Barite precipitation typically occurs when barium rich fluids mix with sulfate rich fluids, however barite found in the modern continental environment suggests that biological activity can play an important role in barite formation by oxidizing sulfur and/or concentrating barium within microenvironments. These activities induce barite precipitation, and carry with them implications for studies of barite genesis. Strontium (Sr) is incorporated into the barite crystal structure during barite formation preserving a radiogenic and stable Sr-isotope signature in barite, providing information about its formation. Here we present Sr-isotope results from three artesian sulfidic springs with ongoing barite precipitation (Zodletone Spring, Oklahoma; Stinking Spring, Utah; and Doughty Springs, Colorado) to explore the controls on stable Sr-isotope fractionation during barite precipitation in a continental setting. Apparent stable Sr-isotope fractionation for all three sites ranged from −0.6‰ to~0.0‰ similar to previously published calculated values for equilibrium conditions and measured values of synthetic barite. However, clear relationships do not exist between water and barite chemistry in the natural systems, indicating that barite does not precipitate directly from solution, but heterogeneously within diverse microenvironments created by microbial biomass or on sediment surfaces. The dynamic microenvironments in a continental setting influence the apparent stable Sr-isotope fractionation during barite precipitation because of changing saturation conditions, Sr concentration and/or precipitation of different mineral phases (e.g., celestine). In order to better understand the geochemistry of barite deposits, future work is necessary to study the controls on radiogenic and stable Sr-isotope signatures of barite in the context of the temporally and spatially dynamic nature of the continental setting.
Barite (BaSO4) is a stable and widely distributed mineral in Earth’s crust. As such, barite has the potential to preserve specific geochemical and morphological characteristics representative of conditions at the time of its formation, which could be useful for interpretations of Earth’s ancient rocks and paleoenvironments. In this study, we used variations in saturation index, solution temperature, solution chemistry, presence of organics, and Mg2+ and Ca2+ ions to investigate variations in barite crystal morphology. Through 42 experiments, we simulated poorly understood, low temperature spring settings similar to Zodletone Spring in Anadarko Basin, Oklahoma. Using SEM/EDS, we identified barite rosettes, rounded barite, euhedral/square-shaped barite, and elliptical barite as the crystal morphologies that directly reflect different formational settings. The X-ray diffraction (XRD) patterns revealed different crystallographic characters of the four distinct barite crystal morphologies; in particular, the samples that precipitated from supersaturated SrSO4 solution exhibited double peaks at 43° 2-Theta, which matched barite with strontium substitution as barite might have incorporated strontium in its structure. Barite crystals that formed in the presence of organics in the initial solution exhibited a double peak at 33° 2-Theta, which was absent in other samples. Confocal Raman microscopy indicated that all of the samples had typical barium sulfate bands, with a few differences in bands between the samples; for example, the 638 cm−1 band showed splitting or a double band between different samples. The samples that precipitated from solution with organics had organic compounds from the experimental solution included in their composition. In both cases, C–H stretches from 2800 cm−1 to 3000 cm−1 were present as well as bands from 1350 cm−1 to 1500 cm−1, which are typical of organic compounds. Based on our experiments, the variation in barite crystal morphologies reflected changes in initial solution chemistry (or environmental settings).
We have documented the presence of celestine (SrSO4) within sediment accumulating at an artesian sulfidic spring (Zodletone Spring, Oklahoma) dominated by barite (BaSO4) precipitation associated with microbiological activity. The distribution and speciation of Sr in solid phases was determined by synchrotron-based micro-X-ray fluorescence spectroscopy and micro-X-ray diffraction, and particle morphology and texture was determined using electron microscopy. In all the natural sulphidic spring samples and lithified tufa sample, celestine was detected in finegrained micron-scale Sr-rich phases but not in euhedral, Sr-poor grains. In parallel laboratorybased precipitation experiments, celestine was not observed even when solutions contained high Sr/Ba. Thermodynamic predictions alone do not account for the presence of micron-scale celestine in the sulfidic spring, and they do not account for the differences in Sr presence and distribution in naturally-occurring versus synthetic grains. While the mechanism is unclear, based on this evidence we hypothesize that the combination of bacterial surfaces and microenvironments within the crusts and microbial mats creates a synergistic effect where Sr is preferentially exchanged over Ba between the overlying stream water and the pore water within the mats allowing celestine to precipitate. Ultimately, our results point to an important role of biological activity for preferential Sr uptake. The presence of micron-scale celestine in ancient barite deposits can therefore potentially be used as a biomarker for conditions similar to modern sulphidic springs.
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