Metastable ikaite (CaCO₃·6H₂O) forms abundantly during winter months along the south shoreline of Mono Lake where shoreline springs mix with lake water. Ikaite precipitates because of its decreased solubility at low temperature and because of orthophosphate-ion inhibition of calcite and aragonite. During the spring some of the ikaite is transformed to anhydrous CaCO₃ and is incorporated into tufa, but most is dispersed by wave action into the lake where it reacts to form gaylussite (Na₂Ca(CO₃)₂· 5H₂O). Spring waters have low pH values, are dominantly Ca-Na-HCO₃, have low radiocarbon activities, and are mixtures of deep-seated geothermal and cold groundwaters. Chemical modeling reveals that precipitation of CaCO₃ can occur over a broad range of mixtures of spring and lake water with a maximum production occurring at 96% spring water and 4% lake water. Under these conditions all the Ca and a significant fraction of the CO₃ of the precipitate is spring supplied. A radiocarbon age of 19,580 years obtained on a natural ikaite sample supports this conclusion. With the springs supplying a large and probably variable portion of the carbonate, and with apparent ¹⁴C age of the carbonate varying from spring to spring, tufa of similar actual antiquity may yield significantly different ¹⁴C dates, making tufa at this location unsuitable for absolute age dating by the radiocarbon method.