Israel Zak

The age curves of sulfur and oxygen isotopes in marine sulfate and their mutual interpretation

Abstract Three hundred new samples of marine evaporite sulfate, of world-wide distribution, were analyzed for δ34S, and 60 of these also for δ18O in the sulfate ion. Detailed δ34S age curves for Tertiary—Cretaceous, Permian—Pennsylvanian, Devonian, Cambrian and Proterozoic times document large variations in δ34S. A summary curve for δ18O also shows definite variations, some at different times than δ34S, and always smaller. The measured δ34S and δ18O correspond to variations in these isotopes in sulfate of the world ocean surface. The variations of δ18O are controlled by input and output fluxes of sulfur in the ocean, three of which are the same that control δ34S: deposition and erosion of sulfate, and deposition of sulfide. Erosion of sulfide differs in its effect on the S and O systems. δ18O in the sulfate does not seem to be measurably affected by equilibration with either seawater or with subsurface waters after crystallization. In principle, the simultaneous application of both δ34S and δ18O age curves should help reduce the number of assumptions in calculations of the cycles of sulfur and oxygen through geological time, and a new model involving symmetrical fluxes is introduced here to take advantage of the oxygen data. However, all previously published models as well as this one lead to anomalies, such as unreasonable calcium or oxygen depletions in the ocean—atmosphere system. In addition, most models are incapable of reproducing the sharp rises of the δ34S curve in the late Proterozoic, the Devonian and the Triassic which would be the result of unreasonably fast net sulfide deposition. This fast depletion could result from an ocean that has not always been mixed (as previously assumed in all model calculations).

Isotope geochemistry of oxygen in the sedimentary sulfate cycle

Abstract A re-evaluation of the isotopic geochemistry of oxygen involved in the exogenic sulfur cycle leads to a model in which δ 18OSO4 of seawater is determined by a dynamic balance of four fluxes: inputs by erosion of evaporite sulfate and by oxidative erosion of sulfide, and outputs of sulfate into evaporites and of carbon dioxide into seawater by bacterial reduction of sulfate to sulfide. The sulfate deposition-erosion loop is closed, but the sulfide loop is open, connected through the fixed reservoir of 18OH2O of seawater. The level of δ 18OSO4 is apparently not appreciably affected by either equilibration with 18OH2O, or by Lloyds proposed fast reduction-oxidation cycle on the sea floor. The possible effect of equilibration during a subsea hydrothermal circulation is unclear from available data. Calculations of hypothetical equilibration of 18 O 16 O SO 4 with 18 O 16 O H 2 O , either from the dehydration of gypsum or from formation waters, give δ 18OSO4 values much higher than any observed in old evaporites. Consequently, these processes were probably not significant in altering the δ 18OSO4 of seawater that is recorded in evaporites.

The geological history of Messinian (upper miocene) evaporites in the Central Jordan Valley (Israel) and how strontium and sulfur isotopes relate to their origin

Evaporites, comprising gypsum, anhydrite, halite and dolomite are described from the Messinian Bira Formation outcrops and from two boreholes (Newe Ur-2 and Zemah-1) in the Central Jordan Valley, Israel. Strontium and sulfur isotopic compositions of the evaporite minerals, and their Sr/Ca and Br/Cl ratios are used to interpret their enviromments of deposition and processes of formation and diagenesis. The brines from which the evaporites precipitated originated from seawater. The processes caused by mixing of surface water, seawater and subsurface brines, resulted in dolomitization and also sulfur reduction. The surface water and subsurface brines reacted with the rocks they drained, including Cretaceous and Eocene carbonates and Neogene basalts. The gypsum deposits in the Central Jordan Valley are interpreted to have formed as a result of evaporation of the magnesium-rich Lake Bira water which became oversaturated with respect to calcite and gypsum. The gypsum was deposited in stratified, relatively closed basins, where a partial reduction of the sulfur resulted in high δ34S of the precipitated gypsum. Gypsum and early diagenetic dolomite formed from the same water bodies. The Bira evaporites in Newe Ur-2 borehole, precipitated from mixtures of sea- and fresh waters with basaltic contribution. The samples from the Lower Gabbro and Halite Unit in the Zemah-1 borehole were deposited from evaporated seawater, which leached basaltic rocks, in closed basins; the Middle Halite Unit formed from seawater, whiles the brines that deposited the Upper Halite Unit leached also basalt rocks.

Bedforms in Salt Deposits of the Dead Sea Brines

ABSTRACT Study of salt deposits emphasizes the mineralogical and geochemical aspects of origin with but little attention to processes and features of physical sedimentation. Deposition from saturated brines along one of the main conveyance canals of the Dead Sea Works near Sedom (southern Israel) indicates, however, that in open-channel flow the halite sediment develops bedforms identical to those reported in alluvial channel deposits. The conveyance canal is trapezoidal, with side slopes of 1:2, bottom width of 17 m, bankfull depth of 1.5 m, and a gradient of 0.00022. Design Mannings n is 0.0223, decreasing to about 0.020 after deposition of the fine halite lining. Under normal operating conditions flow velocity is about 30 cm/s and depth is about 1 m. The conveyed brine, with a densi y of 1.29 and viscosity of 5 cp, is supersaturated with respect to halite, and deposition of halite occurs, aided by temperature fluctuations, mixing, and natural and pump-induced turbulence. The sediment consists of halite with minor amounts of carnallite, and it is very poorly sorted. A conventional interpretation of the grain-size distribution is not applicable, however, since the finer fractions, up to about 100 microns, consist of single, cube, block, or prism-shaped crystals, whereas the coarser material includes mainly porous and irregular clusters of crystals, and larger, ball-like aggregates, up to about 3 mm in diameter. Bedforms encountered along the canal include current crescents and shadows due to deformation of flow around obstacles such as bottom irregularities and salt l mps; transverse, linguoidal, and catenary ripples and megaripples, some of which may easily be mistaken for karrenlike products of solution mass transfer and for features of syneresis; and longitudinal ridge patterns due to three-dimensional systems of streamwise vortices. The comparison of results of Stokesian settling, hindered settling, and pipe flow of brines and of suspensions with different concentrations of solids indicates similar hydraulic behavior of salt/brine and quartz/water systems. After adjustment for particle and fluid properties, the observed bedforms fall into the same hydraulic factor fields as determined for bedforms in alluvial channels. Salt deposits, therefore, clearly belong to the wide group of chemical precipitates, ranging from igneous rocks to industri l slurry deposits, which record the process of mechanical transport and deposition.

Transformation of iron-bearing kaolinite to iron-free kaolinite, geothite, and hematite

Ferruginous clay partings in limestones of the marine, largely evaporitic, Upper Triassic Mohila Formation (Makhtesh Ramon, Israel) contain hexagonal plates and cube-like bodies up to one millimeter across. Analyses by electron microscopy, infrared and Mössbauer spectroscopy, and X-ray powder diffraction indicate that the matrix contains Fe-rich anhedral kaolinite, up to 100 µm in size; the hexagonal plates are composed of euhedral, Fe-free kaolinite covered with well-developed acicular goethite and platy hematite (0.5 to 2 µm in size), and the cubes consist of fine-grained goethite with minor amounts of kaolinite. The anhedral kaolinite appears to be detrital, the hexagonal plates to be authigenic, and the cubes to be pseudomorphs after pyrite. Oxidation appears to have altered Fe-rich kaolinite and pyrite to Fe-free kaolinite, goethite, and hematite and was accompanied by recrystallization and pseudomorphic replacement. The alteration process was slow and was probably induced by a small increase in pH and in the Al/Fe ratio, resulting from oxidation of reduced components (pyrite, ferrous carbonate, organic matter) in a semiclosed, sediment-mud system. Overlying kaolinitic flint clay deposits may be the final product of a similar process.РезюмеРезюме—В морских хемогенных известняках свиты Мохила (верхний триас р-на Махтеш Рамон, Израиль) встречаются тонкие обогащенные железом глинистые прослойки, содержащие гексагональные пластинки и кубики размером до 1 мм. Электронная микроскопия, инфракрасная и моссабауэровская спектроскопия и рентгеновская дифрактометрия позволили установить, что вмещающая порода состоит из ксеноморфных частиц железистого каолинита размером до 100 μм. Гексагональные пластинки сложены идиоморфным безжелезистым каолинитом. Пластинки покрыты хорошо окристаллизованными игольчатыми агрегатами гетита и пластинчатого гематита размером 0,5 — 2,0 μм. Кубики содержат тонкозернистый гетит со следами каолинита. Ксеномо-рфный каолинит по-видимому является обломочным, гексагональные пластинки—аутигенные. Кубики представляют собой псевдоморфозы по пириту. В результате окисления железистый каолинит и пирит были преобразованы в безжелезистый каолинит, гетит и гематит. Этот процесс сопровождался перекристаллизацией и псевдоморфизмом. Процесс замещения был медленным и вероятно был вызван небольшим увеличением рН и отношения Al/Pe как результат окисления пирита, железистых карбонатов и органического вещества в полузамкнутой системе рыхлого ила. Залегающие выше по разрезу слои каолиновых огнеупорных глин возможно являются конечным продуктом аналогичного процесса. [A. Cohen]ResümeeEisenhaltige Tonanteile in Kalken der marinen, hauptsächlich evaporitischen, obertriassischen Mohila Formation (Makhtesh Ramon, Israel) enthalten hexagonale Plättchen und würfelförmige Teilchen bis zu einem Millimeter Durchmesser. Untersuchungen mittels Elektronenmikroskop, Infrarot- und Mössbauerspektroskopie sowie Röntgenpulverdiffraktometrie deuten darauf hin, daß die Matrix Fe-reichen, xenomorphen, bis zu 100 µm großen Kaolinit enthält; die hexagonalen Plättchen sind idiomorphe Fe-freie Kaolinite, die mit gut kristallisiertem nadelförmigem Goethit, und plättchenförmigem Haematit (0,5 bis 2 µm groß) umgeben sind. Die Würfel bestehen aus feinkörnigem Goethit mit geringen Anteilen von Kaolinit. Die xenomorphen Kaolinite scheinen detritisch zu sein, die hexagonalen Kaolinite authigen und die Würfel dürften Pseudomorphosen nach Pyrit darstellen. Durch Oxidation scheint der Fe-reiche Kaolinit und der Pyrit in Fe-freien Kaolinit, Goethit, und Haematit umgewandelt worden zu sein, wobei es gleichzeitig zur Rekristallisation und zur Bildung von Pseudomorphosen kam. Der Umwandlungsprozeß ging langsam vor sich und wurde wahrscheinlich durch eine kleine Erhöhung des pH-Wertes und des Al/Fe-Verhältnisses verursacht, was aus der Oxidation der reduzierten Komponenten (Pyrit, Fe-haltige Karbonate, organisches Material) in einem halbgeschlossenen Sediment-Schlamm-System resultierte. Die überlagernden kaolinitischen Flintclay-Lagerstätten könnten das Endprodukt eines ähnlichen Prozesses sein. [U.W.]RésuméDes lamines d’argiles ferrugineuses dans les calcaires de la formation Mohila, marine et en grande partie évaporitique du Triassique Superieur (Makhtesh Ramon, Israel) contiennent des plaques hexagonales et des cubes mesurant jusqu’à un millimètre de large. Des analyses par microscopie électronique, par spectroscopie infrarouge et de Mossbauer, et par diffraction des rayons-X indiquent que la matrice contient de la kaolinite anhédrale riche en Fe mesurant jusqu’à 100 µm; les plaques hexagonales sons composées de kaolinite euhédrale sans Fe couverte de goethite aciculaire bien dévelopée et d’hématite en plaquettes (mesurant de 0,5 à 2 µm), et les cubes consistent de goethite à fins grains avec des quantités moindres de kaolinite. La kaolinite anhédrale semble être detritique, les plaques hexagonales authigéniques, et les cubes semblent être des pseudomorphes après la pyrite. L’oxidation semble avoir altéré la kaolinite riche en Fe et la pyrite en kaolinite sans Fe, goethite et hématite, et avoir été accompagnée d’une recristallisation et d’un remplacement pseudomorphique. Le procédé d’altération était lent et a été probablement induit par une faible augmentation du pH et de la proportion Al/Fe résultant de l’oxidation de composants réduits (pyrite, carbonate ferreux, matière organique) dans un système sédiment-boue miconfiné. Des dépôts susjacents de “fint-clay” kaolinitiques pourraient être les produits finaux d’un procédé semblable. [D.J.]

Saline waters and residual brines in the Shiraz-Sarvistan basin, Iran

Abstract Origin of saline waters in the Shiraz-Sarvistan area, Iran, is determined by a combined isotopic ( 18 O and D) and chemical characterization. Four types are recognized: (a) fresh water of the anticlinal carbonatic aquifer; (b) fresh and brackish runoff in the synclinal basins; (c) salt springs originating through dissolution of rock salt by type (a) fresh water; and (d) residual brines formed in synclinal closed drainage basins, through evaporation of former water types and loss of the relatively less-soluble salts.

The distribution and behavior of iron in sequences of dolomites, clays and oxides

Abstract Marly beds, containing Fe 2+ - and Fe 3+ -bearing kaolinite and iron oxides in the form of akaganeite and hematite are found, interbedded with Fe 2+ -containing dolomites and pure kaolonite in the Jurassic Ardon Formation, Makhtesh Ramon, Israel. The envisaged reactions leading to the formation of these assemblages are: (1) intensive leaching in a tropical environment, leading to the formation of Fe-rich kaolinites and iron oxides; (2) deposition in a calcium chloride brine environment, reducing in its lower part; and (3) reduction and leaching of Fe in the reducing zone and redeposition in the oxidizing upper zone.

The geological history of pliocene-pleistocene evaporites in Mount Sedom (Israel) and how strontium and sulfur isotopes relate to their origin

Evaporites, comprising of gypsum, anhydrite and halite are described from the Pliocene Sedom Formation, the Caprocks units and the Pleistocene ‘Amora Formation in Mount Sedom, Southern Jordan Valley, Israel. Strontium and sulfur isotopic compositions of the evaporite minerals, and their Sr/Ca and Br/Cl ratios were used to interpret their environments of deposition and processes of formation and diagenesis. Some of the evaporites of the Sedom Formation were deposited from evaporated seawater. Others were deposited from a mixture of seawater and brines. The brines were composed of seawater which penetrated the carbonate rocks of the Rift margins, participated in dolomitization processes and, when hydrologic conditions allowed, seeped out into the Sedom basin and were mixed with evaporated seawater. These processes yielded non-homogeneous fluid masses of mixtures, as indicated by their wide range of87Sr/86Sr ratios (0.70824–0.70905) as compared to the narrow Sr/Ca ratios of the derived evaporites. Their marine origin is indicated by their δ34S values which are around 20‰ The evaporites of the ‘Amora Formation were precipitated from Ca-chloride brines only, which were originally evaporated seawater trapped in the Rift walls in the “Sedom Formation times”, returning to the Sedom basin after its disconnection from the sea. The high Sr content and Sr/Ca ratios indicate that the anhydrites existing today in the Sedom and ‘Amora formations were originally deposited from the evaporated seawater as gypsum which was later recrystallized at depth, at high pressures and temperatures. The caprocks are residual rocks of marine origin, formed by the dissolution of the exposed rock-salt units in the Sedom Formation. They represent mainly the gypsum (or anhydrite) beds intercalated in the rock-salt units of the Sedom Formation in Mount Sedom rather than Ca-sulfates disseminated in the halites.