Controls on aragonite and calcite precipitation in hot spring travertines at Chemurkeu, Lake Bogoria, Kenya

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Subfossil travertines precipitated around hot spring orifices at Chemurkeu on the western shore of Lake Bogoria, Kenya, are composed of calcite, aragonite, and minor dolomite. Aragonite crystal beds, which form 5–10% of the travertine by volume, are formed of large pseudohexagonal prisms, whereas the calcite crystal beds are composed mainly of feather dendrites. Dolomite is only found between aragonite crystals. Boundaries between aragonite and calcite crystals commonly display evidence of dissolution, but there is no evidence to indicate that calcite formed by inversion of aragonite or that the dolomite replaced the aragonite. Thus, the aragonite, calcite, and dolomite are each treated as primary precipitates. Reticulate gelatinous coatings, with a high Si and Mg content, cover most external and internal surfaces of the aragonite and calcite crystals. The travertines may have formed under more humid conditions than today, when the spring waters contained more Ca2+. The physiochemical conditions at the modern springs provide a context for interpreting the factors that controlled the precipitation of the aragonite and calcite. Today, the hot (T > 85 °C) Na–HCO3–Cl spring waters at Chemurkeu, which have a salinity of ~6 g∙L−1 total dissolved solids, a pH of 8.1–9.1, and contain < 2 mg∙L−1 of Ca2+ and < 0.7 mg∙L−1 of Mg2+, are fed by a shallow aquifer [Formula: see text] and a deeper aquifer (T = 170 °C). Modern spring waters, derived from meteoric groundwater, lake water, and condensed steam, are fed mainly from the shallow thermal aquifer. Field, petrographic, and scanning electron microscope evidence obtained from the travertines, coupled with knowledge of the modern springs, indicates that the progressive and cyclic alternation from calcite precipitation to aragonite precipitation to aragonite dissolution which characterizes many travertine successions may have been caused by changes in [Formula: see text] of the spring waters under high temperatures (> 90 °C). The textures in the travertines show that precipitation of the aragonite and calcite crystals was probably abiotic, and episodic rather than continuous. Rapid degassing of CO2 associated with shallow boiling was probably a major factor in crystal growth.