Continental Evaporites Research Articles

Factors controlling lithium accumulation in brines from Upper Cretaceous to Lower Eocene aquifers of a continental evaporite basin

The origin of deep-seated Li-rich brines in a continental evaporite basin remains poorly understood. To clarify hydrogeochemical processes governing the Li enrichment, this study investigated chemical compositions and multi-isotopic (H–O–Li–Sr) features of a set of water samples from Upper Cretaceous to Lower Eocene sedimentary brines, saline groundwaters and oilfield brines, thermal waters and surficial river waters in the Jiangling Basin, central China. Waters from deep reservoirs evolve through multiple hydrogeochemical processes including evaporation, dilution, water-rock reactions and evaporite dissolution. These processes cause a range of Li and Sr contents (5.43–100 mg/L and 32.8–1307 mg/L) and isotope compositions (+8.6 to +19.3‰ and 0.706926–0.712480), indicating contributions from reservoir rock dissolution to δ7Li values and 87Sr/86Sr ratios. Chemical geothermometers applied to brines, saline waters and thermal waters give reservoir temperature values ranging between 70 and 200 °C. Temperature has a distinct effect on advancing host rock-water interactions, while high-temperature waters enhance leaching of Li in rocks compared to the low- and medium-temperature waters. The variable δ7Li values lower than river waters and high Cl/Br ratios in the Upper Cretaceous to Lower Eocene aquifers suggest a result of evaporite dissolution to different degrees, whilst waters in the Upper Cretaceous aquifers have the lowest δ7Li values similar to the thermal waters and the highest Cl/Br ratios suggest a stronger salt dissolution effect. Multi-isotope tracing highlights the role of temperature, reservoir rock types and different hydrogeochemical processes in defining the geochemical and isotopic water composition. Combination of these factors, together with high heat gradients associated with intense volcanic activity eventually results in Li accumulation in deep-seated brines from the Jiangling evaporite basin. Our results contribute to a further comprehensive understanding of hydrogeochemical processes that dominate deep brine formation and Li enrichment in an ancient continental evaporite basin.

Continental evaporites as brine sources for formation of SEDEX and MVT deposits: Implications from boron isotope compositions of tourmaline

In this study, the boron isotopic composition of Paleoproterozoic seawater is reconstructed using δ11B of tourmaline from the Paleoproterozoic Pb-Zn deposits of Rajpura-Dariba, Zawarmala, and Rampura-Agucha in the Aravalli-Delhi Fold Belt (ADFB) as well as non-mineralized tourmaline bearing metapelites from the Mangalwar Complex (MC) and Gangpur Schist Belt (GSB). This is then used to constrain the source of the mineralizing fluids in Sedimentary Exhalative (SEDEX)- and Mississippi Valley Type (MVT)-deposits. Based on textural relations and chemical variations, tourmaline in the Rajpura-Dariba and Zawarmala deposits can be grouped into two generations: syn-mineralization Tur-I and metamorphic Tur-II. In contrast, a single generation of metamorphic tourmaline is identified in the Rampura-Agucha deposit and in the MC. Tur-I from Rajpura-Dariba has XMg of 0.14–0.56, while Tur-II is more magnesian (XMg = 0.58–0.77). Similarly, Tur-I from Zawarmala has a wide compositional range (XMg = 0.22–0.67), whereas Tur-II is Mg-rich (XMg = 0.39–0.67). Tourmalines from Rampura-Agucha and MC are also magnesian (XMg = 0.66–0.70 and 0.59–0.69, respectively). The δ11B values of syn-ore tourmaline from Rajpura-Dariba and Zawarmala vary between −18.8‰ and −16.1‰, and between −6.6‰ and −4.0‰, respectively. Metamorphic tourmalines from Rajpura-Dariba, Zawarmala, Rampura-Agucha and MC have δ11B values between −19.1‰ and −7.7‰.The δ11B of the pelitic sediments in Rajpura-Dariba, Rampura-Agucha, the MC and the GSB were estimated from the δ11B of metamorphic tourmaline hosted in them using mass balance constraints and P-T pseudosection modelling. Applying the B-isotope fractionation factor between Miocene shale and modern clay (Δ11Bmiocene shale − modern clay = −3.8‰ to −2.2‰) to the modelled δ11B of post-diagenetic protoliths of the metapelites, the δ11B of marine sediments for these basins were constrained between −15.1‰ and −10.2‰. Subsequently, using the B-isotope fractionation factor between modern clay and seawater (Δ11Bmodern clay − modern seawater = −45.2‰ to −40.8‰), the δ11B of Paleoproterozoic seawater is estimated to have been between +27.3‰ and +33.4‰.The isotopically light δ11B of syn-mineralization tourmaline from Rajpura-Dariba and Zawarmala (−18.8‰ to −16.1‰ and −6.6‰ to −4.0‰, respectively), and worldwide SEDEX/MVT-type deposits in general (a median value of −9.6‰; compilation of Trumbull et al., 2020), implies that the B was most likely sourced from continental evaporites and not evaporated seawater and/or connate brine. Dissolution of early rift-related continental evaporites most likely act as a major source of basinal brines for the formation of SEDEX/MVT deposits during the sag-phase of basin development.

Hydrodynamic model for independent cold and thermo-mineral twin springs in a stratified continental karst aquifer, Camou, Arbailles Massif, Pyrénées, France

The Camou springs (Arbailles Massif, French Western Pyrenees) display an unusual close association of a typically cold karstic spring that drains the Urgonian western limb of the Arbailles, and a thermo-mineral spring (33.5°C; salinity 17.7 g/L). The latter gains its mineralization at the contact of Triassic evaporites mainly through a deep loop in the Apanicé syncline. The fast upflow of this deep water occurs at the cross of large active lines (the North-Pyrenean thrust located at depth, and the Saison transverse fault). Cave diving in the nearby Maddalen Cave allowed reaching the phreatic passage at the origin of the cold spring, which however also crosses the thermal body in the third sump (S3). Both water bodies are separated by a sharp thermocline. 6 pressure-temperature dataloggers were placed in both water bodies along the thermocline for 6 months. The dataloggers located downstream on either side of the thermocline show at the beginning of flood first a rise of the thermal body, then an invasion of the whole phreatic passage by the cold floodwater, controlled by head pressure changes in the karst aquifer. From observation of these mechanisms, we deduce a hydrodynamic model with a warm plume rising into the cold aquifer, without significant mixing. Such independence of water bodies is explained by the decrease of turbulent rate at the interface, due to the sharp density gradient. The relative absence of mixing does not actually require independent “watertight” routes, both water bodies can thus coexist even in the same conduit. This model locally implies the existence of unknown secondary passages close to the spring, which allow an independent draining of each water body toward separate outlets during low stage. Such type of stratified aquifer linked to density differences is common in coastal karst (Florida, French Calanques…), in the continental evaporite karst (Schlotten of the Harz in Germany, Kungur karst in Ural…), but remains poorly identified in continental carbonate karst areas, mainly because of the difficulty of access. Together with the Mescla spring in French Alpes-Maritimes, the Camou twin springs discharging in the same porch are an outstanding example, allowing a direct study of the stratification and the dynamic of highly contrasted water bodies.

Sr, S, and O Isotope Compositions of Evaporites in the Lanping–Simao Basin, China

Evaporites are widely distributed within continental “red beds” in the Lanping–Simao Basin, west Yunnan, China. Sr (Strontium), S (Sulfur), and O (Oxygen) isotope compositions have been measured on 54 sulfate or/and sulfate-bearing samples collected from Lanping, Nuodeng, Jinggu, Mengyejing, Baozang throughout the Lanping–Simao Basin. The 87Sr/86Sr ratios of all samples (0.708081 to 0.710049) are higher than those of contemporaneous seawater, indicating a significant continental contribution to the drainage basin. Sulfates in the Lanping Basin have higher 87Sr/86Sr ratios (0.709406 to 0.710049) than those (0.708081 to 0.709548) in the Simao Basin. Nevertheless, the δ34S values of gypsums (13.4‰ to 17.6‰) in Lanping and Baozang fall within the range of Cretaceous seawater. Gypsums from a single section in Baozang have trends of decreasing δ34S values and increasing 87Sr/86Sr ratios from base to top, indicating continental input played an increasingly significant role with the evaporation of brines. High δ34S values (20.5‰ to 20.7‰) of celestites in Lanping are probably caused by bacterial sulfate reduction (BSR) process in which 34S were enriched in residual sulfates and/or recycling of Triassic evaporites. The reduced δ34S values of gypsums (9.5‰ to 10.4‰) in Nuodeng could have been caused by oxidation of sulfides weathered from Jinding Pb-Zn deposit. The complex O isotope compositions indicate that sulfates in the Lanping–Simao Basin had undergone sulfate reduction, re-oxidation, reservoir effects, etc. In conclusion, the formation of continental evaporites was likely derived from seawater due to marine transgression during the Cretaceous period. Meanwhile, non-marine inflows have contributed to the basin significantly.

Boron isotopic variations in tourmaline from metacarbonates and associated calc-silicate rocks from the Bohemian Massif: Constraints on boron recycling in the Variscan orogen

Various metacarbonate and associated calc-silicate rocks form minor but genetically significant components of the lithological units in the Bohemian Massif of the Variscan orogen in Central Europe. These rocks vary in terms of their lithostratigraphy, chemical composition and mineral assemblage (dolomite/calcite ratio, silicate abundance). Tourmaline is present in five paragenetic settings within the metacarbonate and calc-silicate units. Type I comprises individual, euhedral, prismatic grains and grain aggregates in a carbonate-dominant (calcite ​± ​dolomite) matrix poor in silicates. Type II is characterized by euhedral to subhedral grains and coarse- to fine-grained aggregates in silicate-rich layers/nests within metacarbonate bodies whereas type III occurs as prismatic grains and aggregates at the contact zones between carbonate and associated silicate host rocks. Type IV is in veins crosscutting metacarbonate bodies, and type V tourmaline occurs at the exocontacts of elbaite-subtype granitic pegmatite. Tourmaline from the different settings shows distinctive compositional features. Typical for type I are Mg-rich compositions, with fluor-uvite ​> ​dravite>> ​magnesio-lucchesiite. Tourmalines from type II silicate-rich layers/nests are highly variable, corresponding to oxy-schorl, magnesio-foitite, Al-rich dravite and fluor-uvite. Typical for type III tourmalines are Ca,Ti-bearing oxy-dravite compositions. The type IV veins feature dravite and fluor-uvite tourmaline compositions whereas type V tourmaline is Li,F-rich dravite. Tourmaline is the only B-bearing phase in paragenetic types I–IV, where it is characterised by two principal ranges of B-isotope composition (δ11B ​= ​−13‰ to −9‰ and −18‰ to −14‰). These ranges correspond to regionally different units of the Moldanubian Zone. Thus, the Svratka Unit (Moldanubian Zone s.l.) contains only isotopically lighter tourmaline (δ11B ​= ​−18‰ to −14‰), whereas metacarbonates in the Polička unit (Teplá–Barrandian Zone) and Olešnice unit (Moravicum of the Moravo-Silesian Zone) has exclusively isotopically heavier tourmaline (δ11B ​= ​−9‰ to −13‰). Tourmalines from metacarbonates in the Variegated Unit cover both ranges of isotope composition. The isotopically light end of the B isotope range may indicate the presence of continental evaporites within individual investigated areas. On the other hand, variations in the range of ~8 δ-units is consistent with the reported shift in B isotopic composition of metasedimentary rocks of the Bohemian Massif due to the prograde metamorphism from very-low grade to eclogite facies. In contrast to the metacarbonate-hosted settings, tourmaline of paragenetic type V from the exocontact of granitic pegmatites displays a significantly heavier range of δ11B (as low as –7.7‰ to –0.6‰), which is attributed to partitioning of 10B to cogenetic axinite and/or different B-signature of the source pegmatite containing tourmaline with heavy δ11B signature.

Late Miocene evaporite geochemistry of Lorca and Fortuna basins (Eastern Betics, SE Spain): Evidence of restriction and continentalization

AbstractThe Lorca and Fortuna basins are two intramontane Neogene basins located in the eastern Betic Cordillera (SE Spain). During the Late Tortonian—Early Messinian, marine and continental evaporites precipitated in these basins as a consequence of increased marine restriction and isolation. Here we show a stratigraphic correlation between the evaporite records of these basins based on geochemical indicators. We use SO4 isotope compositions and Sr isotopic ratios in gypsum, and halite Br contents to characterize these units and to identify the marine or continental source of the waters feeding the evaporite basins. In addition, we review the available chronological information used to date these evaporites in Lorca (La Serrata Fm), including a thick saline deposit, that we correlate with the First Evaporitic Group in Fortuna (Los Baños Fm). This correlation is also supported by micropalaeontological data, giving a Late Tortonian age for this sequence. The Second Evaporitic Group, (Chicamo Fm), and the Third Evaporitic Group (Rambla Salada Fm) developed only in Fortuna during the Messinian. According to the palaeogeographical scheme presented here, the evaporites of the Lorca and Fortuna basins were formed during the Late Tortonian—Early Messinian, close to the Betic Seaway closure. Sulphate isotope compositions and Sr isotopic ratios of the Ribera Gypsum Mb, at the base of the Rambla Salada Fm (Fortuna basin), match those of the Late Messinian selenite gypsum beds in San Miguel de Salinas, in the near Bajo Segura basin (40 km to the East), and other Messinian Salinity Crisis gypsum deposits in the Mediterranean. According to these geochemical indicators and the uncertainty of the chronology of this unit, the assignment of the Rambla Salada Fm to the MSC cannot be ruled out.

Monitoring of playa evaporites as seen with optical remote sensing sensors: case of Chott El Jerid, Tunisia, from 2003 to present

Chott El Jerid in the Zone of Chotts of Tunisia is one of the largest endorheic basins in the world. During the dry period, from May to August, it is generally covered by continental evaporites which result from the desiccation of the lake formed after a flooding event. This lake comprises runoff water from the surrounding relief and also water resurgence. In order to map and monitor these evaporitic surfaces (mineral composition and evolution in space and time), optical multisource, multispectral, and multidate satellite data have been used. Landsat 4–5 TM, Landsat 8 OLI, SPOT 6, and Landsat Surface Reflectance (LSR) constitute the main data set. The central part of the Chott, north and south of the road crossing the Chott over 70 km from Tozeur to Kebili, has been particularly studied, because it corresponds to the major evaporite accumulation zone. These evaporites precipitated as concentric layers (a relatively rare pattern), mainly north of the road, after several recent flooding events during the last 15 years. Winds can play a significant role in the development of the evaporite layers. Image interpretation associated with field data shows that after the final desiccation of the playa lake, the mineralogy of the salt crust comprised an assemblage dominated by halite south of the road and by gypsum north of the road. Halite and gypsum are the only minerals to be identified using satellite remote sensing data. Sulfates such as gypsum can be identified thanks to a drop in reflectance in the short-wave infrared (SWIR) range caused by vibrations of the SO4 group. Gypsum crusts are more widely distributed than halite crusts. LSR data are particularly suitable for multitemporal comparison because they are calibrated and atmospherically corrected. The classical bull’s eye pattern characterizing evaporitic deposits (from carbonates along the rims to halite, gypsum, and finally potassium-magnesium minerals in the center of the basin) is deeply disturbed by the road crossing Chott El Jerid.

Boron, lithium and nitrogen isotope geochemistry of NH4-illite clays in the fossil hydrothermal system of Harghita Bãi, East Carpathians, Romania

Coarse (2.0–0.2μm) and fine (<0.2μm) clay fractions of NH4-illite-smectite (I-S) mixed-layered and K-illite/(NH4, K)-illite (I) mixed phases that vary in age and trace element composition were collected from the fossil hydrothermal system of Harghita Bãi, East Carpathians. Boron and Li isotope ratios were measured by secondary ion mass spectrometry (SIMS), and N by isotope ratio mass spectrometry (IRMS) with the aim to characterize the isotope geochemistry and source of light elements fixed in authigenic NH4-illitic clays.Boron in NH4-I-S clays ranges from 513 to 1457ppm and reached ~1000ppm in the K-I/(NH4, K)-I. The δ11B (‰) measured in NH4-I-S ranges from −12.6 to −22.4 (±0.3‰) and in K-I/(NH4, K)-I is consistently −5.5 to −5.1 (±0.3‰). Boron isotopes systematically become lighter in the NH4-I-S series as temperature increased from 90 to 270°C. Low Li content (1 to 8ppm) was found in illitic clay fractions. The δ7Li (‰) shows negative values ranging from −8.6 to −12.3 (±0.8‰) for the coarser (2.0–0.2μm) NH4-I-S clays and from +4.3 to +14.1 (±1‰) for the finer (<0.2μm) fraction of NH4-I-S and K-I/(NH4, K)-I clays. The N (%) measured in the NH4-I-S clays ranges from 0.70 to 1.50 (±0.2%), whereas in the K-I/(NH4, K)-I is about 0.70 (±0.2%). The δ15N (‰) ranges from +4.8 to +7.4 (±0.6) for most NH4-I-S and NH4, K-I clays, with one outlier for NH4-I-S of +14.6 (±0.6).The δ11B of K-I/(NH4, K)-I clays reflect a magmatic source, whereas the NH4-I-S series is consistent with the influx of isotopically light-B waters derived from hydrothermal leaching of continental evaporites and/or organic-rich sediments. The δ7Li signature measured on K-I/(NH4, K)-I clays also support a magmatic fluid, enriched in heavy Li, followed by precipitation of coarser NH4-I-S from more recent sedimentary contributions of isotopically light Li. This interpretation is also supported by the δ15N, which reflect an influx of waters from an organic sediment origin. The δ15N of +14.6‰ (±0.6) measured on NH4-I-S could be attributed to the presence of meteoric waters mixed with hydrothermal fluids. The isotopic data obtained trace the mobility of magmatic and organic – sedimentary components in the upper continental crust.

Tychite in mantle xenoliths from kimberlites: The first find and a new genetic type

Tychite Na6Mg2(CO3)4(SO3) is a rare natural Na and Mg sulfatocarbonate. It is found only as minor mineral in deposits of saline lakes in the United States, Canada, Uganda, and China. In these continental evaporites tychite has sedimentary genesis. In this study, we report the first occurrence of tychite as a crystal phase in the melt inclusions in olivine from mantle xenoliths of the Udachnaya-East kimberlite pipe. This find provides an evidence for the probability of tychite crystallization from melts; i.e., this rare sulfatocarbonate may have a magmatic origin as well.

In situ metabolism in halite endolithic microbial communities of the hyperarid Atacama Desert.

The Atacama Desert of northern Chile is one of the driest regions on Earth, with areas that exclude plants and where soils have extremely low microbial biomass. However, in the driest parts of the desert there are microorganisms that colonize the interior of halite nodules in fossil continental evaporites, where they are sustained by condensation of atmospheric water triggered by the salt substrate. Using a combination of in situ observations of variable chlorophyll fluorescence and controlled laboratory experiments, we show that this endolithic community is capable of carbon fixation both through oxygenic photosynthesis and potentially ammonia oxidation. We also present evidence that photosynthetic activity is finely tuned to moisture availability and solar insolation and can be sustained for days, and perhaps longer, after a wetting event. This is the first demonstration of in situ active metabolism in the hyperarid core of the Atacama Desert, and it provides the basis for proposing a self-contained, endolithic community that relies exclusively on non-rainfall sources of water. Our results contribute to an increasing body of evidence that even in hyperarid environments active metabolism, adaptation, and growth can occur in highly specialized microhabitats.

A global transition to ferruginous conditions in the early Neoproterozoic oceans

Eukaryotic life expanded during the Proterozoic eon1, 2.5 to 0.542 billion years ago, against a background of fluctuating ocean chemistry2, 3, 4. After about 1.8 billion years ago, the global ocean is thought to have been characterized by oxygenated surface waters, with anoxic and sulphidic waters in middle depths along productive continental margins and anoxic and iron-containing (ferruginous) deeper waters5, 6, 7. The spatial extent of sulphidic waters probably varied through time5, 6, but this surface-to-deep redox structure is suggested to have persisted until the first Neoproterozoic glaciation about 717 million years ago8, 9, 10, 11. Here we report an analysis of ocean redox conditions throughout the Proterozoic using new and existing iron speciation and sulphur isotope data from multiple cores and outcrops. We find a global transition from sulphidic to ferruginous mid-depth waters in the earliest Neoproterozoic, coincident with the amalgamation of the supercontinent Rodinia at low latitudes. We suggest that ferruginous conditions were initiated by an increase in the oceanic influx of highly reactive iron relative to sulphate, driven by a change in weathering regime and the uptake of sulphate by extensive continental evaporites on Rodinia. We propose that this transition essentially detoxified ocean margin settings, allowing for expanded opportunities for eukaryote diversification following a prolonged evolutionary stasis before one billion years ago.

Extremely negative and inhomogeneous sulfur isotope signatures in Cretaceous Chilean manto-type Cu–(Ag) deposits, Coastal Range of central Chile

Chilean manto-type (CMT) Cu(–Ag) hydrothermal deposits share a characteristic association of volcano-sedimentary Jurassic to Lower Cretaceous host rocks, style of mineralization, ore and associated mineralogy and geochemistry, with ore grades typically >1%Cu, that make this family of deposits significant and interesting, both academically and economically. Although often stratabound, geological evidence supports an epigenetic origin for these deposits. We present a detailed stable isotope study of La Serena and Melipilla–Naltahua Lower Cretaceous deposits, central Chile, which reveals extremely negative δ34S values, to −50‰, which are among the lowest values found in any ore deposit. In addition, the range of δ34S values from sulfides in the two areas is very wide: −38.3 to −6.9‰ in La Serena, and −50.4 to −0.6‰ in Melipilla–Naltahua. These new data significantly extended the reported range of δ34S data for CMT deposits. Co-existing sulfates range from 7.9 to 14.3‰, and are exclusive to La Serena deposit. The wide sulfide isotopic range occurs at deposit and hand specimen scale, and suggests a polygenic sulfur source for these deposits, where bacteriogenic sulfide dominates. While sulfur isotope data for the bulk of Jurassic CMT deposits, northern Chile, suggests a predominant magmatic source in their origin (mean=−2.7±1.9‰, 1σ), contributions of a magmatic component is only likely to be involved at Melipilla–Naltahua deposit.The δ13C values obtained for calcites associated with the mineralization range from −20.1 to 0.2‰ also suggesting polygenic carbon sources, with the likely strong involvement of degradation of organic matter and leaching of limestone.Two different genetic models, with involvement of hydrocarbon, are proposed for both areas. For Melipilla–Naltahua, a two-step model can be developed as follows: 1) Framboidal pyrite growth, with very low δ34S, formed by bacterial sulfate reduction in an open system, and with diagenetic degradation of oil-related brines, leaving pyrobitumen. 2) Cu-bearing stage, replacing of framboidal pyrite, inheriting depleted sulfur as low as −50.4‰, together with sulfides directly precipitated from a hydrothermal fluid with δ34S close to 0‰. For La Serena, a single step model fits best, without framboidal pyrite generation. Cu-bearing sulfides were precipitated mainly in veins where Cu plus base metal-bearing hydrothermal fluids mixed with H2S generated by bacterial sulfate reduction in the host rocks. Isotopic evidence clearly illustrates that bacterial activity, perhaps enhanced by hydrothermal activity, was fed by hydrocarbon brines and sulfate remobilized from continental evaporites. It is possible that variable ecological conditions led to different extents of isotopic fractionation, adding to the typical sulfur isotopic heterogeneity of such bacterial systems. For both areas, the Cu-bearing stage occurred during the peak to waning stages of the very low-grade metamorphism that affected the Lower Cretaceous sequence.

Stable isotope geochemistry of magnesite from Holocene salt lake deposits, Taoudenni, Mali

Stable isotope data (δ18O, δ13C) were obtained for magnesite (MgCO3) from Holocene salt lake deposits of the Taoudenni–Agorgott basin, Mali, in which it is the only carbonate mineral present. The deposits have a high glauberite content (Na2Ca(SO4)2), representing a type of continental evaporite formation that is commonly magnesite‐bearing. Samples from seven different levels in a 5 m long sequence were analysed. δ18OV‐PDB values are between +3.17 and +5.91‰, which is lower than might be expected for evaporitic environments, recording reduced 18O enrichment at high salinity. δ13CV‐PDB values are between −1.32‰ and −4.79‰, showing an influence of carbon derived from the decomposition of organic matter rather than from exchange with atmospheric CO2. Covariance between δ18O and δ13C is strong for the lower part of the sequence, recording coupled 18O and 13C enrichment as commonly observed for closed basins. Covariance is lacking for the upper part of the sequence, with only periodic massive magnesite deposition in a more shallow lake. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Continental evaporites and the search for evidence of life on Mars

Continental evaporites are deposits that originate from the evaporation of saline waters in the low areas of saline lakes from all continents, except Europe, and mainly consist of chloride, sulphate and potash minerals. In recent years, the discovery on the Martian surface of hydrated salt minerals, including sulphates and chlorides, interpreted as deriving from the desiccation of preexisting large bodies of water, such as lakes, has provided further convincing evidence of liquid water activity on the surface of Mars and, consequently, it has reinforced the plausibility of finding life. Because evaporites require short‐term aqueous processes for their formation, they can trap and preserve over geologic times a biological record made up of halophilic extremophiles—such as microalgae, bacteria, and their remains—that recent research on Earth has shown to be characterized by unexpectedly high biodiversity. This record may consist of varying types of fossils, including morphological fossils, chemofossils and biominerals. As a consequence, continental evaporite environments and their saline deposits are now a primary target for the near future astrobiology missions devoted to the search for fossil Martian life. Lacustrine evaporite deposits and minerals have, therefore, been identified as primary targets for the NASA–ESA joint programme of the Mars sample return, planned for the end of the current decade. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Nature of ore-forming fluids of the Dal’negorsk deposit: Isotopic and geochemical parameters of the altered host rocks

Oxygen isotopic composition was studied in the altered host rocks of the Dal’negorsk borosilicate deposit in order to establish a boron source and the origin of ore-forming fluids responsible for deposition of economic borosilicate ore. The relationships between oxygen isotopic composition and geochemistry of the altered igneous rocks occupying various structural and temporal positions in the ore zone were studied, including premineral high-potassium minor intrusions located in the zones of datolite mineralization; alkali basalt, gabbro, and breccia from the sedimentary framework of the deposit; and postmineral basaltic andesite, basalt, and dolerite dikes. It was suggested that interaction of aqueous fluid with host rocks brought about not only variation in oxygen isotopic composition but also shifts in geochemistry of these rocks, especially as concerns the chemical elements contained in ore-forming fluid. The disturbance of oxygen isotopic system is typical of all studied rocks: δ18O values sharply decrease indicating interaction with aqueous fluid at elevated temperatures. The lowest δ18O (from −2.9 to +0.1‰) is characteristic of the premineral high-potassium and ultrapotassium minor intrusions from skarn-datolite zone. Igneous rocks from the sedimentary framework of the deposit have δ18O of +2 to −0.9‰ The δ18O of postmineral basaltic andesite, basalt, and dolerite dikes varies from 0 to +7‰ with increasing distance from the ore zone. The oxygen isotopic composition of aqueous fluid evidences its exogenic origin. The geochemical and isotopic characteristics of ore-forming fluid show that it could have been deep-seated subsurface water similar to the contemporary water of the Alpine fold zone, which contain up to 700–1000 mg/l B and is distinguished by high K, Li, Rb, Cs contents and high K/Na ratio. Similar geochemistry is characteristic of the fluid inclusions in quartz from ore zones. It cannot be ruled out that continental evaporites were a source of boron as well. The relationships between δ18O, K-Ar age, and geochemical parameters of premineral and postmineral altered intrusive bodies allow us to suggest that the subsurface B-bearing water discharged through narrow channels controlled by premineral basaltic bodies. The discharge was probably initiated by emplacement of basalt and dolerite dikes.

The geochemistry of Boron-rich groundwater of the Karlovassi Basin, Samos Island, Greece

Abstract The upper Miocene of Karlovassi Basin, Samos Island, Greece, contain continental evaporites such as colemanite, ulexite, celestite, gypsum and thenardite. These evaporites are related with volcanic tuffs, diagenetically altered in a saline-alkaline lake environment. The aim of the present paper is to: a) define the impact of the already known and possible buried borates and other evaporites to the geochemistry of the hydrogeological system of Karlovassi Basin, and; b) to assess the correlation between surface and underground evaporite deposits considering the spatial changes in the concentrations of the examined physicochemical parameters. Fieldwork, laboratory measurements and literature data revealed elevated boron values (2136–33012 ⧎/L) in the central part of Karlovassi Basin. In the same area, high amounts of strontium, sodium, lithium and sulfates also occur. It is proposed that these ions originate from the leaching of evaporites and authigenic minerals such as the Sr-rich clinoptilolite and the boron-bearing potassium feldspar. Boron values are abnormally high for freshwater aquifers, and are indicative of the presence of buried evaporites in the basin with unknown significance.

The Lower Triassic Sorkh Shale Formation of the Tabas block, east central Iran: Succession of a failed-rift basin at the paleotethys margin

The Lower Triassic Sorkh Shale Formation is a dominantly red colored marginal marine succession deposited in the north-south trending Tabas Basin of east central Iran. It is correlated with the unconformity-bounded lower limestone member of the Elika Formation of the Alborz Mountains of northern Iran. The Sorkh Shale is bounded by the pre-Triassic and post-Lower Triassic interregional unconformities and consists mainly of carbonates, sandstones, and evaporites with shale being a minor constituent. Detailed facies analysis of the Sorkh Shale Formation resulted in recognition of several genetically linked peritidal facies that are grouped into restricted subtidal, carbonate tidal flat, siliciclastic tidal flat, coastal plain and continental evaporite facies associations. These were deposited in a low energy, storm-dominated inner-ramp setting with a very gentle slope that fringed the Tabas Block of east central Iran and passed northward (present-day coordinates) into deeper water facies of the Paleotethys passive margin of northern Cimmerian Continent. Numerous carbonate storm beds containing well-rounded intraclasts, ooids and bioclasts of mixed fauna are present in the Sorkh Shale Formation of the northern Tabas Basin. The constituents of the storm beds are absent in the fair weather peritidal facies of the Sorkh Shale Formation, but are present throughout the lower limestone member of the Elika Formation.

Synsedimentary versus metamorphic control of S, O and Sr isotopic compositions in gypsum evaporites from the Cameros Basin, Spain

Sulfate δ 34S and δ 18O and Sr isotope compositions are presented for Berriasian (Upper Jurassic) gypsum evaporites from the Cameros Basin, a continental basin in the Iberian ranges of northern Spain. Solute sources to the ephemeral lakes in which the evaporites formed are dominated by weathering of sediments containing older Keuper marine evaporites and granitic/metamorphic Variscan basement. Strontium isotopic ratios of the continental evaporites ( 87Sr/ 86Sr = 0.707882 to 0.707933) are elevated compared to the Keuper source ( 87Sr/ 86Sr = 0.707605 to 0.707799), most likely reflecting a radiogenic component from basement lithologies. Sulfate δ 34S in the continental evaporites (at around 18.2 ‰ V-CDT) is higher than any possible mixture of solutes to the basin. The degree of 34S enrichment is small (≈ 3.7‰) and is most likely the result of partial bacterial reduction of lake-water sulfate. The same process would also enrich sulfate in 18O, but by only ≈ 1‰. Evaporite sulfate δ 18O (at around 21.7‰ V-SMOW) is, in fact, enriched by ≈ 10‰ relative to sulfate sources to the basin. This large effect is most likely the result of re-equilibration of oxygen isotopes during low grade metamorphism of the basinal sequence. Sulfur and strontium isotopes in the evaporites remained internally buffered and thus unchanged during metamorphism while oxygen isotopes were open to exchange with aqueous fluid and/or interbedded carbonates that constituted a large exchangeable oxygen reservoir. The potential of sulfate oxygen to undergo isotopic exchange at elevated temperatures must be taken into account when interpreting the significance of evaporite sulfate oxygen data from units that have undergone deep burial or low grade metamorphism.

Multiple generations of high salinity formation water in the Triassic Sherwood Sandstone: Wytch Farm oilfield, onshore UK

The origin and heterogeneity of oilfield formation water in the Lower Triassic Sherwood Sandstone at Wytch Farm in the Wessex Basin, UK, have been investigated using production data, detailed water geochemistry and O, S and H stable isotope data. The formation waters are highly saline, NaCl-type brines with TDS values of up to 230,000 mg/L. There is a general decrease in salinity from the flanks of the field to the crest with Cl − decreasing from about 136,000 to 109,000 mg/L. The Cl/Br ratio of the water shows that salinity was largely derived from the dissolution of Upper Triassic continental evaporites found off-structure to the west and north of the field. The water in the field had a meteoric source although variation in δ 2H values suggests that there may be meteoric waters of different ages in the oilfield, reflecting recharge under different palaeoclimatic conditions. At the crest of the field, aqueous SO 4 2 - resulted from dissolution of anhydrite in the reservoir. In contrast, in other parts of the field there is an indication that some of the dissolved SO 4 2 - was derived from oxidation of pyrite at some point on the recharge path of meteoric water to the field. There were two meteoric influx events bringing different Cl − concentrations and different δ 2H values. The first was probably before the Eocene oil influx and could have occurred in the Lower Cretaceous or early Tertiary. The second meteoric influx event probably occurred after or during oil migration into the Wytch Farm structure since the second meteoric water is found at the flanks of the field adjacent to the regions where salt is found in the stratigraphy. The preservation of heterogeneities in oilfield formation water compositions suggests that there has been little aqueous fluid movement or diffusive flux for over 40 million years. Mass flux has been restricted by density stratification within the aquifer and the very low effective permeability for the aqueous phase in the oil-filled reservoir.

Deposition and early alteration of evaporites

SummaryThe depositional settings for primary and early diagenetic evaporite deposits generally fall into three categories: marginal (mixed shallow‐subaqueous and subaerial), shallow and deep subaqueous. These three environmental groupings hold for both marine and nonmarine settings, although the details of continental evaporites may be far more complex than in most marine‐fed water bodies. The primary evaporite morphologies from many continental (playa), hypersaline marine and marine‐marginal depositional settings are reasonably well understood, because of the numerous detailed studies of recent, Holocene and Cenozoic deposits that serve as models for sedimentary interpretation. The sedimentological features that develop in deeper water settings are inferred, based on examination of unaltered Cenozoic deposits. Each environmental setting develops characteristic depositional features and patterns, and one facies grades into the next. Because there may be significant physiochemical changes in water composition during deposition as well as sudden change(s) in both water depth and basinal circulation, description and interpretation of evaporative rocks should not be based on mineralogy alone, but on the distinct sedimentary characteristics of each part of a deposit. In cases where sediments still reflect their primary mineralogy and morphology, most of the environments can now be recognized; however, geochemical studies are commonly required to determine the source(s) of the original water. The determinative geochemical techniques that presently serve as a support to sedimentologic study include studies of fluid inclusions, bromine content of chloride salts, and the stable isotopes of strontium, sulphur, carbon and oxygen. Only after these and perhaps other chemical analyses are considered can the full depositional history of a region be reasonably unravelled.