The role of hydrothermal fluids in sedimentation in saline alkaline lakes: Evidence from Nasikie Engida, Kenya Rift Valley

Abstract

Saline alkaline lakes that precipitate sodium carbonate evaporites are most common in volcanic terrains in semi-arid environments. Processes that lead to trona precipitation are poorly understood compared to those in sulphate-dominated and chloride-dominated lake brines. Nasikie Engida (Little Magadi) in the southern Kenya Rift shows the initial stages of soda evaporite formation. This small shallow (<2 m deep; 7 km long) lake is recharged by alkaline hot springs and seasonal runoff but unlike neighbouring Lake Magadi is perennial. This study aims to understand modern sedimentary and geochemical processes in Nasikie Engida and to assess the importance of geothermal fluids in evaporite formation. Perennial hot-spring inflow waters along the northern shoreline evaporate and become saturated with respect to nahcolite and trona, which precipitate in the southern part of the lake, up to 6 km from the hot springs. Nahcolite (NaHCO 3) forms bladed crystals that nucleate on the lake floor. Trona (Na 2 CO 3 ÁNaHCO 3 Á2H 2 O) precipitates from more concentrated brines as rafts and as bottom-nucleated shrubs of acicular crystals that coalesce laterally to form bedded trona. Many processes modify the fluid composition as it evolves. Silica is removed as gels and by early diagenetic reactions and diatoms. Sulphate is depleted by bacterial reduction. Potassium and chloride, of moderate concentration, remain conservative in the brine. Clastic sedimentation is relatively minor because of the predominant hydrothermal inflow. Nahcolite precipitates when and where pCO 2 is high, notably near sublacustrine spring discharge. Results from Nasikie Engida show that hot spring discharge has maintained the lake for at least 2 kyr, and that the evaporite formation is strongly influenced by local discharge of carbon dioxide. Brine evolution and evaporite deposition at Nasikie Engida help to explain conditions under which ancient sodium carbonate evaporites formed, including those in other East African rift basins, the Eocene Green River Formation (western USA), and elsewhere.

A Comprehensive Methodology for Monitoring Evaporitic Mineral Precipitation and Hydrochemical Evolution of Saline Lakes: The Case of Lake Magadi Soda Brine (East African Rift Valley, Kenya)

“…15,42,48−50 These aggregates form due to the episodic interaction of Carich freshwater with the sodic brine on the lake margin. 15,40,42 Alternatively, as observed in other saline lakes and demonstrated experimentally, Ca−Mg carbonate precipitation could have been mediated by metabolic activities in the lake water. 51−54 Calcite spherule nucleation was observed at the sediment−water interface of highly alkaline lakes with coexisting Ca−Mg-rich hydrochemistry and microbial-driven colloidal substances, forming cemented spherulites in different depositional settings.…”

“…These precipitates before trona must be amorphous phases, probably amorphous silica, representing the precipitation of siliceous phases such as opal-A, magadiite, or gels in Lake Magadi and Little Magadi. 15 Silica gels precipitate in close proximity to the hot springs of Little Magadi lake and near the shorelines under active evaporation. 15,40,48,56 Precipitation of sylvite and glaserite is predicted by our model close to complete desiccation, but they have not been reported in the Lake Magadi basin because complete desiccation has never or very seldomly occurred.…”

“…15 Silica gels precipitate in close proximity to the hot springs of Little Magadi lake and near the shorelines under active evaporation. 15,40,48,56 Precipitation of sylvite and glaserite is predicted by our model close to complete desiccation, but they have not been reported in the Lake Magadi basin because complete desiccation has never or very seldomly occurred. During the rainy season, freshwater supply reaches the lake before brines reach supersaturation with respect to sylvite and glaserite.…”

“…In this work, we present a methodology for in situ studies of sequential evaporation/crystallization of brines from soda lakes in the African Rift Valley. The knowledge of the mineral precipitation sequence in saline and soda lakes is crucial to understanding the hydrochemical evolution of brines, paleoenvironmental conditions (e.g., temperature, pCO 2 ) of the evaporite deposits in the geological record, scaling in geothermal power facilities, and implications for industrial crystallization of these materials. − The assemblage of sodium carbonate minerals precipitated from soda brines are also important for constraining the geochemical conditions of soda oceans in Precambrian Earth, when life is thought to have originated, and other Earth-like planets. , …”

“…Owing to their scientific and economic values, much work has been carried out on the mineral precipitation in soda lakes based either on field data or on thermodynamic modeling. ,− Some recent works have proved that a combination of field research, lab experiments, and computer modeling is key to understanding evaporitic brine evolution and mineral precipitation sequences. ,, So far, there are no experimental investigations on the mineral precipitation sequence in soda lakes. In this work, we present a comprehensive experimental investigation and geochemical modeling of the mineral precipitation sequence and hydrochemical evolution of Lake Magadi brines, using evaporative mineral precipitation experiments monitored by (a) in situ video microscopy and synchrotron X-ray diffraction from acoustically levitated droplets; (b) ex situ Raman spectroscopy, X-ray diffraction, and scanning electron microscopy; and (c) computational modeling.…”