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 (NaHCO₃) forms bladed crystals that nucleate on the lake floor. Trona (Na₂CO₃·NaHCO₃·2H₂O) 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 pCO₂ 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.

中文翻译：

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盐碱湖中热液在沉积中的作用：来自肯尼亚纵谷的纳西基·恩吉达的证据

在半干旱环境中的火山地形中，盐碱湖会沉淀出碳酸钠蒸发物，这是最常见的。与硫酸盐为主和氯化物为主的湖卤水相比，导致天然碱沉淀的过程知之甚少。肯尼亚南部裂谷的Nasikie Engida（小马加迪）显示出苏打汽化物形成的初始阶段。这个小的浅水湖（<2 m深； 7 km长）被碱性温泉和季节性径流补给，但是与邻国马加迪湖不同的是多年生的。这项研究旨在了解Nasikie Engida的现代沉积和地球化学过程，并评估地热流体在蒸发岩形成中的重要性。北部海岸线上的多年生温泉流入水蒸发后，就纳科石和天然碱而言变得饱和，沉淀在湖的南部，距温泉最多6公里。碳酸氢钠（NaHCO₃）形成在湖底成核的叶片状晶体。Trona（Na ₂ CO ₃ ·NaHCO ₃ ·2H ₂ O）从木筏和针状晶体的底部有核灌木丛中沉淀出来，并在横向上聚集成层状的天然碱。许多过程都会随着流体成分的变化而改变。二氧化硅以凝胶形式以及早期的成岩反应和硅藻被去除。硫酸盐通过细菌减少而耗尽。中等浓度的钾和氯化物在盐水中保持保守。由于大量的热液流入，碎屑沉积相对较小。碳酸氢钠在何时何地沉淀p CO ₂很高，特别是在湖底泉水附近。纳吉基·恩吉达（Nasikie Engida）的结果表明，温泉排放至少使该湖维持了2年，而蒸发气的形成受到二氧化碳局部排放的强烈影响。Nasikie Engida的盐水演化和蒸发岩沉积有助于解释古代碳酸钠蒸发岩形成的条件，包括其他东非裂谷盆地，始新统绿河组（美国西部）和其他地区的碳酸盐蒸发岩的形成条件。