Paragenetic history of multiple mineral phases in ooids from Great Salt Lake, UT

Abstract:

The shallow saline waters of the Great Salt Lake (GSL), Utah are host to modern ooid sand, consisting of peloidal or detrital silicate nuclei particles surrounded by cortices characterized by radial, concentric, or radial-concentric fabrics. Previous studies had identified the occurrence of clay minerals within the cortices of GSL ooids, but these non-carbonate phases have been largely ignored in subsequent studies. This study aims to reveal the paragenetic history of co-occurring carbonate and silicate minerals within GSL ooid cortices. Understanding the sequence of mineral formation in lacustrine ooid fabrics can help resolve questions about relationships between lake water chemistry, microbial activity, and ooid formation and growth, and provide context for interpreting the occurrence of non-carbonate minerals within the cortices of ooids reported in ancient lacustrine strata. We used transmitted light microscopy, electron microprobe element mapping, scanning electron microscopy (SEM), and synchrotron x-ray fluorescence (XRF) mapping and sulfur K-edge absorption spectroscopy (XAS) to characterize the compositions and spatial relationships between mineral phases in GSL ooids. Large radially-oriented euhedral aragonite crystals were found to cut into Mg-silicate mineral phases and unconformably traverse laminar stratigraphy, suggesting that Mg-silicate precipitation must have preceded the growth of large radial aragonite fabrics. XAS spectra and XRF image maps revealed that Mg-silicate zones coincided with elemental sulfur (S₀), which we interpreted to suggest that sulfate-reducing bacteria played a role in Mg-silicate precipitation. Our observations suggest that the paragenetic history of GSL ooids is more complicated than previously appreciated and that this complexity might directly reflect microbial influences on ooid formation