DOLOMITIZATION OF LATE PLEISTOCENE TUFA MOUNDS OF THE GREAT SALT LAKE

Late Pleistocene, 2+-m-thick aragonite tufa mounds on the western shoreline of the Great Salt Lake are partially replaced by a peculiar form of dolomite. The dolomite replaces the original microbialite fabric and aragonite cement with near-perfect fabric preservation. In fact, dolomite was not initially recognized in the field or in thin section because the dolomite is readily soluble in cold 10% HCl, and stains pink with Alizarin Red-S, and lacks rhombohedral form even as a cement. The mineralogy was first detected by XRD analysis. Re-examination of the thin section found two distinct fabrics: aragonite microbialites with chisel-tipped, bladed aragonite cement and dolomitized microbialites with near-spheroidal dolomite cement. Scanning-electron microscopy shows that the crystal faces of the dolomite cement are not pitted and have numerous sharp-edged steps and kinks. The quasi-rhombohedral, rounded form of the dolomite cement is clearly the result of crystal-growth phenomena and not post-crystallization dissolution. Microprobe analyses (n = 92) of the dolomite yield an average 46 mole % MgCO₃, in agreement with the 44 to 45 mole % MgCO₃ content determined from x-ray diffraction (30.8° 2-theta position of the 104 peak). The Great Salt Lake dolomite is one of the poorest ordered dolomites yet documented, with relative ordering of 28.5% and a reflection ratio of 0.9503 compared to a well ordered standard dolomite. Elevated Sr abundances in the dolomite indicate that dolomite formed in a rock-buffered diagenetic system, where much of the bicarbonate and Ca were supplied by dissolution of aragonite. Mg was supplied by the lake water. The modern Great Salt Lake has an Mg/Ca ratio of ~14 and a Mg abundance of ~4200 ppm. Preservation of the tufa fabric by dolomite was accomplished by the closely related dissolution of aragonite and precipitation of dolomite in a fluid undersaturated with respect to aragonite and supersaturated with respect to Ca-rich dolomite. Formation of spheroids might reflect a high growth rate from a high-alkalinity fluid that was undersaturated with respect to other carbonate minerals. Maintenance of this growth rate might have be aided by the Ca-enrichment and cation disordering of the precipitating dolomite phase and the large surface area of a sphere compared to that of an inscribed rhomb.