The sediment along the shorelines of Great Salt Lake (GSL), Utah is dominated by ooids, concentrically-coated carbonate sand grains. Two characteristics differentiate GSL ooids from typical modern marine ooids: well-developed radial aragonite microfabrics and the ubiquitous occurrence of a Mg-silicate phase. The radial microfabrics have formed the basis of conceptual models applied to understand the formation of radial fabrics in ancient ooids, but the formation of the Mg-silicates, and the relationship between Mg-silicates and radial aragonite crystals have received little attention. The occurrence of Mg-silicates in GSL ooids is surprising because GSL lake water pH is ~8.3, too low for Mg-silicate precipitation (requires pH>8.7). We use transmitted light microscopy, element mapping via wavelength-dispersive x-ray spectroscopy with electron microprobe, scanning electron microscopy, and synchrotron x-ray fluorescence (XRF) mapping and sulfur K-edge absorption spectroscopy to explore the spatial relationships between the mineral phases in GSL ooids. We observe large euhedral aragonite crystals penetrating Mg-silicate zones and cutting across laminar cortices, suggesting that the characteristic radial aragonitic fabrics in GSL ooids, traditionally interpreted as a primary structure, are enhanced, or in some cases entirely created via neomorphism. XRF maps reveal that Mg-silicate zones co-occur with elemental sulfur (S0), which we interpret as a metabolic intermediate of microbial sulfur cycling. This co-occurrence supports our hypothesis that microbial sulfur cycling plays a key role in the formation of GSL ooids by locally shifting pH beyond the threshold for Mg-silicate precipitation. This compositional fingerprint could serve as a biosignature in ancient lacustrine strata where Mg-silicates co-occur with carbonate minerals.
