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

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.     

Determination of the solubility products of sodium carbonate minerals and an application to trona deposition in Lake Magadi (Kenya)

The ion-interaction model of PITZER (1973), is very effective in deriving stability relationships at high concentrations for the system Na-Cl-HCO₃-CO₃-OH-H₂O. The solubility products of the main sodium carbonates have been calculated from solubility data between 5 and 50°C. The stability diagram in log p_co2 — temperature coordinates and the invariant points deduced from the newly determined data are in good agreement with the most recent measurements.These results are used to calculate the activities of the major dissolved species in Lake Magadi brines (Kenya). The thermodynamic treatment confirms the main conclusions reached earlier by Eugster (1970, 1980) mainly from field observations. Trona precipitation occurs at equilibrium while natron is likely to form when the temperature decreases below 25°C. After the salt deposition the CO₂ supply from the atmosphere is too slow to allow equilibrium between the atmosphere and the brines. In the next stages of evaporative concentration thermonatrite and halite precipitate. The deposition of the latter salts along with the observed HCO^?₃ depletion suggest that fractional crystallization is likely to control trona deposition.     

The shallow ground water chemistry of arsenic,fluorine, and major elements: Eastern Owens Lake,California

Owens Lake in SE California became essentially dry by the 1920s after the Los Angeles Aqueduct was constructed and diversion of water from the Owens River began. Frequent dust storms at Owens Lake produce clouds of efflorescent salts which present human health hazards as a result of their small particle size and elevated concentrations of As and SO₄. This study was conducted to characterize the evolution of major elements in ground water in eastern Owens Lake and to examine the factors controlling the concentrations of dissolved As and F. Evapoconcentration of shallow ground waters at the lakebed surface produces high pH, high alkalinity brines with major ion compositions that are consistent with those predicted by the Hardie–Eugster Model. Evaporite minerals identified in the surface salts using XRD were halite (NaCl), thenardite (Na₂SO₄), trona (Na₃H(CO₃)₂·2H₂O), pirssonite (Na₂Ca(CO₃)₂·2H₂O), and nesquehonite (MgCO₃·3H₂O). Significant correlations between both As and F with Li in shallow ground waters indicate that As and F are not partitioned into surface salts until very high salinities are reached (>9.0 m). Leaching experiments show that As and F can be readily released from lakebed salts when exposed to natural precipitation. Conservative behavior of As and F results from the high pH values and low Ca activities of shallow ground waters that contribute to: (1) redox stability of As(V) even at moderately reducing conditions, (2) a decrease in the adsorption affinities of As and F to mineral surfaces, (3) undersaturation with respect to fluorite (CaF₂(s)).     

Geochemical Evolution of Great Salt Lake, Utah, USA

The Great Salt Lake (GSL) of Utah, USA, is the largest saline lake in North America, and its brines are some of the most concentrated anywhere in the world. The lake occupies a closed basin system whose chemistry reflects solute inputs from the weathering of a diverse suite of rocks in its drainage basin. GSL is the remnant of a much larger lacustrine body, Lake Bonneville, and it has a long history of carbonate deposition. Inflow to the lake is from three major rivers that drain mountain ranges to the east and empty into the southern arm of the lake, from precipitation directly on the lake, and from minor groundwater inflow. Outflow is by evaporation. The greatest solute inputs are from calcium bicarbonate river waters mixed with sodium chloride-type springs and groundwaters. Prior to 1930 the lake concentration inversely tracked lake volume, which reflected climatic variation in the drainage, but since then salt precipitation and re-solution, primarily halite and mirabilite, have periodically modified lake-brine chemistry through density stratification and compositional differentiation. In addition, construction of a railway causeway has restricted circulation, nearly isolating the northern from the southern part of the lake, leading to halite precipitation in the north. These and other conditions have created brine differentiation, mixing, and fractional precipitation of salts as major factors in solute evolution. Pore fluids and diagenetic reactions have been identified as important sources and especially sinks for CaCO₃, Mg, and K in the lake, depending on the concentration gradient and clays.     

河南吴城天然碱矿床地质特征及戍因探讨

吴城碱矿是世界罕见的古代天然碱矿床之一,属于典型的陆相碳酸盐型盐湖沉积。矿床赋存于一个早第三纪断陷盆地中,该盆地发育了厚约2400余米的陆相碎屑—蒸发岩。天然碱距地表650—900余米,呈多层状产出,下部为天然碱矿层,上部则为含岩盐天然碱层。矿床盐类矿物组合以天然碱为主,次为石盐、重碳酸钠盐。共生矿物有磷钠钙石、氯碳钠镁石,未见通常在碳酸盐湖中出现的硫酸盐矿物——芒硝。     

江西41 4含钽、铌钠长石化花岗岩风化壳中三水铝石的发现及其成因研究

The Wucheng trona deposit, one of the rare ancient trona deposits in the world, is genetically of typical terrestrial facies carbonate sediments of brine lake. Contained in an Eogene block basin composed of terrigenous clastic-evaporite rocks some 2400 m in thickness, it occurs 650 to over 900 m beneath the earth's surface in the form of multilayers, the lower part being pure trona beds while the upper part consisting of trona beds interbedded with halite streaks. Saline minerals are trona and to a much less amount, halite and nahcalite, associated with such minerals as shortite and northupite. Mirabilite, a rather common mineral in the carbonate salt lake, has not yet been found here. The unusually developed sedimentary cyclothem of the host rock, predo-minantly argillaceous dolomite- kuchersite- (halite-bearing) trona, makes up a most striking feature of this deposit, and the trona bed lies invariably on the kuchersite-dolomite facies. Although belonging to one of the minerogenetic series of saline deposit, the trona deposit, instead of being simply a product of the evaporation of brine, has its specific mineral-forming conditions and sedimentation mechanism. Trona is a typical product of the terrestrial facies carbonate lake. lts formation, therefore, requires the persistent supply of large quantities of Na-rich carbonate type water which, being actually surface water or ground water circulating through the metamorphic and igneous rocks at the periphery of the basin, constitutes an indispensable factor in the sedimentation of trona by its concentration and evaporation. Besides, the coicentration of CO_{8<.sub>^-2 and HCO8^- in the solution is related to the partial pressure of CO2. Only when sufficient CO2 is unceasingly supplied can nahcalite and trona be precipitated. CO2, in turn, depends on the presence of large amounts of organism for its formation. These factors are prerequisites for the precipitation of the trona deposit, making it distinctly different from other saline deposits. Other formation conditions of the Wucheng trona deposit, such as the existence of a closed basin controlled by tectonics, the alternate arid and semiarid climates, are quite analogous to those of other saline deposits. Some controversial problems concerning this kind of deposit are also put for ward in this paper for further investigation, including the feasibility of taking the abundance of sulfate minerals as a criterion of distinguishing ancient tronas from recent ones, the influence of material resources and climate conditions upon the variation of ore types, and the origin of the highly-concentrated sodium brine in areas adjacent to the Wucheng deposit.}     

Geochemical and isotopic constraints on the interaction between saline lakes and groundwater in southeast Australia

Major ion and stable isotope geochemistry allow groundwater/surface-water interaction associated with saline to hypersaline lakes from the Willaura region of Australia to be understood. Ephemeral lakes lie above the water table and locally contain saline water (total dissolved solids, TDS, contents up to 119,000 mg/L). Saline lakes that lack halite crusts and which have Cl/Br ratios similar to local surface water and groundwater are throughflow lakes with high relative rates of groundwater outflows. Permanent hypersaline lakes contain brines with TDS contents of up to 280,000 mg/L and low Cl/Br ratios due to the formation of halite in evaporite crusts. These lakes are throughflow lakes with relatively low throughflow rates relative to evaporation or terminal discharge lakes. Variations in stable isotope and major ion geochemistry show that the hypersaline lakes undergo seasonal cycles of mineral dissolution and precipitation driven by the influx of surface water and evaporation. Despite the generation of highly saline brines in these lakes, leakage from the adjacent ephemeral lakes or saline throughflow lakes that lack evaporite crusts is mainly responsible for the high salinity of shallow groundwater in this region.     

Hydrogeochemistry in the Ngorongoro Crater,Tanzania, and implications for land use in a World Heritage Site

The chemistry of surface and ground waters in the Ngorongoro Crater, Tanzania, home to thousands of large mammals and a World Heritage Site, is controlled by the volcanic host rock lithology, evaporative concentration, mineral precipitation and redissoluton, and biological factors. Three groups of waters are informally differentiated based on their ranges of concentration: (1) dilute inflow (meteoric runoff and springs from short flowpaths: pH<8, Cl⁻<10 mg/l); (2) concentrated inflow (concentrated runoff and springs from long flowpaths: pH=8–9, Cl⁻=10–100 mg/l); and (3) brackish waters (pools and Lake Makat: pH>9, Cl⁻>100 mg/l). Evaporative concentration and biological activity in swamps commonly produce strong geochemical gradients between dilute sources and peripheral concentrated ephemeral wetlands. Dilute inflow is found in the Lerai, Munge, and Oljoro Nyuki streams, and several large springs near the Crater wall such as Ngoitokitok and Seneto. Concentrated inflow is found in downstream reaches of the Munge stream, discharging from springs away from the Crater wall such as Engitati and Mti Moja, and in dry-season pools. Brackish waters are found discharging from springs on the southern margin of Lake Makat, in mudflats surrounding marshes, in ephemeral pools, and in the lake itself. Although few hydrologic data are available, the persistence of relatively fresh water in vegetated wetlands is consistent with lower sedge-dominated wetland evapotranspiration rates compared with open water. This suggests that wetlands may play an important role in ensuring fresh water availability in the basin, and it demonstrates the need for future hydrologic study. Most of the Ngorongoro waters originate as rainfall outside the Crater, and travel into the basin as surface or ground water flow, emphasizing the need for a watershed-scale approach to land management.     

Depositional environments and diagenesis in Lake Parakeelya: a Cambrian alkaline playa from the Officer Basin, South Australia

Laminated and desiccated siliceous dolostones, dolomitic mudstones and dolomitic sandstones in the Cambrian Parakeelya Alkali Member of the Observatory Hill Formation accumulated in an alkaline playa. Four facies are recognized (from lake centre to lake edge): lake, saline mudflat, dry mudflat and sandflat facies. The facies occur in cycles. Cycles of tens of metres thickness record the gradual expansion and contraction of the playa. Superimposed smaller cycles of tens of centimetres thickness record minor oscillations in the position of the strandline in a shallow lake. The dominance of saline mudflat, dry mudflat and sandflat facies indicates that the lake was rimmed by broad flat areas with negligible relief. The high δ¹⁸O values of primary and penecontemporaneous diagenetic carbonates of +24 to +28‰ (SMOW) indicate strong evaporation of ground and surface waters within the lake system. Calcite pseudomorphs of the sodium carbonate minerals trona and shortite have δ¹⁸O values between + 19 and + 22.5‰, and contain fluid inclusions with variable salinites and homogenization temperatures up to around 110°C. This suggests that the euhedral alkaline evaporites were dissolved by heated waters; calcite pseudomorphs then precipitated from a mixed solution formed by the interaction of these incoming fluids with the relatively saline interstitial brines. The sodium bicarbonate solutions formed by dissolution of the evaporites would have been dispersed by the basinal brines so that despite the closed drainage, further groundwater concentration did not take place.     

Studies of quaternary saline lakes—II. Isotopic and compositional changes during desiccation of the brines in Owens Lake,California, 1969–1971

Owens Lake is an alkaline salt lake in a closed basin in southeast California. It is normally nearly dry, but in early 1969, an abnormal runoff from the Sierra Nevada flooded it to a maximum depth of 2·4 m. By late summer of 1971, the lake was again nearly dry and the dissolved salts recrystallized. Changes in the chemistry, pH, and deuterium content were monitored during desiccation.During flooding, salts (mostly trona, halite, and burkeite) dissolved slowly from the lake floor. Their concentration in the lake waters increased as evaporation removed water and salts again crystallized, but winter temperatures caused precipitation of some salts and the following summer warming caused their solution, resulting in seasonal variations in the concentration patterns of some ions. The pH values (9·4–10·4) changed with time but showed no detectable diurnal pattern.The deuterium concentration increased during evaporation and appeared to be in equilibrium with vapor leaving the lake according to the Rayleigh equation. The effective α(D/H in liquid/D/H in vapor) decreased as salinity increased; the earliest measured value was 1·069 [as total dissolved solids (TDS) of lake waters changed from 136,200 to 250,400 mg/1]and the last value (calc.) was 1·025 (as TDS changed from 450,000 to 470,300 mg/1). Deuterium exchange with the atmosphere was apparently small except during late desiccation stages when the isotopic contrast became great. Eventually, atmospheric exchange, combined with decreasing α and lake size and increasing salinity, stopped further deuterium concentration in the lake. The maximum contrast between atmospheric vapor and lake deuterium contents was about 110%.     

Characterizing Major Controls on Spatial and Seasonal Variations in Chemical Composition of Surface and Pore Brine of Maharlu Lake,Southern Iran

Maharlu Lake with Na–Cl water type is the terminal point of a closed basin in southern Iran. A total of 10 water samples from two rivers discharging to the lake and 78 water samples of surface and pore brine of Maharlu Lake have been collected from different depths (surface, 20, 50 and 100 cm) of four sampling stations along the lake during a period of lake water-level fluctuation (November 2014–July 2015). To investigate chemical interaction between lake surface water and shallow pore water and to understand the major factors governing chemical composition of Maharlu brine, concentrations of major and minor (boron, bromide and lithium) solutes, pH and total dissolved solids have been measured in collected water samples. Saturation indices of evaporite minerals in collected water samples have been also calculated. The chemical behavior of dissolved solutes and evaporative evolution of the lake brine during a hydrological period have been simulated using PHREEQC. The results of our investigations indicated that chemical composition of lake surface water and pore brine of Maharlu Lake are mainly connected with lake water-level fluctuations and distance from input rivers (and depth), respectively. Hydrochemical investigations and statistical analysis showed that the brines chemistry of Maharlu is mainly controlled by three processes: brine evaporative evolution, dissolution–precipitation and diagenetic evolution of secondary carbonates.     

内蒙古伊盟地区现代碱湖地质特征和形成条件分析

内蒙古伊克昭盟地区分布着众多小型现代盐湖。湖内沉积有天然碱、芒硝和石盐等化工矿产。各碱湖地层构成碎屑-粘土-蒸发岩沉积旋回,沉积厚度在10m 左右,基岩为白垩纪砂岩。结晶碱层以块状、厚层状为主,其分布受湖盆形态控制,一般埋深0.1～0.4m。伊克昭盟地区碱湖的形成是构造活动的结果。经长期风化剥蚀,地表和地下水向古河道和风蚀洼地聚集了基岩或剥蚀区地层的有关组分,并携带到湖盆中,使湖水转变为成水、卤水,后来由于气候骤然变冷,湖水中 Na_2CO_3或 Na_2SO_4过饱和而分别结晶为Na_2CO_3·10H_2O 或 Na_2SO_4·10H_2O,从而形成碱湖沉积。     

Origin and evolution of major salts in the Darling pans,Western Cape,South Africa

Sediment and water samples from 12 saline pans on the semi-arid west coast of South Africa were analysed to determine the origin of salts and geochemical evolution of water in the pans. Pans in the area can be subdivided into large, gypsiferous coastal pans with 79–150 g/kg total dissolved salt (TDS), small inland brackish to saline (2–64 g/kg TDS) pans and small inland brine (168-531 g/kg TDS) pans that have a layer of black sulphidic mud below a halite crust. The salinity of coastal pan waters varies with the seasonal influx of dilute runoff and dissolution of relict Pleistocene marine evaporite deposits. In contrast, inland pans are local topographic depressions, bordered on the north by downslope lunette dunes, where solutes are concentrated by evaporation of runoff, throughflow and groundwater seepage. The composition of runoff and seepage inflow waters is determined by modification of coastal rainfall by weathering, calcite precipitation and ion exchange reactions in the predominantly granitic catchment soils. Evaporation of pan waters leads to precipitation of calcite, Mg–calcite, dolomite, gypsum and halite in a distinct stratigraphic succession in pan sediments. Bicarbonate limits carbonate precipitation, Ca limits gypsum precipitation and Na limits halite precipitation. Dolomitisation of calcite is enhanced by the high Mg/Ca ratio of brine pan waters. Brine pan waters evolve seasonally from Na–Cl dominated brines in the wet winter months to Mg–Cl dominated brines in the dry summer months, when 5–20 cm thick halite crusts cover pan surfaces. Pan formation was probably initiated during a drier climate period in the early Holocene. More recent replacement of natural vegetation by cultivated land may have accelerated salt accumulation in the pans.     

Origin and evolution of oilfield brines from Tertiary strata in western Qaidam Basin: Constraints from <Superscript>87</Superscript>Sr/<Superscript>86</Superscript>Sr, δD, δ<Superscript>18</Superscript>O, δ<Superscript>34</Superscript>S and water chemistry

Chemistry of major and minor elements, ⁸⁷Sr/⁸⁶Sr, δD, δ¹⁸O and δ³⁴S of brines were measured from Tertiary strata and Quaternary salt lakes in the western Qaidam Basin. The water chemistry data show that all oilfield brines are CaCl₂ type. They were enriched in Ca²⁺, B³⁺, Li⁺, Sr²⁺, Br⁻, and were depleted in Mg²⁺, SO₄ ²⁻, which indicated that these brines had the characteristics of deeply circulated water. The relationship between δD and δ¹⁸O shows that all data of these brines decline towards the Global Meteoric Water Line (GWL) and Qaidam Meteoric Water Line (QWL), and that the intersection between oilfield brines and Meteoric Water Lines was close to the local spring and fresh water in the piedmont in the western Qaidam Basin. The results suggest that oilfield brines has initially originated from meteoric water, and then might be affected by water-rock metamorphose, because most oilfield brines distribute in the range of metamorphosing water. The ⁸⁷Sr/⁸⁶Sr values of most oilfield brines range from 0.71121 to 0.71194, and was less than that in salt lake water (>0.712), but close to that of halite in the study area. These imply that salt dissolution occurred in the process of migration. In addition, all oilfield brines have obviously much positive δ³⁴S values (ranging from 26.46‰ to 54.57‰) than that of salt lake brines, which was caused by bacterial sulfate reduction resulting in positive shift of δ³⁴S value and depleteed SO₄ ²⁻ in oilfield brines. Combined with water chemical data and δD, δ¹⁸O, ⁸⁷Sr/⁸⁶Sr, δ³⁴S values, we concluded that oilfield brines mainly originate from the deeply circulated meteoric waters, and then are affected by salt dissolution, water-rock metamorphose, sulfate reduction and dolomitization during the process of migration. These processes alter the chemical compositions of oilfield brines and accumulate rich elements (such as B, Li, Sr, Br, K and so on) for sustainable utilization of salt lake resources in the Qaidam Basin.     

Impact of lake‐level changes on the formation of thermogene travertine in continental rifts: Evidence from Lake Bogoria,Kenya Rift Valley

Travertine is present at 20% of the ca 60 hot springs that discharge on Loburu delta plain on the western margin of saline, alkaline Lake Bogoria in the Kenya Rift. Much of the travertine, which forms mounds, low terraces and pool‐rim dams, is sub‐fossil (relict) and undergoing erosion, but calcite‐encrusted artefacts show that carbonate is actively precipitating at several springs. Most of the springs discharge alkaline (pH: 8·3 to 8·9), Na‐HCO₃ waters containing little Ca (<2 mg l^?1) at temperatures of 94 to 97·5°C. These travertines are unusual because most probably precipitated at temperatures of >80°C. The travertines are composed mainly of dendritic and platy calcite, with minor Mg‐silicates, aragonite, fluorite and opaline silica. Calcite precipitation is attributed mainly to rapid CO₂ degassing, which led to high‐disequilibrium crystal morphologies. Stratigraphic evidence shows that the travertine formed during several stages separated by intervals of non‐deposition. Radiometric ages imply that the main phase of travertine formation occurred during the late Pleistocene (ca 32 to 35 ka). Periods of precipitation were influenced strongly by fluctuations in lake level, mostly under climate control, and by related changes in the depth of boiling. During relatively arid phases, meteoric recharge of ground water declines, the lake is low and becomes hypersaline, and the reduced hydrostatic pressure lowers the level of boiling in the plumbing system of the hot springs. Any carbonate precipitation then occurs below the land surface. During humid phases, the dilute meteoric recharge increases, enhancing geothermal circulation, but the rising lake waters, which become relatively dilute, flood most spring vents. Much of the aqueous Ca²⁺ then precipitates as lacustrine stromatolites on shallow firm substrates, including submerged older travertines. Optimal conditions for subaerial travertine precipitation at Loburu occur when the lake is at intermediate levels, and may be favoured during transitions from humid to drier conditions.     

Major Ion Geochemistry of Nam Co Lake and its Sources, Tibetan Plateau

The major cations and anions from lake water samples and its sources, including glacier snow, precipitation, stream, and swamp water in the Nam Co basin, central Tibetan Plateau, were studied. The concentrations of the major ions varied significantly in the five environmental matrices. Generally, the mean concentrations of most ions are in the order of lake water > swamp water > stream water > precipitation > snow. Rock weathering is the dominant process controlling the chemical compositions of the stream and swamp waters, with carbonate weathering being the primary source of the dissolved ions. The Nam Co lake water is characterized by high Na⁺ concentration and extremely low Ca²⁺ concentration relative to other ions, resulting from evapoconcentration and chemical precipitation within the lake. Comparison with the water chemistry of other lakes over the Tibetan Plateau indicated that Nam Co is located in a transition area between non-saline lakes and highly saline lakes. The relatively low concentration of total dissolved solids is possibly due to the abundant inflow of glacial meltwater and relatively high annual precipitation.     

Chert and its sodium-silicate precursors in sodium-carbonate lakes of East Africa

Chert has formed from two sodium-silicate minerals, magadiite (NaSi₇,O₁₃(OH)₃·3H₂O) and kenyaite (NaSi₁₁O_20.5(OH)₄·3H₂O), in uppermost Pleistocene deposits of lakes Magadi and Natron in Kenya and Tanzania. The chert consists of finely crystalline quartz and characteristically forms nodules of irregular shape with white coatings having reticulate surface patterns. Similar nodules are widespread in lower and middle Pleistocene lacustrine deposits in the vicinity of Lake Magadi, Lake Natron, and Olduvai Gorge. Although magadiite and kenyaite are absent in the lower and middle Pleistocene deposits, the chert in these beds probably formed from a sodium-silicate precursor. All of the chert-bearing sediments were deposited in saline, alkaline lakes rich in dissolved sodium carbonate-bicarbonate.Magadiite (and chert) may form either thin, widespread deposits or localized masses which may be cross-cutting. Thin, widespread layers of magadiite have been precipitated by mixing of silica-rich brine with fresh water in a chemically stratified lake; localized masses may have been formed by interaction of brine with fresher water entering the floor or margin of the lake. Magadiite and kenyaite can alter to chert in contact with sodium-carbonate brine and possibly by leaching with relatively fresh water over a period of 20,000 years or less.The siliceous zeolites clinoptilolite and erionite predominate in trachyte tuffs associated with magadiite and chert; less-siliceous phillipsite predominates in trachyte tuffs of chert-free sequences.     

Geochemistry of great Salt Lake,Utah II: Pleistocene-Holocene evolution

Sedimentologic and biostratigraphic evidence is used to develop a geochemical model for Great Salt Lake, Utah, extending back some 30,000 yrs. B.P. Hydrologie conditions as defined by the water budget equation are characterized by a lake initially at a low, saline stage, rising by about 17,000 yrs. B.P. to fresh water basin-full conditions (Bonneville level) and then, after about 15,000 yrs. B.P., dropping rapidly to a saline stage again, as exemplified by the present situation.Inflow composition has changed through time in response to the hydrologie history. During fresh-water periods high discharge inflow is dominated by calcium bicarbonate-type river waters; during saline stages, low discharge, NaCl-rich hydrothermal springs are significant solute sources. This evolution in lake composition to NaCl domination is illustrated by the massive mirabilite deposition, free of halite, following the rapid drawdown until about 8,000 years ago, while historic droughts have yielded principally halite.Hydrologic history can be combined with inferred inflow composition to derive concentration curves with time for each major solute in the lake. Calcium concentrations before the drawdown were controlled by calcite solubility, and afterwards by aragonite. Significant amounts of solutes are removed from the lake by diffusion into the sediments. Na⁺, Cl^? and SO₄^2? are also involved in salt precipitation. By including pore fluid data, a surprisingly good fit has been obtained between solute input over the time period considered and the amounts actually found in lake brines, pore fluids, salt beds and sediments. Excess amounts are present for calcium, carbonate and silica, indicating detrital input.