence of microbes (Widanagamage et al., 2014). It is suggested that saturation index and the temperature of the solution are the two major controls on strontium distribution coefficient, Kd(Sr) which indirectly influence stable Sr isotope fractionation during barite precipitation. Authigenic barite samples precipitated in modern continental settings (warm water springs) were examined to elucidate processes controlling mass dependent fractionation of Sr during barite precipitation. Barite precipitation mechanisms at these spring sites are biologically mediated. Barite crystal morphology changes with rate of diffusion and rate of precipitation. It is suggested that sulfate concentration in the solution is more important in barite crystal morphology than temperature (Kowacz et al., 2007). However, my study suggests that temperature influences barite crystal morphology more than Ba2+/SO42- ratio in the solution. None of the geochemical or physicochemical parameters show a direct correlation with stable Sr isotope fractionation during barite precipitation in continental setting. However, microbial processes are identified as an important parameter for stable Sr isotope fractionation and the changes in micro environments need to be studied closely to understand the factors controlling stable Sr isotope fractionation in continental setting. Sr heterogeneity within barite crystal structure is considered a potentially important factor in stable Sr isotope fractionation during barite precipitation. Localized co-precipitation of multiple mineral phases (e.g., celestine) during barite precipitation has been identified in continental barite at synchrotron facilities, which could be important in stable Sr isotope fractionation. Precipitated barite from these spring water systems eventually presents in tufa deposits. Tufa samples were collected from each study site. Barite was identified in each tufa sample. The morphology of these barite crystals differs from the morphology of natural barite crystals from the sediments in the active spring site and the stream. Future study should measure stable Sr isotope ratios in tufa barite to understand the potential fractionation during early diagenesis. The information that is synthesized in this research on stable Sr-isotope fractionation during barite precipitation (both natural and synthetic) is useful to understand potential relationships and associated kinetic isotope effects in bio-geochemical processes. These isotopic signatures could potentially be used to explore paleo-environmental conditions in early Earth.