Replacement of Aragonite by Quasi-Rhombohedral Dolomite in a Late Pleistocene Tufa Mound, Great Salt Lake, Utah, U.S.A.

Abstract

ABSTRACT Fabric-preserving quasi-rhombohedral dolomite forms as cement and replacement of aragonite in a tufa mound 2+ m thick on the western shoreline of the Great Salt Lake. The rounded crystals range in size from 5 to 20 µm in diameter. Crystal faces are not pitted and have numerous sharp-edged steps and kinks. The quasi-rhombohedral forms are the result of crystal-growth phenomena and not of 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 position of the 104 peak). The Great Salt Lake dolomite is poorly ordered, with relative ordering of 28.5% and a reflection ratio of 0.9503 compared to a well ordered standard dolomite. The Great Salt Lake dolomite has the poorest ordering of any natural dolomite recorded to date. More than half of the electron-microprobe analyses show elevated Sr abundance, indicating that dolomite formed in a rock-buffered diagenetic system, where much of the bicarbonate and Ca were supplied by dissolution of aragonite and Mg from the lake water. 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 been 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.