Amitai Katz

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.

The dolomitization of CaCO3: an experimental study at 252–295°C

Abstract The results of experiments on the hydrothermal dolomitization of calcite (between 252 and 295°C) and aragonite (at 252°C) by a 2 M CaCl 2 -MgCl 2 aqueous solution are reported and discussed. Dolomitization of calcite proceeds via an intermediate high (ca. 35 mole %) magnesian calcite, whereas that of aragonite is carried out through the conversion of the reactant into a low (5.6 mole %) magnesian calcite which in turn transforms into a high (39.6 mole %) magnesian calcite. Both the intermediate phases and dolomite crystallize through a dissolution-precipitation reaction. The intermediate phases form under local equilibrium within a reaction zone surrounding the dissolving reactant grains. The volume of the reaction zone solution can be estimated from Sr 2+ and Mg 2+ partitioning equations. In the case of low magnesian calcite growing at the expense of aragonite at 252°C, the total volume of these zones is in the range of 2 × 10 −5 to 2 × 10 −4 1., out of 5 × 10 −3 1., the volume of the bulk solution. The apparent activation energies for the initial crystallization of high magnesian calcite and dolomite are 48 and 49 kcal/mole, respectively. Calcite transforms completely into dolomite within 100 hr at 252°C. The overall reaction time is reduced to approximately 4 hr at 295°C. The transformation of aragonite to dolomite at 252°C occurs within 24 hr. The nature of the reactant dictates the relative rates of crystallization of the intermediate phases and dolomite. With calcite as reactant, dolomite growth is faster than that of magnesian calcite; this situation is reversed when aragonite is dolomitized. Coprecipitation of Sr 2+ with dolomite is independent of temperature (within analytical error) between 252 and 295°C. Its partitioning, with respect to calcium, between dolomite and solution results in distribution coefficients in the range of 2.31 × 10 −2 to 2.78 × 10 −2 .

Diagenesis in live corals from the Gulf of Aqaba. I. The effect on paleo-oceanography tracers

Abstract The effect of early diagenesis on trace element abundance in coral skeleton was studied in live coral heads (Porites) from the Nature Reserve Reef (NRR), Elat, Gulf of Aqaba, northern Red Sea. Petrography of the corals shows diagenetic features of dissolution, recrystallization, and secondary aragonite precipitation (pore filling), which are most extensive in the oldest part of the coral. Coral porewaters were extracted with a special setup and were analyzed for chemical composition. The total alkalinity and Sr deficit in pore water as compared to reef water is consistent with both precipitation of secondary aragonite and recrystallization of primary skeleton. The present rate constant of pore filling by secondary aragonite was estimated by a water replacement experiment to be 0.0015 y−1, which equals to pore filling rate of 1.5 ± 0.3 kg aragonite per year. The corals show clear seasonal fluctuations in Sr/Ca ratios that are interpreted as reflecting changes in sea surface temperature (SST). Yet, the secondary aragonite is characterized by a significantly higher Sr/Ca ratio than the average ratio in primary aragonite. Thus, measuring a mixed sample of pristine and secondary aragonite may produce erroneous (about 2°C lower) SST estimates by the Sr/Ca thermometers. It appears that the Sr/Ca ratio, a major proxy for paleo-environmental marine studies, is sensitive to subtle pore-filling and replacement of the original coral matrix by secondary aragonite in the marine environment.

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 formation of natural cryogenic brines

Abstract The source of salts in the Ca-chloridic, hypersaline brines (up to 190 g Cl L −1 ) occurring in crystalline basement rocks in the Canadian, Fennoscandian and Bohemian Shields and their evolution have been investigated and reported. The Cl-Br-Na relationship indicates that these waters have been concentrated from seawater, by freezing during glacial times. The Na/Cl ratio (0.25 to 0.35) in the more saline fluids is compatible with cooling down to −30°C, where the most saline waters have been concentrated by a factor of 25 to 30 relative to the parent seawater. The brines formed from seawater within cryogenic troughs, along the subarctic continental margins, around ice sheets. The depressions within which the brines formed are the cryogenic analogues of the classic, evaporitic lagoon. One million years suffice to saturate with brine a 2000km-radius by 1km-depth rock volume at an H 2 O removal rate of only 2.8 mm/yr. Density-induced brine migration on a continental scale takes place via fissures below the ice. Our calculations, that were performed on a hypothetical ice sheet with dimensions compatible with the Laurentide ice sheet, demonstrate that during 1m.y., a 60m thick cryogenic sediment section could have formed. However, the precipitated minerals (mirabilite and hydrohalite) are repeatedly dispersed by the advance and retreat of the ice sheet, dissolved by melt water-seawater mixtures, and eroded during postglacial uplift, leaving almost no trace in the geological record. The cryogenic brines formed intermittently during and between glacial periods. The repeating advance and retreat of the ice sheets exerted a major control on the direction and intensity of brine flow. The cryogenic concentration of seawater and the migration of brine towards the center of the glaciostatic depression occurred mainly during the build up of the ice sheet, while reversal of the water flow from the center of the cryogenic basin outwards happened upon deglaciation. The flow of the waters in the subsurface was, inevitably, accompanied by significant dilution with melt water from the ice sheets. Using a “granitic” U concentration of 4 ppm and a (Ca-Mg mass balance based) rock/water ratio anywhere between 3.4 and 6.8 kg L −1 , a few hundred thousand years of brine-rock interaction are sufficient for the growth of 129 I in the most saline Canadian Shield brine to its present concentration (3.4×10 8 atoms 129 I L −1 ). Hence, both the formation of the saline fluids and their emplacement in their present sites occurred most likely within the Pleistocene. The young age calculated for cryogenic brines in crystalline shields and the dynamic water flow therein should raise concern about the planning and construction of high-grade nuclear waste repositories in such rocks, which are already under way.

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.

Ra isotopes and Rn in brines and ground waters of the Jordan-Dead Sea Rift Valley: Enrichment, retardation, and mixing

Abstract Thirty-six springs and wells from the Dead Sea Rift Valley were periodically sampled and analyzed. The latter included full chemical analyses, 222 Rn and 226 Ra (by α-counting emanometry), as well as 228 Ra/ 226 Ra and 224 Ra/ 228 Ra activity ratios (γ spectrometry). Sampling stretched over almost 2 yr. Several hundred Rn-Ra analyses and close to a hundred isotopic ratios were measured. Most waters in the Rift Valley have both elevated Ra (several to 750 dpm/L) and Rn (a few hundred to almost 60,000 dpm/L) content. In practically all samples 222 Rn activity considerably exceeds that of its parent 226 Ra. The Ra content is the result of all Dead Sea Rift Valley waters being mixtures of fresh water with saline brines. Ra is efficiently extracted from surrounding rocks into the brine end member. 228 Ra/ 226 Ra ratios are exceptionally low −0.07 to 0.9, mostly less than 0.2. This apparently reflects the U over Th enrichment in the source rocks that contribute the Ra. Ra enrichment (both 228 Ra and 226 Ra) is locally correlated throughout the Rift Valley with water salinity. This correlation can be used to constrain the age of the brine-freshwater mixing process. In one of the hydrologic subsystems studied (the Fuliya block), the mixing of the shallow ground water must have occurred in less than 200 to 300 yr ago, probably before no more than some 30 yr. High radon activities in surface waters along the Dead Sea Rift Valley result from the formation of radium-enriched linings on the aquifer rock surface. The mixing of the radium-extracting brines with fresh water leads to continual adsorption of radium as water salinity decreases. The unique combination of fast upward flow and continual mixing in the Dead Sea Rift Valley accounts for a constant replenishment of radium in the waters. This causes a gradual buildup of a radium lining on the aquifer walls until eventually a steady-state surface activity is established.