Multiple fluid components of salt diapirs and salt dome cap rocks, Gulf Coast, U.S.A.

Salt diapirs contain a few percent of anhydrite that accumulated as residue to form anhydrite cap rocks during salt dissolutions. Reported ⁸⁷Sr/⁸⁶Sr ratios of these salt-hosted and cap rock anhydrites in the Gulf Coast, U.S.A., indicate their derivation from Middle Jurassic seawater. However, a much wider range of ⁸⁷Sr/⁸⁶Sr ratios, incorporating a highly radiogenic component in addition to the Middle Jurassic component, has been found in several Gulf Coast salt domes. This wide range of ⁸⁷Sr/⁸⁶Sr ratios of anhydrite within the salt stocks records Sr contributions from both marine water and formation water that has equilibrated with siliciclastics. During cap rock formation this anhydrite either recrystallized in the presence of, or was cemented by, a low-Sr fluid with a Late Cretaceous seawter-type Sr isotope ratio or simply lost Sr during recrystallization. Later, the cap rock was invaded by warm saline brines with high Sr isotope ratios from which barite and metal sulfides were precipitated. Subsequently, low-salinity water hydrated part of the anhydrite bringing to six the total number of fluids that interacted througout the history of salt dome and cap rock growth. The progenitor of these salt diapirs, the Louann Formation, is generally thought to have formed from marine water evaporated to halite and, rarely, higher evaporite facies. Salt domes in the East Texas, North Louisiana, and Mississippi Salt Basins have ⁸⁷Sr/⁸⁶Sr and δ³⁴S values that corroborate a Mid-Jurassic age for the mother salt. However, salt domes in the Houston and Rio Grande Embayments of the Gulf Coast Basin have ⁸⁷Sr/⁸⁶Sr ration ranging to values higher than both Middle Jurassic seawater and all Rb-free marine Phanerozoic rocks. These anomalous ⁸⁷Sr/⁸⁶Sr ratios are probably derived from radiogenic Sr-bearing fluids that equilibrated with siliciclastic rocks and invaded the salt either prior to, or during, diapirism. Potential sources of the radiogenic ⁸⁷Sr component include clay and/or feldspar (located either in older units beneath the Louann Formation or younger units flanking the salt diapirs) and K-salts within the Louann evaporites. Because partial Sr exchange in anhydrite had to take place in a fluid medium, admittance of radiogenic ⁸⁷Sr-bearing fluids into the salt may have led to diapirism by lowering the shear strength of the crystalline salt. The slight number of anomalous ⁸⁷Sr/⁸⁶Sr values in the interior basins indicates that anomalous values are related to areally discrete structural or stratigraphic controls that affected only the Gulf Coast Basin.     

Rare earth elements and sulfur and strontium isotopes of upper Cretaceous phosphorites in Egypt

Upper Cretaceous Phosphorites from different localities in Egypt were analyzed for their rare earth elements (REEs) contents and sulfur and strontium isotopes to examine the effect of depositional conditions versus diagenesis on these parameters.The negative Ce and Eu anomalies of the study phosphorites suggest its formation under reducing conditions. However, chondrite-normalized REEs patterns show relative enrichments of LREEs over the HREEs, which is obviously different from the seawater REEs pattern suggesting post-depositional modifications on the REEs distributions during diagenesis. The difference in the REEs concentrations and Ce anomalies among the study localities as well as the similarity between the REEs patterns of these phosphorites and associated black shales might support this interpretation.The concentration of structural SO₄²⁻ (0.6-3.7%) and their δ³⁴S values (+0.5 to -20‰) in the upper Cretaceous phosphorites in Egypt suggest the formation of these phosphorites in the zone of sulfate reduction. On the other hand, the sulfur isotopes in the pyrite from the study phosphorites (δ³⁴S = +4.6‰ − 23‰ with an average of −7.7‰) are attributed to the influence of seawater from which pyrite was formed during diagenesis. The difference between the δ³⁴S values in the phosphorites (all are positive values) and those in the associated pyrite (mostly negative values) reflect an asymmetric sulfate and sulfide sulfur isotopic composition due to the formation of francolite (source of sulfate) and pyrite (source of sulfide) in different conditions and/or process.The ⁸⁷Sr/⁸⁶Sr values of the upper Cretaceous phosphorites in Egypt are very close to the marine values during the Campanian-Maastrichtian time and their average (0.707622) is more or less comparable to the average ⁸⁷Sr/⁸⁶Sr values of the Cretaceous-Eocene Tethyan phosphorites. This suggests no post-depositional alteration (i.e. diagenetic effect) on the Sr isotopic composition of these phosphorites.     

Origin of quartz geodes from Laño and Tubilla del Agua sections (middle–upper Campanian,Basque‐Cantabrian Basin,northern Spain): isotopic differences during diagenetic processes

Quartz geodes and nodular chert have been found within middle–upper Campanian carbonate sediments from the Laño and Tubilla del Agua sections of the Basque‐Cantabrian Basin, northern Spain. The morphology of geodes together with the presence of anhydrite laths included in megaquartz crystals and spherulitic fibrous quartz (quartzine‐lutecite), suggest an origin from previous anhydrite nodules. The anhydrite nodules at Laño were produced by the percolation of marine brines, during a period corresponding to a sedimentary gap, with δ³⁴S and δ¹⁸O mean values of 18.8‰ and 13.6‰ respectively, consistent with Upper Cretaceous seawater sulphate values. Higher δ³⁴S and δ¹⁸O mean values of 21.2‰ and 21.8‰ recorded in the Tubilla del Agua section are interpreted as being due to a partial bacterial sulphate reduction process in a more restricted marine environment. The idea that sulphates may have originated from the leaching of previously deposited Keuper sulphate evaporites with subsequent precipitation as anhydrite, is rejected because the δ³⁴S, δ¹⁸O and ⁸⁷Sr/⁸⁶Sr values of anhydrite laths observed at both the Tubilla del Agua and Laño sections suggest an origin from younger marine brines. Later calcite replacement and precipitation of geode‐filling calcite is recorded in both sections, with δ¹³C and δ¹⁸O values indicating the participation of meteoric waters. Synsedimentary activity of the Peñacerrada diapir, which lies close to the Laño section, played a significant role in the local shallowing of the basin and the formation of quartz geodes. In contrast, eustatic shallowing of the inner marine series of the Tubilla del Agua section led to the generation of morphologically similar quartz geodes. Copyright © 2002 John Wiley & Sons, Ltd.     

Micro-scale S isotope studies of the Kharaelakh intrusion, Noril’sk region, Siberia: Constraints on the genesis of coexisting anhydrite and sulfide minerals

The coexistence of magmatic anhydrite and sulfide minerals in non-arc-related mafic magmas has only rarely been documented. Likewise the S isotope fractionation between sulfate and sulfide in mafic rocks has infrequently been measured. In the Kharaelakh intrusion associated with the world-famous Noril’sk ore district in Siberia coexisting magmatic anhydrite and sulfide minerals have been identified. Sulfur isotope compositions of the anhydrite-sulfide assemblages have been measured via both ion microprobe and conventional analyses to help elucidate the origin of the anhydrite-sulfide pairs. Magmatic anhydrite and chalcopyrite are characterized by δ³⁴S values between 18.8‰ and 22.8‰, and 9.3‰ and 13.2‰, respectfully. Coexisting anhydrite and chalcopyrite show Δ values that fall between 8.5‰ and 11.9‰. Anhydrite in the Kharaelakh intrusion is most readily explained by the assimilation of sulfate from country rocks; partial reduction to sulfide led to mixing between sulfate-derived sulfide and sulfide of mantle origin. The variable anhydrite and sulfide δ³⁴S values are a function of differing degrees of sulfate reduction, variable mixing of sulfate-derived and mantle sulfide, incomplete isotopic homogenization of the magma, and a lack of uniform attainment of isotopic equilibrium during subsolidus cooling. The δ³⁴S values of sulfide minerals have changed much less with cooling than have anhydrite values due in large part to the high sulfide/sulfate ratio. Variations in both sulfide and anhydrite δ³⁴S values indicate that isotopically distinct domains existed on a centimeter scale. Late stage hydrothermal anhydrite and pyrite also occur associated with Ca-rich hydrous alteration assemblages (e.g., thomsonite, prehnite, pectolite, epidote, xonotlite). δ³⁴S values of secondary hydrothermal anhydrite and pyrite determined by conventional analyses are in the same range as those of the magmatic minerals. Anhydrite-pyrite Δ values are in the 9.1-10.1‰ range, and are smaller than anticipated for the low temperatures indicated by the silicate alteration assemblages. The small Δ values are suggestive of either sulfate-sulfide isotopic disequilibrium or closure of the system to further exchange between ∼550 and 600 °C. Our results confirm the importance of the assimilation of externally derived sulfur in the generation of the elevated δ³⁴S values in the Kharaelakh intrusion, but highlight the sulfur isotopic variability that may occur in magmatic systems. In addition, our results confirm the need for more precise experimental determination of sulfate-sulfide sulfur isotope fractionation factors in high-T systems.     

Constraining the marine strontium budget with natural strontium isotope fractionations (Sr/Sr∗, δSr) of carbonates, hydrothermal solutions and river waters

We present strontium (Sr) isotope ratios that, unlike traditional ⁸⁷Sr/⁸⁶Sr data, are not normalized to a fixed ⁸⁸Sr/⁸⁶Sr ratio of 8.375209 (defined as δ^88/86Sr = 0 relative to NIST SRM 987). Instead, we correct for isotope fractionation during mass spectrometry with a ⁸⁷Sr-⁸⁴Sr double spike. This technique yields two independent ratios for ⁸⁷Sr/⁸⁶Sr and ⁸⁸Sr/⁸⁶Sr that are reported as (⁸⁷Sr/⁸⁶Sr∗) and (δ^88/86Sr), respectively. The difference between the traditional radiogenic (⁸⁷Sr/⁸⁶Sr normalized to ⁸⁸Sr/⁸⁶Sr = 8.375209) and the new ⁸⁷Sr/⁸⁶Sr∗ values reflect natural mass-dependent isotope fractionation. In order to constrain glacial/interglacial changes in the marine Sr budget we compare the isotope composition of modern seawater ((⁸⁷Sr/⁸⁶Sr∗, δ^88/86Sr)_Seawater) and modern marine biogenic carbonates ((⁸⁷Sr/⁸⁶Sr∗, δ^88/86Sr)_Carbonates) with the corresponding values of river waters ((⁸⁷Sr/⁸⁶Sr∗, δ^88/86Sr)_River) and hydrothermal solutions ((⁸⁷Sr/⁸⁶Sr∗, δ^88/86Sr)_HydEnd) in a triple isotope plot. The measured (⁸⁷Sr/⁸⁶Sr∗, δ^88/86Sr)_River values of selected rivers that together account for ∼18% of the global Sr discharge yield a Sr flux-weighted mean of (0.7114(8), 0.315(8)‰). The average (⁸⁷Sr/⁸⁶Sr∗, δ^88/86Sr)_HydEnd values for hydrothermal solutions from the Atlantic Ocean are (0.7045(5), 0.27(3)‰). In contrast, the (⁸⁷Sr/⁸⁶Sr∗, δ^88/86Sr)_Carbonates values representing the marine Sr output are (0.70926(2), 0.21(2)‰). We estimate the modern Sr isotope composition of the sources at (0.7106(8), 0.310(8)‰). The difference between the estimated (⁸⁷Sr/⁸⁶Sr∗, δ^88/86Sr)_input and (⁸⁷Sr/⁸⁶Sr∗, δ^88/86Sr)_output values reflects isotope disequilibrium with respect to Sr inputs and outputs. In contrast to the modern ocean, isotope equilibrium between inputs and outputs during the last glacial maximum (10-30 ka before present) can be explained by invoking three times higher Sr inputs from a uniquely “glacial” source: weathering of shelf carbonates exposed at low sea levels. Our data are also consistent with the “weathering peak” hypothesis that invokes enhanced Sr inputs resulting from weathering of post-glacial exposure of abundant fine-grained material.     

Mineralogical and isotopic record of biotic and abiotic diagenesis of the Callovian-Oxfordian clayey formation of Bure (France)

The Callovian-Oxfordian (COx) clayey unit is being studied in the Eastern part of the Paris Basin at depths between 400 and 500 m depth to assess of its suitability for nuclear waste disposal. The present study combines new mineralogical and isotopic data to describe the sedimentary history of the COx unit. Petrologic study provided evidence of the following diagenetic mineral sequence: (1) framboidal pyrite and micritic calcite, (2) iron-rich euhedral carbonates (ankerite, sideroplesite) and glauconite (3) limpid calcite and dolomite and celestite infilling residual porosity in bioclasts and cracks, (4) chalcedony, (5) quartz/calcite. Pyrite in bioturbations shows a wide range of δ³⁴S (−38‰ to +34.5‰), providing evidence of bacterial sulphate reduction processes in changing sedimentation conditions. The most negative values (−38‰ to −22‰), measured in the lower part of the COx unit indicate precipitation of pyrite in a marine environment with a continuous sulphate supply. The most positive pyrite δ³⁴S values (−14‰ up to +34.5‰) in the upper part of the COx unit indicate pyrite precipitation in a closed system. Celestite δ³⁴S values reflect the last evolutionary stage of the system when bacterial activity ended; however its deposition cannot be possible without sulphate supply due to carbonate bioclast dissolution. The ⁸⁷Sr/⁸⁶Sr ratio of celestite (0.706872-0.707040) is consistent with deposition from Jurassic marine-derived waters. Carbon and oxygen isotopic compositions of bulk calcite and dolomite are consistent with marine carbonates. Siderite, only present in the maximum clay zone, has chemical composition and δ¹⁸O consistent with a marine environment. Its δ¹³C is however lower than those of marine carbonates, suggesting a contribution of ¹³C-depleted carbon from degradation of organic matter. δ¹⁸O values of diagenetic chalcedony range between +27‰ and +31‰, suggesting precipitation from marine-derived pore waters. Late calcite crosscutting a vein filled with chalcedony and celestite, and late euhedral quartz in a limestone from the top of the formation have lower δ¹⁸O values (∼+19‰), suggesting that they precipitated from meteoric fluids, isotopically close to present-day pore waters of the formation. Finally, the study illustrates the transition from very active, biotic diagenesis to abiotic diagenesis. This transition appears to be driven by compaction of the sediment, which inhibited movement of bacterial cells by reduction of porosity and pore sizes, rather than a lack of inorganic carbon or sulphates.     

Magnesium isotope systematics of the lithologically varied Moselle river basin, France

Magnesium and strontium isotope signatures were determined during different seasons for the main rivers of the Moselle basin, northeastern France. This small basin is remarkable for its well-constrained and varied lithology on a small distance scale, and this is reflected in river water Sr isotope compositions. Upstream, where the Moselle River drains silicate rocks of the Vosges mountains, waters are characterized by relatively high ⁸⁷Sr/⁸⁶Sr ratios (0.7128-0.7174). In contrast, downstream of the city of Epinal where the Moselle River flows through carbonates and evaporites of the Lorraine plateau, ⁸⁷Sr/⁸⁶Sr ratios are lower, down to 0.70824.Magnesium in river waters draining silicates is systematically depleted in heavy isotopes (δ²⁶Mg values range from −1.2 to −0.7‰) relative to the value presently estimated for the continental crust and a local diorite (−0.5‰). In comparison, δ²⁶Mg values measured in soil samples are higher (∼0.0‰). This suggests that Mg isotope fractionation occurs during mineral leaching and/or formation of secondary clay minerals. On the Lorraine plateau, tributaries draining marls, carbonates and evaporites are characterized by low Ca/Mg (1.5-3.2) and low Ca/Sr (80-400) when compared to local carbonate rocks (Ca/Mg = 29-59; Ca/Sr = 370-2200), similar to other rivers draining carbonates. The most likely cause of the Mg and Sr excesses in these rivers is early thermodynamic saturation of groundwater with calcite relative to magnesite and strontianite as groundwater chemistry progressively evolves in the aquifer. δ²⁶Mg of the dissolved phases of tributaries draining mainly carbonates and evaporites are relatively low and constant throughout the year (from −1.4‰ to −1.6‰ and from −1.2‰ to −1.4‰, respectively), within the range defined for the underlying rocks. Downstream of Epinal, the compositions of the Moselle River samples in a δ²⁶Mg vs. ⁸⁷Sr/⁸⁶Sr diagram can be explained by mixing curves between silicate, carbonate and evaporite waters, with a significant contribution from the Vosgian silicate lithologies (>70%). Temporal co-variation between δ²⁶Mg and ⁸⁷Sr/⁸⁶Sr for the Moselle River throughout year is also observed, and is consistent with a higher contribution from the Vosges mountains in winter, in terms of runoff and dissolved element flux. Overall, this study shows that Mg isotopes measured in waters, rocks and soils, coupled with other tracers such as Sr isotopes, could be used to better constrain riverine Mg sources, particularly if analytical uncertainties in Mg isotope measurements can be improved in order to perform more precise quantifications.     

The geographic distribution of Sr isotopes from surface waters and soil extracts over the island of Bornholm (Denmark) – A base for provenance studies in archaeology and agriculture

In this paper we report the Sr isotope signatures, and Sr, Al and Na concentrations of 30 surface waters (lakes/ponds and rivers/creeks) and 19 soil sample extracts from the island of Bornholm (Denmark) and present a categorized ⁸⁷Sr/⁸⁶Sr value distribution map that may serve as a base for provenance studies, including archaeological migration and authenticity proof for particular food products. The Sr isotopic compositions of surface waters range from ⁸⁷Sr/⁸⁶Sr = 0.7097–0.7281 (average 0.7175 ± 0.0049; 1σ), whereas 0.1 M HNO₃, 0.05 M HNO₃, and 0.01 M CaCl₂ soil extracts range from ⁸⁷Sr/⁸⁶Sr = 0.7095–0.7197 and define somewhat lower but statistically indistinguishable averages of 0.7125 ± 0.003 (1s). These compositions are lower than the values expected from the Precambrian granitoid basement (⁸⁷Sr/⁸⁶Sr = 0.758–0.944), and from the overlying, mainly clastic Paleozoic sediments. Combined Sr isotope composition vs. Sr, Na and Al concentration relationships of soil extracts imply that lowering of the isotopic composition of leachable Sr on Bornholm results as a consequence of significant admixture to this fraction of Sr deposited as marine salts (aerosols), and that rainwater only has a minor influence on the Sr budget of the surface waters. Positively correlated Al/Na and [1/Sr] vs. ⁸⁷Sr/⁸⁶Sr relationships in soil extracts and surface waters indicate that the surface run-off on Bornholm is characterized by two predominant sources, namely marine aerosols (sea salts) with high Sr and low ⁸⁷Sr/⁸⁶Sr values, and a source with lower [Sr] delivering radiogenic Sr to the surface waters, which we equate with Sr leached from the products of mineral weathering (soils).A feasibility study for using Sr isotopic compositions of surface waters and soil extracts as a proxy for bioavailable Sr signatures was performed with a few samples collected in the vicinity of the eleventh century AD Ndr. Grødbygård cemetery site in SW Bornholm, from where Sr isotope compositions of modern fauna samples and tooth enamel of humans buried in the cemetery have been reported. Waters and soil extracts studied herein from around this site range from ⁸⁷Sr/⁸⁶Sr = 0.7104–0.7166 and correspond to Sr compositions extracted from snail shells in this area which span a range of ⁸⁷Sr/⁸⁶Sr = 0.7095–0.7160. Some human tooth enamel is characterized by more radiogenic values (⁸⁷Sr/⁸⁶Sr up to 0.718) which points to a possible provenance of these humans from the granite–gneiss terrain in the north of the island and/or to immigration of these humans in their childhood from other places (for example from mainland Sweden) to Bornholm. If the total compositional range of ⁸⁷Sr/⁸⁶Sr = 0.709–0.718 (n = 44) recorded in human enamel from the Ndr. Grødbygård site is considered representative for the variation of bioavailable Sr on Bornholm, then our soil leachate and surface water data entirely covers this range. We therefore propose that the combination of Sr isotope analyses of surface waters and soil leachates are an easy, fast and relatively cost efficient way to characterize a local bioavailable ⁸⁷Sr/⁸⁶Sr signature, and consequently propose that the overall average of ⁸⁷Sr/⁸⁶Sr = 0.7153 ± 0.0048 (1σ; n = 50) can be taken as a band for bioavailable Sr fractions suitable to discriminate between local and non-local signatures in provenance studies in the field of archaeology and for food and plant authenticity control in agricultural applications.     

Sulfur isotope systematics of the 1982 El Chichón trachyandesite: an ion microprobe study

Sulfur isotopic compositions were determined by ion microprobe for 36 spots on anhydrite crystals in trachyandesitic pumices erupted from El Chichón Volcano in 1982. Individual anhydrite crystals are homogeneous in δ³⁴S, within the ±1‰ (2σ) uncertainty of the method, but crystal-to-crystal variations are large (+2.5 to +10.9‰). The mean δ³⁴S for anhydrite (+6.4 ± 2.1‰, 1σ) is significantly lower than earlier results for bulk anhydrite separates (+9.0 to +9.2‰). The difference between the mean δ³⁴S values in these two populations may reflect a grain-size effect, with heavier sulfur concentrated in smaller anhydrite crystals, few of which were analyzed by ion microprobe. Variations in δ³⁴S show no correlation with complex textures in anhydrite revealed by cathodoluminescence color. Ion-microprobe analyses of δ³⁴S were also obtained on six ovoid-shaped inclusions of pyrrhotite, chalcopyrite, and/or intermediate sulfide solid solution hosted by silicate or oxide crystals, interpreted to be magmatic (δ³⁴S = −0.1 to +2.7‰; mean +0.7‰), and on four irregularly shaped multiphase sulfide fragments in the matrix, interpreted as xenocrystic, which range widely in δ³⁴S (−3.7 to +5.5‰). We evaluate four different mixing scenarios involving (1) magmatic anhydrite and sedimentary sulfate, (2) magmatic anhydrite and hydrothermal anhydrite, and anhydrite and coexisting sulfide crystals precipitated in different domains of a common magma reservoir that were affected by (3) different degrees of degassing or (4) different degrees of crustal sulfur contamination. The model involving physical contamination of sedimentary sulfate is considered untenable. The other three models are considered to be viable, but none of them can explain all observations. The results of this study and other recent investigations prompt a re-evaluation of the sulfur budget for the 1982 El Chichón eruption. We estimate that 2.2 × 10¹³ g of S was emitted, and that 58 wt.% of the sulfur was present as anhydrite prior to eruption, with the remainder in a vapor phase, with H₂S/SO₂ ≈ 9. The bulk magmatic δ³⁴S value for the 1982 El Chichón trachyandesite is estimated as +4.1 to +5.8‰, typical of the relatively heavy sulfur isotopic compositions that characterize subduction-related magmas.     

Sr, C, and O isotope geochemistry of Ordovician brachiopods: a major isotopic event around the Middle-Late Ordovician transition

Here we present Sr, C, and O isotope curves for Ordovician marine calcite based on analyses of 206 calcitic brachiopods from 10 localities worldwide. These are the first Ordovician-wide isotope curves that can be placed within the newly emerging global biostratigraphic framework. A total of 182 brachiopods were selected for C and O isotope analysis, and 122 were selected for Sr isotope analysis. Seawater ⁸⁷Sr/⁸⁶Sr decreased from 0.7090 to 0.7078 during the Ordovician, with a major, quite rapid fall around the Middle-Late Ordovician transition, most probably caused by a combination of low continental erosion rates and increased submarine hydrothermal exchange rates. Mean δ¹⁸O values increase from −10‰ to −3‰ through the Ordovician with an additional short-lived increase of 2 to 3‰ during the latest Ordovician due to glaciation. Although diagenetic alteration may have lowered δ¹⁸O in some samples, particularly those from the Lower Ordovician, maximum δ¹⁸O values, which are less likely to be altered, increase by more than 3‰ through the Ordovician in both our data and literature data. We consider that this long-term rise in calcite δ¹⁸O records the effect of decreasing tropical seawater temperatures across the Middle-Late Ordovician transition superimposed on seawater δ¹⁸O that was steadily increasing from ≤−3‰ standard mean ocean water (SMOW). By contrast, δ¹³C variation seems to have been relatively modest during most of the Ordovician with the exception of the globally documented, but short-lived, latest Ordovician δ¹³C excursion up to +7‰. Nevertheless, an underlying trend in mean δ¹³C can be discerned, changing from moderately negative values in the Early Ordovician to moderately positive values by the latest Ordovician. These new isotopic data confirm a major reorganization of ocean chemistry and the surface environment around 465 to 455 Ma. The juxtaposition of the greatest recorded swings in Phanerozoic seawater ⁸⁷Sr/⁸⁶Sr and δ¹⁸O at the same time as one of the largest marine transgressions in Phanerozoic Earth history suggests a causal link between tectonic and climatic change, and emphasizes an endogenic control on the O isotope budget during the Early Paleozoic. Better isotopic and biostratigraphic constraints are still required if we are to understand the true significance of these changes. We recommend that future work on Ordovician isotope stratigraphy focus on this outstanding Middle-Late Ordovician event.     

Oxygen,sulfur, and strontium isotope and fluid inclusion studies of barite deposits from the Iglesiente-Sulcis mining district,Southwestern Sardinia,Italy

This paper deals with barite from stratiform, karst, and vein deposits hosted within Lower Paleozoic rocks of the Iglesiente-Sulcis mining district in southwestern Sardinia. For comparison sulfates from mine waters are studied. Stratiform barite displays ³⁴S=28.8–32.1, ¹⁸O=12.7–15.6, and ⁸⁷Sr/⁸⁶Sr=0.7087, in keeping with an essentially Cambrian marine origin of both sulfate and strontium. Epigenetic barite from post-Hercynian karst and vein deposits is indistinguishable for both sulfur and oxygen isotopes with ³⁴S=15.3–26.4 and ¹⁸O=6.6–12.5; ⁸⁷Sr/⁸⁶Sr ratios vary 0.7094–0.7140. These results and the microthermometric and salinity data from fluid inclusions concur in suggesting that barite formed at the site of mineralization by oxidation of reduced sulfur from Cambrian-Ordovician sulfide ores in warm, sometimes hot solutions consisting of dilute water and saline brine with different ¹⁸O values. The relative proportion of the two types of water may have largely varied within a given deposit during the mineralization. In the karst barite Sr was essentially provided by carbonate host rocks, whereas both carbonate and Lower Paleozoic shale host rocks should have been important sources for Sr of the vein barite. Finally, ³⁴S data of dissolved sulfate provide further support for the mixed seawater-meteoric water composition of mine waters from the Iglesiente area.     

A δS isoscape of total sulphur in soils across Northern Ireland

A map of the spatial variation in isotopic composition of a substrate, or isoscape, provides a tool to address a range of research questions, such as the use of isotope fingerprinting to identify the origin of compounds. The focus of this study was to establish a soil S isotope map for Northern Ireland in order to investigate spatial variability within the isotopic composition of total soil sulphur (S). The sample resolution was about one sample every 141 km². δ³⁴S values were grouped based on soil type and soil parent material. Total soil S δ³⁴S data were grouped according to soil type, to evaluate whether a soil’s characteristics affect its isotopic composition. Gleyed soils had a mean δ³⁴S value of +6.3 ± 6.1‰, lower than the means for other soil types. A trend towards higher δ³⁴S values (mean δ³⁴S of +13.8 ± 6.0‰) was observed in soils with a high organic matter content. No trends in δ³⁴S value were observed for other soil types. There were no discernible correlations between total soil S δ³⁴S and soil parent material, with the exception of shale and mudstones. The majority of soils derived from shale and mudstones had δ³⁴S values at or close to the mean of +4.9‰. A spatially coherent pattern of relatively high soil total S δ³⁴S values in the west and north, and low in the SE, is discernible in Northern Ireland. Based on the observed spatial distribution, it has been concluded that the quantity and source (anthropogenic or marine) of wet atmospheric S deposition is a controlling factor on regional variation of soil δ³⁴S in Northern Ireland. This S isoscape will assist studies of S cycling in Northern Ireland and may assist in the creation of an ‘isotopic fingerprint’ for a potential ‘input’ source needed to interpret data in traceability studies.     

Controls on the spatial and temporal variability of vadose dripwater geochemistry: Edwards aquifer, central Texas

A 4-yr study of spatial and temporal variability in the geochemistry of vadose groundwaters from caves within the Edwards aquifer region of central Texas offers new insights into controls on vadose groundwater evolution, the relationship between vadose and phreatic groundwaters, and the fundamental influence of soil composition on groundwater geochemistry. Variations in Sr isotopes and trace elements (Mg/Ca and Sr/Ca ratios) of dripwaters and soils from different caves, as well as phreatic groundwaters, provide the potential to distinguish between local variability and regional processes controlling fluid geochemistry, and a framework for understanding the links between climatic and hydrologic processes.The Sr isotope compositions of vadose cave dripwaters (mean ⁸⁷Sr/⁸⁶Sr = 0.7087) and phreatic groundwaters (mean ⁸⁷Sr/⁸⁶Sr = 0.7079) generally fall between values for host carbonates (mean ⁸⁷Sr/⁸⁶Sr = 0.7076) and exchangeable Sr in overlying soils (mean ⁸⁷Sr/⁸⁶Sr = 0.7088). Dripwaters have lower Mg/Ca and Sr/Ca ratios, and higher ⁸⁷Sr/⁸⁶Sr values than phreatic groundwaters. Dripwater ⁸⁷Sr/⁸⁶Sr values also inversely correlate with both Mg/Ca and Sr/Ca ratios. Mass-balance modeling combined with these geochemical relationships suggest that variations in fluid compositions are predominantly controlled by groundwater residence times, and water-rock interaction with overlying soils and host aquifer carbonate rocks. Consistent differences in dripwater geochemistry (i.e., ⁸⁷Sr/⁸⁶Sr, Mg/Ca, and Sr/Ca) between individual caves are similar to compositional differences in soils above the caves. While these differences appear to exert significant control on local fluid evolution, geochemical and isotopic variations suggest that the controlling processes are regionally extensive. Temporal variations in ⁸⁷Sr/⁸⁶Sr values and Mg/Ca ratios of dripwaters from some sites over the 4-yr interval correspond with changes in both aquifer and climatic parameters. These results have important implications for the interpretation of trace element and isotopic variations in speleothems as paleoclimate records, as well as the understanding of controls on water chemistry for both present-day and ancient carbonate aquifers.     

四川盆地东部地区早中三叠世蒸发岩的锶同位素特征及其地质意义

锶同位素已经成为全球海平面变化、造山运动、古气候和古环境等全球地质事件研究与对比的有效工具之一。本文以四川盆地东部地区早中三叠世蒸发岩的野外剖面和钻孔岩心为主要研究对象,测试了碳酸盐岩、硫酸盐岩和石盐岩的锶同位素组成,并建立了相应的锶同位素演化曲线。研究结果显示,碳酸盐岩的⁸⁷Sr/⁸⁶Sr值平均为0.707 895,硬石膏岩的⁸⁷Sr/⁸⁶Sr值平均为0.708 174,石盐岩的⁸⁷Sr/⁸⁶Sr值平均为0.708 177,同时碳酸盐岩的⁸⁷Sr/⁸⁶Sr值从早三叠世的0.707 413快速增加到中三叠世早期的0.708 515,而后呈现下降趋势。从总体上看,这些⁸⁷Sr/⁸⁶Sr值与全球早中三叠世同期的⁸⁷Sr/⁸⁶Sr值数据接近,说明四川盆地东部地区早中三叠世钾盐的物质来源大部分为海水,并沉积于海相沉积环境,同时由于火山...     

Evidence for mass-dependent isotopic fractionation of strontium in a glaciated granitic watershed

The stable isotope composition of strontium (expressed as δ^88/86Sr) may provide important constraints on the global exogenic strontium cycle. Here, we present δ^88/86Sr values and ⁸⁷Sr/⁸⁶Sr ratios for granitoid rocks, a 150 yr soil chronosequence formed from these rocks, surface waters and plants in a small glaciated watershed in the central Swiss Alps. Incipient chemical weathering in this young system, whether of inorganic or biological origin, has no resolvable effect on the ⁸⁷Sr/⁸⁶Sr ratios and δ^88/86Sr values of bulk soils, which remain indistinguishable from bedrock in terms of Sr isotopic composition. Although due in part to the chemical heterogeneity of the forefield, the lack of a resolvable difference between soil and bedrock isotopic composition indicates that these soils have thus far witnessed minimal net loss of Sr; a low degree of chemical weathering is also implied by bulk soil chemistry. The isotopic composition of Sr in streamwater is more radiogenic than median soil, reflecting the preferential weathering of biotite in the catchment; streamwater δ^88/86Sr values, however, are indistinguishable from bulk soil δ^88/86Sr values, implying that no resolvable fractionation of Sr isotopes takes place during release to the weathering flux in the Damma forefield. Analyses of plant tissue reveal that plants (Rhododendron and Vaccinium) preferentially assimilate the lighter isotopes of Sr such that their δ^88/86Sr values are significantly lower than those of the soils in which they grow. Additionally, δ^88/86Sr values of foliar and floral tissues are lower than those of roots, contrary to observations for Ca, for which Sr is often used as an analogue in weathering studies. We suggest that processes that discriminate against Sr in favour of Ca, due to the different nutritional requirement of plants for these two elements, are responsible for the observed contrast.     

The origin and mechanisms of salinization of the lower Jordan river

The chemical and isotopic (⁸⁷Sr/⁸⁶Sr, δ¹¹B, δ³⁴S_sulfate, δ¹⁸O_water, δ¹⁵N_nitrate) compositions of water from the Lower Jordan River and its major tributaries between the Sea of Galilee and the Dead Sea were determined in order to reveal the origin of the salinity of the Jordan River. We identified three separate hydrological zones along the flow of the river:

(1)

A northern section (20 km downstream of its source) where the base flow composed of diverted saline and wastewaters is modified due to discharge of shallow sulfate-rich groundwater, characterized by low ⁸⁷Sr/⁸⁶Sr (0.7072), δ³⁴S_sulfate (−2‰), high δ¹¹B (∼36‰), δ¹⁵N_nitrate (∼15‰) and high δ¹⁸O_water (−2 to-3‰) values. The shallow groundwater is derived from agricultural drainage water mixed with natural saline groundwater and discharges to both the Jordan and Yarmouk rivers. The contribution of the groundwater component in the Jordan River flow, deduced from mixing relationships of solutes and strontium isotopes, varies from 20 to 50% of the total flow.

(2)

A central zone (20-50 km downstream from its source) where salt variations are minimal and the rise of ⁸⁷Sr/⁸⁶Sr and SO₄/Cl ratios reflects predominance of eastern surface water flows.

(3)

A southern section (50-100 km downstream of its source) where the total dissolved solids of the Jordan River increase, particularly during the spring (70-80 km) and summer (80-100 km) to values as high as 11.1 g/L. Variations in the chemical and isotopic compositions of river water along the southern section suggest that the Zarqa River (⁸⁷Sr/⁸⁶Sr∼0.70865; δ¹¹B∼25‰) has a negligible affect on the Jordan River. Instead, the river quality is influenced primarily by groundwater discharge composed of sulfate-rich saline groundwater (Cl^-=31-180 mM; SO₄/Cl∼0.2-0.5; Br/Cl∼2-3×10^-3; ⁸⁷Sr/⁸⁶Sr∼0.70805; δ¹¹B∼30‰; δ¹⁵N_nitrate ∼17‰, δ³⁴S_sulfate=4-10‰), and Ca-chloride Rift valley brines (Cl^-=846-1500 mM; Br/Cl∼6-8×10^-3; ⁸⁷Sr/⁸⁶Sr∼0.7080; δ¹¹B>40‰; δ³⁴S_sulfate=4-10‰). Mixing calculations indicate that the groundwater discharged to the river is composed of varying proportions of brines and sulfate-rich saline groundwater. Solute mass balance calculations point to a ∼10% contribution of saline groundwater (Cl⁻=282 to 564 mM) to the river. A high nitrate level (up to 2.5 mM) in the groundwater suggests that drainage of wastewater derived irrigation water is an important source for the groundwater. This irrigation water appears to leach Pleistocene sediments of the Jordan Valley resulting in elevated sulfate contents and altered strontium and boron isotopic compositions of the groundwater that in turn impacts the water quality of the lower Jordan River.

     

Petrogenesis of the Kirkpatrick Basalt,Solo Nunatak,northern Victoria Land,Antarctica, based on isotopic compositions of strontium,oxygen and sulfur

Chemical and isotopic compositions of Jurassic tholeiites of the Kirkpatrick Basalt Group from Solo Nunatak, northern Victoria Land, indicate that these rocks are contaminated with crustal material. The basalts are fine grained and contain phenocrysts of augite, pigeonite, hypersthene and plagioclase. The flows on Solo Nunatak are chemically more similar to average tholeiite than flows from Mt. Falla and Storm Peak in the Central Transantarctic Mountains (TAM) which appear to be more highly differentiated. Initial ⁸⁷Sr/⁸⁶Sr ratios of the flows on Solo Nunatak are high (>0.710) and are similar to those reported for the Kirkpatrick Basalt in the Central TAM. Whole-rock δ¹⁸O values are also high, ranging from +6.0 to +9.3‰ and correlate positively with initial ⁸⁷Sr/⁸⁶Sr ratios, similar to the Kirkpatrick Basalt in the Central TAM. The correlation between initial ⁸⁷Sr/⁸⁶Sr ratios and δ¹⁸O values is explained as the result of simultaneous fractional crystallization and assimilation of a crustal contaminant. Sulfur isotope compositions vary between limits of δ³⁴S= -4.01 to +3.41‰ Variations in (δ³⁴S probably resulted from outgassing of SO₂ under varying oxygen fugacities. Laboratory for Isotope Geology and Geochemistry (Isotopia), Contribution No. 71     

Baseline determination of the atmospheric Pb,Sr and Nd isotopic compositions in the Rhine valley,Vosges mountains (France) and the Central Swiss Alps

The Pb, Sr and Nd isotopic compositions of biomonitors (lichen, moss, bark) and soil litter from different regions in the Rhine valley, as well as of <0.45 μm particles separated out of ice of the Rhône and Oberaar glaciers and lichens from the Swiss Central Alps, have been determined in order to deduce the natural baseline of the atmospheric isotopic compositions of these regions, which are suggested to be close to the isotopic compositions of the corresponding basement rocks or soils at the same sites. ²⁰⁶Pb/²⁰⁷Pb and ⁸⁷Sr/⁸⁶Sr isotope ratios are positively correlated. Most polluted samples from traffic-rich urban environments have the least radiogenic Pb and Sr isotopic compositions with ²⁰⁶Pb/²⁰⁷Pb and ⁸⁷Sr/⁸⁶Sr ratios of 1.11 and 0.7094, respectively. These ratios are very different from those of the atmospheric baseline for the Vosges mountains and the Rhine valley (²⁰⁶Pb/²⁰⁷Pb: 1.158–1.167; ⁸⁷Sr/⁸⁶Sr: 0.719–0.725; ε_Nd: −7.5 to −10.1). However, this study indicates that the baseline of the atmospheric natural Pb and Sr isotopic compositions is affected by anthropogenic (traffic, industrial and urban) emissions even in remote areas. Lichen samples from below the Rhône and Oberaar glaciers reflect the baseline composition close to the Grimsel pass in the Central Swiss Alps (⁸⁷Sr/⁸⁶Sr: 0.714 − 0.716; ε_Nd: −3.6 to −8.1). The ¹⁴³Nd/¹⁴⁴Nd isotope ratios are highly variable (8ε units) and it is suggested that the variation of the ¹⁴³Nd/¹⁴⁴Nd is controlled by wet deposition and aerosols originating from the regional natural and industrial urban environments and from more distant regions like the Sahara in North Africa. The least anthropogenetically affected samples collected in remote areas have isotopic compositions closest to those of the corresponding granitoid basement rocks.