Determination of Rare Earth Element Isotopic Compositions Using Sample-Standard Bracketing and Double-Spike Approaches

Rare earth elements (REEs) have been found to have numerous uses to trace geological and cosmochemical processes through analyses of elemental patterns, radioactive decay, nucleosynthetic anomalies, and cosmogenic effects. Stable isotopic fractionation is one aspect of REE geochemistry that has been seldom studied, with most publications focusing on the development of analytical methodologies for individual REEs, and most applications concerning terrestrial igneous rocks. In this study, we present a method to systematically analyze stable isotopic fractionations of 8 REEs, including Ce, Nd, Sm, Eu, Gd, Dy, Er, and Yb, using sample-standard bracketing (SSB) and double-spike (DS) approaches. All REEs are separated and purified using a fluoropolymer pneumatic liquid chromatography (FPLC) system. We introduce procedures for identifying and correcting some isobaric interferences in double-spike data reduction. Several geostandards, including igneous rocks and sediments, are analyzed using SSB and DS methods. The results indicate that REE isotopic fractionation in igneous processes is limited, except for Eu. Other REEs can still be isotopically fractionated by low-temperature processes and kinetic effects at a high temperature

Investigating the enigma of an irregular groundwater age pattern in a confined, presumed “fossil” complex aquifer through mixing cell flow modeling

Significant fluctuations in the groundwater (GW) age along the eastern flow path of the Nubian Sandstone Aquifer's (NSA), as derived from Krypton-81 groundwater dating, have suggested that this aquifer, located in Israel's Negev Desert and previously presumed to be a fossil, is not entirely isolated but mixes with younger and even recent water. The intermittent rejuvenation and drastic increases in the GW age across short distances most likely imply hydraulic connectivity with the surrounding aquifers, which contribute both younger and more ancient water to the NSA. The current study aims at modeling the GW flow system to locate and quantify its water sources despite the aquifer's hydrogeological complexity and the scarcity of hydrological data. We implemented the Mixing Cell Modeling (MCM) approach, understanding that the alternating rejuvenations and increases in the GW age downstream of the NSA's eastern flow trajectory reflect the mixing of the NSA's groundwater with young and old GW bodies, respectively. Thus, prompted by the 81Kr water age distribution, yet independent of the Kr radioisotope data, a multi-tracer mixing cell flow model was adopted based on a set of balance equations of water, dissolved minerals, and stable environmental isotopes. The findings indicate that (1) there is a small, yet substantial, intrusion of old brackish GW from a deep-seated, highly pressurized aquifer into the NSA in the northeastern Negev; (2) the rejuvenation of GW in the NSA is due to significant mixing with water from nearby overlying carbonate and chert aquifers, and (3) the NSA is substantially replenished through the Nubian Sandstone (NS) outcrops along the Negev Desert anticlines. Most GW intrusions into the NSA occur near the intersections of the eastern flow path with some of the Negev's major faults and synclines, such as the Paran and Ramon Fault zones and the Zin Syncline. In light of the relatively young GW age at the end of the NSA's western flow path in the northern Negev, and based on the similarities in the hydrogeological structures in the Negev and northern Sinai Deserts, we propose that similar mixing processes with GW from the overlying carbonate aquifers and direct GW recharge through the NS outcrops also occur in the northern Sinai Peninsula. The approach presented in this study might apply to examining recharge processes and hydraulic connectivity in other aquifers that were formerly classified as “fossil,” such as the immense NSA found in the Arabian (Jordan & Saudi Arabia) and the Western (Egypt) Deserts

L'azote, le néon et le xénon dans le manteau : sources, processus et hétérogénéités

Texte intégral accessible uniquement aux membres de l'Université de LorraineThis study is devoted for (i) the precise ana)yses of noble gas isotopic compositions in plume-related sample from Kola, Russian and (ii) a detailed study on nitrogen isotopic composition in peridotite xenoliths by stepwisecombustion. High 20Ne/22Ne ratio was observed in one of the sam pies, suggesting the presence of solar nebula during the accretion of the earth and its incorporation in the deep mantle. The ratios of radiogenicnoble gases (4He-21Ne-136Xe) in Kola samples implies that whole mantle region was subject to primitive degassing over 560 Ma. A d15N value as low as -17 [pour mille] was observed for the first time as a bulk composition of terrestrial silicate. ln mantle xenoliths, the N2/Ar ratios were significantly enhanced compared to that of thefluid inclusions. An implication of this observation is that the behaviour of nitrogen and argon in the geochemical processes may occasionally differ.Des avances significatives dans notre compréhension de l'origine et du cycle des volatils terrestres nécessitent de ; (i) préciser le mode d'acquisition du néon solaire dans le manteau, (ii) déterminer les chronologies du dégazage des différents réservoirs mantéliques et (iii) comprendre le comportement de l'azote dans le manteau. Cette étude a réalisé (i) des analyses précises de gaz rare mantéliques dans les roches plutoniques de Kola et (ii) des analyses détaillées sur les xénolites péridotitiques utilisant une méthode de combustion par palier de température. Un rapport élevé en 20Ne/22Ne a été observé, suggérant la présence probable de la nébuleuse proto-solaire pendant l'accrétion de la Terre, et un piégeage de cette composante dans le manteau terrestre. La corrélation entre les rapports 3He/22Ne et les degrés d'enrichissement sur les rapports 21Ne*/22Ne indique que le phénomène de fusion partielle serait à l'origine de l'hétérogénéité sur les deux rapports. Les rapports entre les gaz rares radiogéniques (4He-21Ne-136Xe) de Kola indiquent que ce réservoir a perdu du xénon issu de la fission du 244Pu (T1/2 = 82 Myr), par rapport à un réservoir chondritique. Ce résultat impliquerait que la plupart des régions du manteau aient été sujettes au dégazage primitif à l'origine de l'atmosphère. Le modèle élaboré dans cette étude (basé sur un processus suivant une cinétique du premier ordre) permet d'estimer la durée de ce dégazage à plusieurs centaines de Myr. Une valeur de [delta] 15N à -17 [pour mille] a été observée pour la première fois sur la composition totale d'un minéral silicaté (phlogopite) terrestre, cependant la plupart les xénolites étudiés sont caractérisées par une valeur [delta] 15N >0 [pour mille], distincte de celles des MORBs et des diamants ([delta] 15N - -3.5 [pour mille]). L'origine de cette hétérogénéité isotopique considérable reste encore inexpliquée et nécessitent une étude plus approfondie. Dans les xénolites péridotitiques, les rapports N2[delta]/Ar sont significativement supérieurs à ceux obtenu dans les inclusions fluides utilisant une méthode de broyage pour l'extraction du gaz. Ceci peut suggérer un comportement différent de l'argon et l'azote dans les cycles géochimiques. L'existence vraisemblable de NH4+ dans les phlogopites conforte cette hypothèse. Les implications de ces résultats sur l'âge de fermeture de l'atmosphère, la contribution du dégazage primitif à l'atmosphère, et la signification des différences d'âge entre le système Hf-W et le système I-Pu-U-Xe sont discutés, ainsi que la nécessité d'études expérimentales

Nitrogen in peridotite xenoliths: lithophile behavior and magmatic isotope fractionation

In order to document the origin and speciation of nitrogen in mantle-derived rocks and minerals, the N and Ar contents and isotopic compositions were investigated for hydrous an

Determination of Rare Earth Element Isotopic Compositions Using Sample-Standard Bracketing and Double-Spike Approaches

Publication status: PublishedRare earth elements (REEs) have been found to have numerous uses to trace geological and cosmochemical processes through analyses of elemental patterns, radioactive decay, nucleosynthetic anomalies, and cosmogenic effects. Stable isotopic fractionation is one aspect of REE geochemistry that has been seldom studied, with most publications focusing on the development of analytical methodologies for individual REEs, and most applications concerning terrestrial igneous rocks. In this study, we present a method to systematically analyze stable isotopic fractionations of 8 REEs, including Ce, Nd, Sm, Eu, Gd, Dy, Er, and Yb, using sample-standard bracketing (SSB) and double-spike (DS) approaches. All REEs are separated and purified using a fluoropolymer pneumatic liquid chromatography (FPLC) system. We introduce procedures for identifying and correcting some isobaric interferences in double-spike data reduction. Several geostandards, including igneous rocks and sediments, are analyzed using SSB and DS methods. The results indicate that REE isotopic fractionation in igneous processes is limited, except for Eu. Other REEs can still be isotopically fractionated by low-temperature processes and kinetic effects at a high temperature

Recent seawater intrusion into deep aquifer determined by the radioactive noble-gas isotopes 81Kr and 39Ar

Radioactive noble-gas isotopes tracers 81Kr and 39Ar are used for the first time to measure the residence times of deep (∼1000 m) saline coastal groundwater, and to determine its connection mode with the sea. The average rate of seawater intrusion into the deep aquifer in Israel, located near the Mediterranean Sea, is estimated. 81Kr-ages of the saline water samples, found to be younger than 40 ka, contradict previously estimated ages of up to several million years based on hydrogeological considerations. The new results imply a stronger and more recent connection between the aquifer and the sea, and indicate that the intrusion occurred during the sea-level rise that began about 20 ka ago. These coastal aquifers need to be managed with caution because lowering of the adjacent fresh water level due to over pumping could accelerate seawater intrusion in a relatively short time. This study demonstrates the suitability of these two noble-gas tracers for the examination of hydrogeological systems in general and for the study of seawater intrusion in particular

Field Degassing as a New Sampling Method for 14C Analyses in Old Groundwater

AbstractRadiocarbon (14C) activity in groundwater can be used to determine subsurface residence time up to ∼40 kyr, providing crucial information on dynamic properties of groundwater and on paleoclimate. However, commonly applied sampling methods for dissolved inorganic carbon (DIC-14C) are prone to low level of modern atmospheric contamination, resulting in underestimation of groundwater ages that cluster around 30–40 kyr. We extract CO2gas from groundwater using a device originally developed for studies of noble gas radionuclides. Carbon is collected in the gas phase, eliminating the possibility of fostering microbial activities and aqueous chemical reactions during sample storage. This method collects CO2-14C and radiokrypton (81Kr and85Kr) samples simultaneously. The presence of any shorter-lived85Kr is used to evaluate the degree of atmospheric contamination during sampling or mixing of young groundwater. Most groundwater samples showed lower CO2-14C activities than those of DIC-14C, presumably due to the absence of atmospheric contamination. Samples with81Kr age exceeding 150 kyr have no detectable CO2-14C except where mixing sources of young groundwater is suspected. These field data serve as confirmations for the reliability of the newly presented sample collection and CO2-14C method, and for the outstanding roles of radiokrypton isotopes in characterizing old groundwater.</jats:p

Identifying recharge processes into a vast "fossil" aquifer based on dynamic groundwater 81Kr age evolution

Water in deep aquifers in arid regions is often considered to be “fossil” when modern recharge rates are negligible relative to the reservoir capacity. Over the past five decades, the Nubian Sandstone Aquifer (NSA) in the arid region of the Sinai Peninsula (Egypt) and the Negev Desert (Israel) has been considered to contain fossil water based on 14C dating, which revealed 14C ages of about 30 kyr over most of the aquifer. However, this relatively homogeneous age distribution contradicts the expected increase in groundwater age in the direction of decreasing piezometric head along the flow trajectories. Here, dating results with the longer-lived 81Kr radioisotope (t1/2 = 229 ± 11 kyr) are presented, highlighting a wide age range of 40 kyr to 630 kyr in the confined sections of the aquifer, all with very low 14C activity (<1 pmC). Elevated 81Kr and 14C activities were only observed within or close to the system's recharge areas. These findings support a new perception of groundwater replenishment during different epochs from the early mid-Pleistocene to the Holocene. By tracking the downstream age evolution, rejuvenation was identified in places where the confinement had been breached. At other locations, the existence of an older groundwater body contributing to the aquifer was detected by means of strongly depleted 81Kr activity. High spatial heterogeneity in groundwater ages close to the discharge zone of the system is attributed to pronounced age stratification with depth. Calculated ages in the more isolated sections of the system were used to assess regional flow velocity, hydraulic conductivity, and their agreement with present recharge rates. We conclude that groundwater ages should be reevaluated with 81Kr in regional aquifers where low 14C activities prevail. With an effective age range beyond one million years, this may enable the reconstruction of recharge history well into the Pleistocene and provide crucial information for the management of groundwater resources