Juan J. Pueyo

Oxygen and sulphur isotope compositions as indicators of the origin of Mesozoic and Cenozoic evaporites from Spain

Abstract The general investigation of oxygen and sulphur isotope compositions of the main evaporitic formations of Mesozoic and Cenozoic ages from Spain, for primary purposes, had to differentiate between marine and continental sulphate evaporites. Furthermore, the isotope data would also make it possible to determine the source of sulphate of the continental evaporites. The δ-values measured for the marine evaporites of Spain of Triassic, Lias, Cretaceous and Eocene age, are in good agreement with the general oxygen and sulphur isotopic curves of the oceanic sulphate. The continental evaporites deposited in the Ebro, Tajo, Calatayud and Teruel basins, since Paleocene up to Miocene times, exhibit wide ranges of δ-values both through time and space. The main source of sulphate reworked by continental waters was Triassic evaporitic sulphate, although another one was locally Cretaceous evaporite. In addition, in continental formations the local conditions of drainage and of in situ bacterial activity acted to modify the isotope composition of the sulphate source, thus imprinting specificity of the isotopic signature of each basin.

Characterization of model evaporitic environments through the study of lipid components

Abstract A saline circuit has been studied as a model of coastal evaporite system. The lipid composition of diverse salt ponds encompassing calcite, gypsum and halite domains has been determined. Most of the lipid materials has been found in the carbonate samples and is related to algal/cyanobacterial debris. Lipids in the gypsum domain are of heterotropic microbialorigin and extreme halophilic bacteria constitute the main lipid contribution in the halite samples. n-Alkane distributions with high predominance of n-docosane constitute a previously described feature of sedimentary evaporitic conditions that is charateristics of the intermediate calcite/gypsum samples. In the calcite domain, the presence of C20 highly branched isoprenid olephines, tetrahymanol and the large amounts of phytol constitute likely precursors of lipids usually found in evaporitic environments (i.e. C20 highly branched isoprenoid alkanes, gammacerane and high phytane/pristane ratios). Their occurence point to dehydration and hydrogenations as two main diagenetic processes leading to the formation of “evaporitic molecular markers”.

ISOPRANYLGLYCEROL DIETHERS IN NON-ALKALINE EVAPORITIC ENVIRONMENTS

Abstract Gas chromatographic-mass spectrometric analyses of Tertiary and recent halite samples using high-temperature gas Chromatographic columns have shown that O-phytanyl-O-sesterterpanylglycerol and bis-O-phytanylglycerol are common constituents in halites deposited in non-alkaline systems. These isopranylglycerol diether distributions are in contrast with those found in carbonates and most sulphates deposited in solar salterns or in sediments of mesohaline lagoons, which contain only the C20-C20 homolog. The investigation of the distributions of these compounds in thirteen species of non-alkaliphilic halobacteria has revealed that bis-O-phytanylglycerol occurs as the single homolog in most cases, namely Halobacterium, Haloferax, and Haloarcula spp., but O-phytanyl-O-sesterterpanylglycerol has been found in one Halococcus species. The presence of this C20-C25 diether in this genus of extremely halophilic archaebacterial cocci expands the limited number of organisms known previously as potential precursors of this isopranylglycerol, alkaliphilic halobacteria, and Methanosarcina spp. and indicates that its widespread occurrence in recent and Tertiary halites is related with contributions from non-alkaliphilic halobacteria.

Evolution and geochemical signatures in a Neogene forearc evaporitic basin: the Salar Grande (Central Andes of Chile)

Abstract The Salar Grande is a 45-km-long (N–S axis), 4–5-km-wide, fault-bounded (pull-apart) Neogene forearc evaporitic basin, located in the Cordillera de la Costa of northern Chile whose sedimentary record is composed almost exclusively of a massive halite salt body. Underground waters coming from the east (now the Precordillera and Altiplano) were progressively enriched in solutes by interaction with volcanic rocks. These fluids drained into a lacustrine system located in the Depresion Central (in the Llamara–Quillagua area) and thick sequences of diatomites, carbonates, evaporites and clastic sediments were deposited. These lacustrine areas eventually dried out and became a salar with anhydrite and halite. During this stage (Late Miocene–Pliocene), evolved brines of the Llamara–Quillagua area migrated eastwards through structural paths, reaching the Salar Grande basin which acted as a final sink. A proof of this connection is the anhydrite level that links the western Llamara and southeastern Salar Grande areas. The accessory minerals and geochemical signatures in the rock salt are a record of the inputs arriving to the former lake and reflect variations in climate, and in tectonic and volcanic activity. The arrangement of halite textures along the sequence shows an evolution from a very shallow ephemeral saline lake (some cm–dm deep on average) to a salar where interstitial processes controlled by phreatic brines dominate. This evolution is coherent with a slight increase in the terrigenous content of the rock salt, and documents the increasing aridity of the climate as a response to the tectonic uplift of the Andean Ranges. In Pliocene times pore brines were definitively lost as a response to the downfall of the water table generated by the beginning of the exorheism due to the opening of the Loa River canyon towards the sea. The aim of this paper is to describe the Salar Grande through the study of its geological setting and its salt record. A detailed description of the salar will enable us to reach valuable conclusions about environmental conditions in the area during the Neogene, prior to the opening of the hydrologic system to the Pacific Ocean.

Solute inputs in the Salar de Atacama (N. Chile)

Abstract The provenance of saline inputs to the Salar de Atacama has been studied by means of the isotopic composition of sulphates dispersed in drilling core samples and the analysis of the water of surface inflows. The pattern shown by the ratios of 87Sr/86Sr and the isotopic composition of sulphur and oxygen in sulphates, suggests a mixing of two water inputs: one coming from the north-east of the basin, higher in 87Sr/86Sr, δ34S and δ18O and the other from the south-west, lower in 87Sr/86Sr, δ34S and δ18O. Analysis of the recent inflows to the Salar have confirmed the existence of two solute sources: northern and eastern inputs show an enrichment in those elements related to hydrothermalism and volcanism, in relation to water present in the west of the basin.

Relación de aguas superficiales y subterráneas en el área del lago Chungará y lagunas de Cotacotani, norte de Chile: un estudio isotópico

El lago Chungara y las lagunas de Cotacotani corresponden a cuerpos de aguas superficiales localizados al noroeste de la Cuenca del Lauca, en el Altiplano del norte de Chile. Todas las aguas superficiales y subterraneas tienen una salinidad baja a moderada con conductividades electricas variables entre 48,7 y 3090 µS/cm. Los lagos estan conectados a acuiferos situados en los flancos de los edificios volcanicos proximos, asi como en los depositos de brechas de avalancha del volcan Parinacota. Las composiciones isotopicas de d18O y dD y su relacion con las concentraciones de cloruro de las aguas del lago Chungara, lagunas de Cotacotani, manantiales afluentes y del rio Chungara, demuestran que: a. las aguas del lago Chungara presentan una composicion quimica e isotopica homogenea tanto en la horizontal como en la vertical, lo que indica una buena mezcla de las aguas en el lago; b. una parte importante de la recarga hidrica que alimenta las lagunas de Cotacotani procede de aguas del lago Chungara, canalizada a traves de un flujo lateral subterraneo y c. el agua subterranea que da origen a los manantiales de la zona tiene su recarga principal en las precipitaciones que se registran durante los meses de primavera-verano (octubre-marzo). Los altos contenidos de tritio (3H) medidos en las aguas de los manantiales del sector indican que existe una recarga actual (durante los ultimos decenios) al sistema acuifero. La aplicacion de modelos de parametro agregado para la interpretacion del tiempo de residencia del agua en el acuifero indica que el modelo de flujo de piston es el que mejor se ajusta a las caracteristicas isotopicas de estas aguas subterraneas.

Polyhalite occurrence in the Werra (Zechstein, upper Permian) peribaltic basin of Poland and Russia: Evaporite facies constraints

Polyhalite is a common constituent of many ancient evaporite sequences, especially Permian and Neogene ones, that is related to the Na−K−Mg−Cl−SO4 type of marine brines in those time intervals. There are four polyhalite deposits in the Zechstein of northern Poland, and more than ten polyhalite-bearing areas in the adjacent part of Russia, and they are commonly accompanied by K−Mg chlorides. Most polyhalite occurrences are related to the upper part of the Lower Werra Anhydrite and in most cases, polyhalite deposits are concentrated at the sulfate platform close to its boundary with platform slope, where they can pass horizontally into polyhalite beds occurring in the Oldest Halite. The bromine content in samples of the Oldest Halite range from 40–120 ppm and the composition of fluid inclusions in halite are characteristic of halite precipitated from seawater concentrated to the early and middle stages of halite precipitation. The δ18O and δ34S values for sulfates are 10.03‰–13.50‰ and 10.03‰–12.14‰, respectively, and the δ37Cl values for halites from −0.1‰ to +0.4‰ support their marine origin. Bromine distribution in the Oldest Halite and the occurrence of anhydrite intercalations indicate fluctuations of the brine density during the Oldest Halite deposition. The formation of polyhalite was preceded by the syndepositional dehydration of the original gypsum deposit and it appears that the anhydrite was then transformed to polyhalite by reaction with marine brines more evolved than those from which precipitated precursor calcium sulfate minerals. These concentrated brines could have been derived from the evaporation of marine brines and/or inflow of K- and Mg-rich brines that were formed in nearby shallow salt pans occurring in sulfate platform areas and thus sulfate platform areas and adjacent slopes of those platforms were predestined for polyhalite formation.

Mineralogía y evolución de las salmueras madres en el yacimiento de nitratos Pedro de Valdivia, Antofagasta, Chile.

RESUMEN Los yacimientos de nitrato de Chile estan constituidos por complejas paragenesis de minerales salinos que rellenan la porosidad y los espacios abiertos de rocas de naturaleza diversa. El estudio de las secuencias mineralogicas de relleno de los diversos tipos de porosidad en la roca encajante del yacimiento Pedro de Valdivia, ha permitido obtener informacion sobre la evolucion de las salmueras madres de las paragenesis salinas. Dicha evolucion se pone de manifiesto por la precipitacion de secuencias minerales progresivamente mas solubles: silicatos (ceolitas); calcita; sulfatos calcicos, sodicos (potasicos) y magnesicos; nitrato-sulfatos sodicos y potasico-magnesicos; nitratina, y pequenas cantidades de yodatos, yodato-sulfatos sodicos y potasico-magnesicos; cromatos, boratos y percloratos. En la secuencia general de precipitacion, se han podido diferenciar dos lineas evolutivas: una de naturaleza sodica, caracterizada por la asociacion glauberita-darapskita- (hectorfloresita), y otra, de naturaleza sodico-potasico-magnesica, caracterizada por la presencia de bloedita-polihalita-humberstonita-nitro-(fuenzalidaita). Las salmueras madres del yacimiento estan relacionadas con el sistema freatico a partir del cual precipitaron las asociaciones salinas. Se atribuye a las mineralizaciones una edad oligoceno-miocena (con edad minima de 6 Ma). Las principales caracteristicas fisiograficas durante dicho periodo habrian sido similares a las actuales y el clima, probablemente, mas humedo. En la genesis de las salmueras madres de los yacimientos de nitrato han debido intervenir procesos termales y reciclaje por lixiviacion de sales previamente formadas en el ambiente arido del Desierto de Atacama. La evaporacion es el unico mecanismo, lo suficientemente efectivo, como para saturar las salmueras en minerales muy solubles. Sin embargo, los nitratos no deben ser considerados como verdaderas evaporitas, dada su disposicion rellenando fisuras (y porosidades), la casi ausencia de crecimientos desplazativos (configurando nodulos o enterolitos) y de otras estructuras caracteristicas de las evaporitas.

The Effect of Geological Parameters on Radiation Damage in Rock Salt: Application to Rock Salt Repositories

Laboratory irradiations at a constant dose rate of 15 kGy/h at 100 C have enabled the study of the radiation damage in several types of rock salt. Total doses ranged from 20 kGy to 48.9 MGy. Two methods (optical absorption and release of hydrogen by reaction with water) have been used to measure the concentration of radiation-induced defects. Their concentration was compared with the dose and the chemical and mineral composition of rock salt samples, using multivariate statistical techniques. The results show a log-linear increase in the concentration of colloidal sodium with dose, whereas the F-centers concentration remains nearly constant. Moreover, there is a clear influence of the mineral composition of the rock salt in the radiation damage, leading to defect concentrations varying over one order of magnitude for the same dose. Rock salt with small amounts of accessory minerals presents the lowest defect concentration. Experimental data have been compared with the theoretical predictions obtained by the Jain-Lidiard model. For doses higher than 1 MGy, both values are of the same order of magnitude.

The influence of mineralogy and texture in the water content of rock salt formations. Its implication in radioactive waste disposal

The consideration of the use of salt formations as possible radioactive waste disposal sites led us to attempt to determine the extent to which their brine content could influence the performance of the disposal system. Bedded rock salt from the Cardona, Zaragoza and Guendulain Fms., as well as diapiric rock salt from northern and southeastern Spain, have been selected and their water contents characterized by thermogravimetry. Free water content (intergranular water and water in fluid inclusions) in the studied formations ranges from 0.01 to 1.24% weight. In addition, the presence of hydrated minerals increases the amount of total water in the rock (up to 3.50%). Clear differences between the studied formations are observed in the total amount of water and in the form of water entrapment in rock salt. The results obtained have allowed the classification of the studied rock salt formations with respect to their free-brine content as water-poor (around 0.1% in average), intermediate water-rich (around 0.2%) and water-rich (higher than 0.3%). The petrographical features of the rock salt, such as mineralogical content and halitic textures, play an important role in its water content. Whilst brine in fluid inclusions is related to the halite texture, intergranular brine depends mainly on the content of clay and sulphate minerals.