Mineral precipitation and hydrochemical evolution through evaporitic processes in soda brines (East African Rift Valley)

Soda lakes of the East African Rift Valley are hyperalkaline, hypersaline extremely enriched in Na+, K+, Cl?, CO32?, HCO3?, and SiO2. In this paper, we investigate chemical evolution these production sediments by salt precipitation via evaporation. Water samples from tributary springs three (Magadi, Nasikie Engida Natron) have been experimentally studied in-situ X-ray diffraction during evaporation experiments to characterize sequence mineral precipitation. These data complemented ex-situ studies, analyses thermodynamic hydrochemical calculations producing detailed information on activity all solution species saturation state minerals potentially generated given composition. Major precipitating sodium carbonates/bicarbonates as well halite. The CO3/HCO3 ratio, controlled pH, is main factor defining Na?carbonates sequence: lake brines where > 1, trona precipitates first whereas hot springs, ? nahcolite instead trona, which forms later partial dissolution nahcolite. Precipitation possible only at lower pH values (pCO2 higher than ?2.7) explaining distribution current stratigraphic sequences. Later, evaporation, thermonatrite precipitates, normally same time halite, a very high (>11.2) after significant depletion HCO3? due soluble increases brine contributing hyperalkaline character lakes. Villiaumite, sylvite, alkaline earth carbonates, fluorapatite silica also predicted precipitate, but most them not observed experiments, either because small amount produced, kinetic effects delaying nucleation some phases, or biologically induced chemistry that considered our calculations. Even cases, composition corresponding ions allows for discussion their accumulation eventual phases. coupling geochemical modelling key understanding hydrogeochemical hydroclimatic conditions soda lakes, evaporite settings, oceans early Earth other extraterrestrial bodies.