A. Starinsky

Strontium behavior in the aragonite-calcite transformation: An experimental study at 40–98°C

The behavior of strontium during the replacement of aragonite by calcite, in a closed system between 40°C and 98°C, has been experimentally investigated. The experiments were conducted in CaCl2 solutions, with and without NaCl. The distribution coefficient of strontium in calcite (λSr2+C) was found to be affected only slightly by temperature changes, and almost insignificantly by the presence of NaCl. λSr2+C values at 0.01 mCa2+ (its concentration in normal sea water) are: 0.055 at 40°C and 0.058 at 98°C. These results indicate that the low (around 500 ppm) concentration of strontium in ancient limestones could have been brought about by aragonite-to-calcite transformation in a system open to sea water, and are not necessarily indicative of replacement in fresh waters.

Strontium isotopic, chemical, and sedimentological evidence for the evolution of Lake Lisan and the Dead Sea

Precise strontium isotope ratios, combined with chemical analyses and sedimentological information, are used to monitor the water sources and the evolution of the Dead Sea and its late Pleistocene precursor, Lake Lisan (70-18 kyr B.P.). The materials analyzed include bulk aragonite, water-leached soluble salts, and residual aragonite and gypsum from the Lisan Formation in the Perazim Valley (near the SW shore of the Dead Sea). The residual aragonite and the associated soluble salts display systematic fluctuations in 17Sr86Sr ratios between 0.70803 and 0.70806 and from 0.70805 to 0.70807, respectively. In individual soluble salt-residual aragonite pairs, the soluble salt displays a higher 87Sr86Sr ratio. Gypsum samples yield 17Sr86Sr ratios similar to the soluble salts from adjacent layers in the section. This shows that, in individual samples, the source of Sr in aragonite was distinct from that in soluble salts and the gypsum. The sterility of the Lisan sediments, their strictly nonbioturbated fine lamination, and their high content of chloride salts indicate that Lake Lisan was a saline, or even hypersaline water body. In the absence of alternative sources of HCO3− and S042− the abundance of primary aragonite and gypsum in the Lisan column reflects an import of very large volumes of freshwater into the otherwise saline lake, resulting in a density stratification of this water body. The history of the upper water layer and that of the lower brine is reflected in the chemical and strontium isotope composition of the aragonite and in that of the associated soluble salts and in the gypsum samples, respectively. Whereas the bicarbonate and much of the Ca2+ required for aragonite crystallization were supplied by the freshwater, the complementary Ca2+ (and Sr 2+) were added by the lower brine. The upper water layer of Lake Lisan acted as a SO42− capacitor during the lakes rise periods. It was removed therefrom, as prominent gypsum beds, upon climatic-induced (drier period) mixing or even complete overturn of the lake. The evolution of Lake Lisan took place between two distinct modes. The first was characterized by an extensive supply of freshwater and resulted in a rise of the lakes level, a (density) layered structure, and precipitation of aragonite. The second mode was marked by a diminishing freshwater input, resulting in mixing or complete overturn of its water, and precipitation of gypsum. These two modes reflect the climatic evolution of the region in the late Pleistocene which fluctuated between drier and wetter periods. The transition to the Holocene is accompanied by the dry up of Lake Lisan and its contraction to the present Dead Sea.

The origin and evolution of Canadian Shield brines: evaporation or freezing of seawater? New lithium isotope and geochemical evidence from the Slave craton

Abstract New chemical and isotopic data for deep seated calcium chloride brine from the Miramar Con gold mine, Yellowknife N.W.T., strongly suggest that the brine salinity is of marine origin. Diagnostic marine properties include uniformly elevated Br/Cl ratios typical of seawater concentrated beyond halite saturation, and Li/Br ratios (0.0254–0.0325) and δ 6 Li compositions (−32.1 to −36.3‰) similar to seawater (−32.3‰). The mean δ 6 Li for all mine water samples of −35.1‰ may reflect minor uptake of Li by secondary minerals. This interpretation is supported by analyses of altered metabasalt from fault zones which is enriched in Li but depleted in δ 6 Li (−14.7 to −15.6‰) relative to the unaltered metabasalt (−5.4‰). The mechanism responsible for concentrating the hypersaline brine end member is not unequivocal as evidence exists to support both evaporative and cryogenic processes. On the one hand, the Devonian sedimentary record in the Great Slave Lake region, in conjunction with Yellowknife brine isotopic compositions ( δ 2 H and δ 34 S SO 4 ) that are similar to various Devonian fluids, support an evaporative origin. On the other hand, the Na/Cl–Br/Cl relationship in the brine strongly suggests a cryogenic mechanism. Regardless of the concentrative mechanism, the chemical data indicate that the Yellowknife parent brine was concentrated 28- to 30-fold relative to seawater. The extreme depletion of Mg and enrichment of Ca in the brines, accompanied by Sr/Ca ratios similar to that of seawater, are accounted for by dolomitization of an aragonite-rich marine sediment by the brine before infiltration into the crystalline basement rocks. Subsequent alteration of silicate minerals in the shield added additional Ca and Sr to the brine as indicated by their radiogenic 87 Sr / 86 Sr ratios (up to 0.7147). Based on mineral balance calculations, the major mineral products of the cryogenic and evaporitic concentration and evolution paths are significantly different. The cryogenic evolution results in some 15% mirabilite, 60% hydrohalite, and 18% dolomite whereas the major minerals formed from the evaporitic evolutionary sequence are 36% halite, 8% gypsum, 17% dolomite, and 30% albite. The great similarity between the calcium chloride brine from Yellowknife and other such Canadian Shield brines indicates that they may share a common marine origin.

Chemical and boron isotope compositions of non-marine brines from the Qaidam Basin, Qinghai, China

The general chemistry and boron isotope composition were investigated in fresh waters and hypersaline brines from the Qaidam Basin, northern Qinghai-Xizang (Tibet) Plateau, China. This basin is a large, tectonically active system, isolated from the ocean and composed of thick clastic and evaporite sediments. The modern playas are subject to intense evaporation and are characterized by hypersaline brines as well as potash and borate evaporites. The chemical composition of the dissolved solutes in the modern brines and waters reveals three main sources: (1) inflow of hot springs enriched in sodium, sulphate and boron. Evaporation of these waters leads to a high Na/Cl ratio (>1), a NaClSO4 brine and an evaporite mineral assemblage of halite-mirabilite-borate (Lakes Daqaidam and Xiaoqaidam); (2) inflow surface river waters which are modified by preferential dissolution of halite and potassium and magnesium salts characterized by a Na(Mg)Cl solute type with low Na/Cl (<1), BrCl,LiCl and BCl ratios; and (3) Ca-chloridic subsurface brines which are controlled by both salt dissolution and dolomitization processes. Evaporation and salt crystallization of mixtures of the latter two types leads to a “marine-like” brine (e.g., NaMgCl type, NaCl ratio ⪡ 1) and mineral assemblages similar to that predicted for progressive evaporation of seawater (e.g., Qarhan playa: halite-sylvite-carnallite-bischofite). The δ11 B-values of the input waters to the Qaidam Basin (range of −0.7 to + 10.9%. vs. NBS-951 and brines from salt lakes (+0.5 to + 15.0%.) are similar to those of associated granitic rocks (δ11B= −2.3 to +3.7%.; n=3) and hence indicate the non-marine origin of these fluids. The highest δ11B-values are associated with fluids with low BLi ratios, indicating selective removal of elemental boron and 10B by adsorption onto clay minerals. The magnitude of 11B enrichment due to adsorption is ∼15–20%., and thus non-marine brines are well distinguished from marine-derived brines (δ11B of +39 to +59%.) preserving the large isotopic difference (∼40%.) of their source waters. It is proposed to use this distinctive isotopic signature for tracing the origin of ancient evaporite environments.

The role of seawater freezing in the formation of subsurface brines

Abstract Several mechanisms (evaporation, water-rock interaction, ultra-filtration) have been suggested to explain the evolution of ubiquitous Ca-chloride subsurface brines. In the present paper, the freezing of seawater in polar regions, and in even wider areas during glacial periods, is proposed as an additional possible path of brine formation. Four detailed seawater freezing experiments to −14°C (resulting in a concentration factor of about 5) were carried out, and Na, K, Ca, Mg, Sr, Cl, SO4, and Br were analysed in the residual brines and in the ice. Br and Sr, whose behavior during the freezing of seawater is reported here for the first time, show a conservative behavior throughout the studied temperature range. Our data and earlier literature show that the high salinities, which are common in subsurface brines (>300 g/l), may be obtained by the removal of H2O as ice in the primary glacial environment. The decrease in the Na Cl ratio is caused by the crystallization of mirabilite (Na2SO4 · 10H2O), supplemented by hydrohalite (NaCl · 2H2O). Sulfate is removed both in mirabilite and by bacterial reduction. The brine then migrates to the subsurface, heats-up under the local geothermal gradient, and interacts with the adjacent rocks. At this stage, it may be diluted by meteoric waters, its Mg Ca ratio decreases (dolomitization and chloritization), the SO 4 Cl ratio varies according to the local gypsum-anhydrite equilibrium conditions, and the Ca (SO 4 + HCO 3 ) ratio increases as a result of dolomitization or chloritization. The interaction with rocks in the subsurface may affect both the original 87 Sr 86 Sr and the 18 O 16 O ratios of the brine. Although several of the processes which lead to the formation of Ca-chloride brines are common for both the evaporative and the freezing models, the Na-Br-Cl relationship in a given brine can be used to discriminate between the two modes of brine evolution. Several subsurface brines from the Canadian Shield and one brine from Finland are used as examples of the seawater freezing model, and an explanation is proposed for the necessary mass production of brines in glacial environments.

Strontium in rainwater from Israel: Sources, isotopes and chemistry

Abstract Eighteen rain samples from Israel have been analyzed for their chemical composition and87Sr/86Sr ratios. The Sr-isotopic ratios lie in the range 0.7078 and 0.7092, and the Sr concentrations vary from 1 × 10−4 to 9 × 10−4 meq Sr/l. Soluble salts in rainwater are inherited from three major natural sources, seaspray, Recent marine minerals and mineral dust eroded from rock outcrops and soil. A mixing model is formulated to apply the chemical composition of rain (Cl− and Sr2+) and its isotopic87Sr/86Sr ratio, for the identification and estimation of the Sr sources. All the samples fall within the mixing space predicted by the model for the three end members mentioned above. The data indicate that the most important non-seaspray source contributing dissolved salts to the rains in Israel comprises a mixture of Senonian to Eocene chalk (and its weathering products) and Recent marine minerals, from local and imported sources. Most of the samples (67%) contain 50% or more non-seaspray Sr (i.e., Sr dissolved from dust or Recent marine minerals), whereas 56% of the samples display87Sr/86Sr ratios lower than 0.7090. The rest represent mixtures of seaspray and Recent marine minerals.

The impact of brine-rock interaction during marine evaporite formation on the isotopic Sr record in the oceans: Evidence from Mt. Sedom, Israel

The effect of brine-rock interaction on the composition of strontium in evaporitic basins and its impact on the 87Sr/86Sr ratios in contemporaneous seawater are examined for the Sedom (Dead Sea Rift Valley, or DSR), the Messinian (Mediterranean) and the Louann (Gulf of Mexico) evaporites. For that purpose, mineralogical, chemical and isotopic (Sr, S) analyses were performed on the Sedom Fm. evaporites (halite, anhydrite and dolomite). 87Sr/86Sr ratios are distinctively lower in the Sedom evaporites (dolomites: 0.7082–0.7083; halites: 0.7083–0.7087) than in the contemporaneous late Pliocene seawater (≈0.709). At the same time the sulfur isotope ratios (δ34S ≈ 20‰) are consistent with deposition from late Cenozoic seawater. This duality, together with the variation of strontium isotopes between the dolomites and halites can be explained by modification of the 87Sr/86Sr ratio in the lagoon water by influx of Ca-Chloride brines. The brines were formed by dolomitization of marine carbonates of the DSR Cretaceous wall rocks (where 87Sr/86Sr ∼ 0.7077). Brine-rock interaction can similarly explain the anomalous 87Sr/86Sr ratios in the Messinian and Louann evaporites. It is concluded that this process causes significant changes in the 87Sr/86Sr ratios of evaporitic lagoons. A water and strontium mass balance of the Sedom data is used to show the impact on the strontium oceanic budget. Extrapolation to larger evaporitic basins indicates that the combined global riverine and hydrothermal influx of strontium can be matched by halite or gypsum precipitating lagoon of 2–3.5 × 105 km2. Examples for such evaporitic sites include the Messinian, Louann and Zechstein basins.

Thermodynamic constraints on Dead Sea evaporation: can the Dead Sea dry up?

Abstract The relation between climatic parameters (relative air humidity) and the water activity of the Dead Sea water determines the possible maximum evaporation of the lake. Using the Pitzer thermodynamic approach, the activity of the Dead Sea water during evaporation was calculated and compared to the present relative air humidity above the water. Long-term (1992–1997) quasi-continuous meteorological data acquired at sea provide detailed information on the patterns and trends of the relative humidity above the lake. Present climatic conditions allow the Dead Sea water to evaporate down to a water activity of 0.50, corresponding to the lowest air humidity measured over the lake. This water activity falls in the range of halite precipitation, while carnallite precipitation starts somewhat lower ( a H 2 O =0.49). Our dynamic model predicts that for air humidity as low as 50% (reflecting present climate conditions), the Dead Sea level may drop to as low as −500 m (i.e., 500 m below mean sea level). At that point, the lake will have a volume of 88 km 3 and a surface area of 526 km 2 . For the sake of comparison, at the beginning of 1977, after the southern basin of the Dead Sea was separated from the northern basin, the level of the lake was −402 m, its volume was 146 km 3 , and its surface area was 815 km 2 .

Behaviour of strontium in subsurface calcium chloride brines: Southern Israel and Dead Sea rift valley

Calcium chloride brines are, as a rule, relatively rich in strontium, but the enrichment is usually limited and is found to be related to the concentration of calcium. The limiting mechanisms were evaluated as a model which comprises simple interactions between minerals and solutions. Based on the known ranges of strontium concentration in minerals, mineral solubilities and partition coefficients of strontium (both poorly known in certain cases), six fields of SrCa molar ratios were defined in terms of participating minerals and processes: (a) 0.38−1.56 × 10− 3 by dolomitization of calcite; (b) 1.5−2.2 × 10− 2 due to dolomitization of aragonite; (c) 0.4−1.4 × 10− 2 as a result of solution-reprecipitation of calcite; (d)0.12−0.20 through transformation of aragonite to calcite; (e)0.10−0.60 through equilibrium of the pair calcite-strontianite; and (f)0.01−0.08 by equilibrium with gypsum and celestite. The model was applied to the analysis of two groups of brines from southern Israel which are originated in the coastal plain (group C) and in the rift valley (group R). The low MgCa ratios of both water groups point to dolomitization as the main subsurface modifying process. SrCa ratios of brines belonging to group C are consistent with dolomitization of aragonitic surface sediments at the beginning of their evolution. Brines of group R bear evidence to a similar pathway at the beginning of their evolution, but most of them were further affected by interaction with limestone.

40Ar39Ar dating of combustion metamorphism (“Mottled Zone”, Israel)

Tertiary surface combustion of Cretaceous bituminous sediments in Israel produced high-T, low-P metamorphic assemblages, which were dated by conventional KAr and 40Ar39Ar techniques. Meaningful ages were obtained from samples of high metamorphic grade and K content higher than 0.8%, containing sanidine, anorthite, gehlenite and zeolites. In these minerals the KAr system has been reset, and equilibrated with atmospheric Ar, yielding a mean age of 3 ± 1 Ma. 40Ar39Ar plateau and isochron ages for four whole-rock samples range between 2.3 and 4.0 Ma. One gehlenite rock yielded an 40Ar39Ar age of 16 ± 0.2 Ma (1σ). These results indicate a major combustion metamorphic event around 3 Ma, probably preceded by an event around 16 Ma. The early event took place upon initial exposure of the protolith in the Miocene, whereas the second followed the removal of Neogene sediments and re-exposure of the bituminous rocks.