Rare earth elements,neodymium and strontium isotopic systematics in mineral waters: evidence from the Massif Central,France

Rare Earth Elements (REEs), and Sr and Nd isotope distributions, have been studied in mineralized waters from the Massif Central (France). The CO₂-rich springs are characterized by a neutral pH (6–7) associated with total dissolved solids (TDS) from 1 to 7 g l⁻¹. The waters result from the mixing of very mineralized water pools, thought to have equilibrated at a temperature of around 200°C with superficial waters. These two mineral water pools evidenced by Sr isotopes and dissolved REEs could reflect 2 different stages of water–rock interaction and an equilibrium with different mineral assemblages.The concentrations of individual dissolved REEs and total dissolved REEs (ΣREE), in the mineral waters examined, vary over several orders of magnitude but are not dependent on the main parameters of the waters (TDS, T°C, pH, Total Organic C). The dissolved REE concentrations presented as upper continental crust normalized patterns show HREE enrichment in most of the samples. The time evolution of REE patterns does not show significant fluctuations except in 1 borehole, located in the Limagne d’Allier area, which was sampled on 16 occasions over an 18 month period. Ten samples are HREE-enriched, whereas 6 samples show flat patterns.The aqueous speciation of REEs shows that CO²⁻₃ complexes dominate (>80%) over the free metal, F⁻, SO²⁻₄ and HCO⁻₃ complexes. The detailed speciation demonstrates that the fractionation of REEs (i.e. the HREE enrichment) in CO₂-rich and pH neutral fluids is due essentially to the predominance of the CO²⁻₃ complexes.The Sr isotopic composition of the mineral waters in the Massif Central shows different mixing processes; in the Cézallier area at least 3 end-member water types exist. The most dilute end-member is likely to originate as poorly mineralized waters with minimal groundwater circulation. Two other mineralized end-members are identified, although the link between the geographical location of spring outflow and the mixing proportion between the 2 end-members is not systematic. The range in ϵ_Nd(0) for mineralized waters in the Massif Central correlates well with that of the known parent rocks except for 4 springs. One way to explain the ϵ_Nd(0) in these instances is a contribution from drainage of volcanic rocks. The isotopic systematics help to constrain the hydrogeological models for this area.     

二元混合体系的端元Sm-Nd模式年龄计算方法

报道二元混合体系的端元Sm-Nd模式年龄的计算方法,即利用二元混合体系的Rb-Sr、Sm-Nd同位素分析结果,可以计算2个端元的Sm-Nd同位素组成,两端元在混源样品中的质量百分比,以及地壳残留时间。同时计算了广西花山复式花岗岩的端元模式年龄,结果显示该岩体的物质来源于2500Ma华南古陆壳和622Ma从亏损地幔分异出来的大洋地壳的混合物。     

沉积岩系互稀释混合元素丰度关系

二元混合成国岩石元素丰度（ｘ，ｙ）遵守分式线性函数定律:式中ａ，ｂ，ｃ，ｄ为丰度关系常数，它们只和岩系中两个已知岩石（１和２）中元素丰度（ｘ_１，ｙ_１；ｘ_２，ｙ_２）有关，并等于其镜像∑变换（混合变换）（ａ，ｂ，ｃ，ｄ）＝∑∑|ｘ_１，ｙ_１；ｘ_２，ｙ_２|（２）元素丰度关系常数中，如果ｂ与Ｃ之比值小于零时，元素丰度ｙ＝Ｘ呈互稀释混合关系。     

哈密土墩矿区高光谱影像蚀变矿物识别初步研究

文章以哈密土墩矿区为例,利用航空高光谱HyMAP数据,从单矿物光谱与混合像元光谱之间的相似概率和模型化参数出发,发展了基于光谱混合组成的极大相关高光谱矿物识别方法,初步识别出该区与成矿关系密切的6种蚀变矿物及其空间分布趋势,并为野外地质工作与薄片鉴定所证实。该研究初步表明,高光谱遥感可为地质学中矿物的空间分布研究提供有用的新技术。     

Refinements in a site-mixing model for illites: local electrostatic balance and the quasi-chemical approximation

A quasi-chemical model for illites has been derived, and local electrostatic balance has been added to a random regular solution site-mixing model for illites (Stoessell, 1979). Each model assumes similar order-disorder conditions for both the end-members micas and the solid solution. Thermodynamic properties of illites predicted by the random, electrostatic, and quasi-chemical models are compared as a function of composition. For natural illite compositions, molar entropies of mixing in the electrostatic model are about 1 entropy unit less than those in the random model. Intermediate values are given by the quasi-chemical model. Each model predicts an increased entropy of mixing in dominantly trioctahedral illites as compared to dioctahedral illites. Each model also predicts destabilization of trioctahedral illites using absolute molar exchange energies greater than 2 RT/Z_x, where Z_x is the number of adjacent cation interactions per site in the Xth site class. The most negative free energies of mixing are predicted by the quasi-chemical model. Intermediate values predicted by the random model are apparently the result of error cancellation due to overestimation of both the entropy and enthalpy of mixing.     

Solid solutions of trace Eu(III) in calcite: Thermodynamic evaluation of experimental data over a wide range of pH and pCO2

The thermodynamics of dilute Eu-calcite solid solutions formed under widely different pH-pCO₂ conditions at T = 25°C and p = 1 bar were investigated using three sets of Eu(III) uptake experiments, two of which were taken from the literature: (a) recrystallization in synthetic cement pore water at pH ∼ 13 and pCO₂ ∼ 10⁻¹³ bar (this work); (b) coprecipitation in 0.1 M NaClO₄ at pH ∼ 6 and pCO₂ ∼ 1 bar; (c) coprecipitation in synthetic seawater at pH ∼ 8 and pCO₂ ranging from 3 × 10⁻⁴ to 0.3 bar.Solid solution formation was modeled using the Gibbs energy minimization (GEM) method. In a first step (“forward” modeling), we tested ideal binary solid solution models between calcite and the Eu end-members Eu₂(CO₃)_3, EuNa(CO₃)₂, Eu(OH)CO₃ or Eu(OH)₃, for which solids with independently measured solubility products exist. None of these four binary solid solutions was capable of reproducing all three experimental datasets simultaneously. In a second step (“inverse” modeling), ideal binary solid solutions were constructed between calcite and the candidate Eu end-members EuO(OH), EuH(CO₃)₂ and EuO(CO₃)_0.5, for which no independent solubility products are available. For each single data point and each of these end-members, a free energy of formation with inherent activity coefficient term ( = G_α^o + RT lnγ_α) was estimated from “dual thermodynamic” GEM calculations. The statistical mean of was then calculated for each of the three datasets. A specific end-member was considered to be acceptable if a standard deviation of ± 2 kJ mol⁻¹ or less resulted for each single dataset, and if the mean -values calculated for the three datasets coincided. No binary solid solution with any of the seven above mentioned end-members proved to satisfy these criteria.The third step in our analysis involved consideration of ternary solid solutions with CaCO₃ as the major end-member and any two of the seven considered Eu trace end-members. It was found that the three datasets can only be reproduced simultaneously with the ternary ideal solid solution EuH(CO₃)₂ - EuO(OH) - CaCO₃, setting = −1773 kJ mol⁻¹ and = −955 kJ mol⁻¹, whereas all other end-member combinations failed. Our results are consistent with time-resolved laser fluorescence data for Cm(III) and Eu(III) indicating that two distinct species are incorporated in calcite: one partially hydrated, the other completely dehydrated. In conclusion, our study shows that substitution of trivalent for divalent cations in carbonate crystal structures is a more complex process than the classical isomorphic divalent-divalent substitution and may need consideration of multicomponent solid solution models.     

Raman spectra of silicate garnets

The single-crystal polarized Raman spectra of four natural silicate garnets with compositions close to end-members almandine, grossular, andradite, and uvarovite, and two synthetic end-members spessartine and pyrope, were measured, along with the powder spectra of synthetic pyrope-grossular and almandine-spessartine solid solutions. Mode assignments were made based on a comparison of the different end-member garnet spectra and, in the case of pyrope, based on measurements made on additional crystals synthesized with ²⁶Mg. A general order of mode frequencies, i.e. R(SiO₄)>T(metal cation)>T(SiO₄), is observed, which should also hold for most orthosilicates. The main factors controlling the changes in mode frequencies as a function of composition are intracrystalline pressure (i.e. oxygen-oxygen repulsion) for the internal SiO₄-vibrational modes and kinematic coupling of vibrations for the external modes. Low frequency vibrations of the X-site cations reflect their weak bonding and dynamic disorder in the large dodecahedral site, especially in the case of pyrope. Two mode behavior is observed for X-site cation vibrations along the pyrope-grossular binary, but not along the almandine-spessartine join. Received: 3 December 1996 / Revised, accepted: 13 April 1997     

Chemical and mineralogical evidence of the occurrence of mantle metasomatism by carbonate-rich melts in an oceanic environment (Santiago Island, Cape Verde)

Lavas from Santiago Island attest to a complex magmatic history, in which heterogeneous mantle source(s) and the interactions of advecting magmas with thick metasomatised oceanic lithosphere played an important role in the observed isotopic and trace element signatures. Young (<3.3 Ma) primitive lavas from Santiago Island are characterised by pronounced negative K anomalies and trace element systematics indicating that during partial melting D_K>D_Ce. These features suggest equilibration with an oceanic lithospheric mantle containing K-rich hydrous mineral assemblages, consistent with the occurrence of amphibole + phlogopite in associated metasomatised lherzolite xenoliths, where orthopyroxene is partially replaced by newly formed olivine + (CO₂ + spinel + carbonate inclusion-rich) clinopyroxene. Metasomatism induced a decrease in $ a ^{{{\text{melt}}}}_{{{\text{SiO}}_{{\text{2}}} }} $ and Ti/Eu ratios, as well as an increase in fO ₂ , Ca/Sc and Sr/Sm in the Santiago magmas, suggesting a carbonatitic composition for the metasomatic agent. Santiago primitive lavas are highly enriched in incompatible elements and show a moderate range in isotopic compositions (⁸⁷Sr/⁸⁶Sr?=?0.70318–0.70391, ¹⁴³Nd/¹⁴⁴Nd?=?0.51261–0.51287, ¹⁷⁶Hf/¹⁷⁷Hf?=?0.28284–0.28297). Elemental and isotopic signatures suggest the involvement of HIMU and EM1-type mantle end-members, in agreement with the overall isotopic characteristics of the southern Cape Verde Islands. The overall geochemical characteristics of lavas from Santiago Island allow us to consider the EM1-like end-member as resulting from the involvement of subcontinental lithospheric mantle in the genesis of magmas on Santiago.     

Carbon isotope analyses of n-alkanes in dust from the lower atmosphere over the central eastern Atlantic

Atmospheric dust samples collected along a transect off the West African coast have been investigated for their lipid content and compound-specific stable carbon isotope compositions. The saturated hydrocarbon fractions of the organic solvent extracts consist mainly of long-chain n-alkanes derived from epicuticular wax coatings of terrestrial plants. Backward trajectories for each sampling day and location were calculated using a global atmospheric circulation model. The main atmospheric transport took place in the low-level trade-wind layer, except in the southern region, where long-range transport in the mid-troposphere occurred. Changes in the chain length distributions of the n-alkane homologous series are probably related to aridity, rather than temperature or vegetation type. The carbon preference of the leaf-wax n-alkanes shows significant variation, attributed to a variable contribution of fossil fuel- or marine-derived lipids. The effect of this nonwax contribution on the δ¹³C values of the two dominant n-alkanes in the aerosols, n-C₂₉ and n-C₃₁ alkane, is, however, insignificant. Their δ¹³C values were translated into a percentage of C₄ vs. C₃ plant type contribution, using a two-component mixing equation with isotopic end-member values from the literature. The data indicate that only regions with a predominant C₄ type vegetation, i.e. the Sahara, the Sahel, and Gabon, supply C₄ plant-derived lipids to dust organic matter. The stable carbon isotopic compositions of leaf-wax lipids in aerosols mainly reflect the modern vegetation type along their transport pathway. Wind abrasion of wax particles from leaf surfaces, enhanced by a sandblasting effect, is most probably the dominant process of terrigenous lipid contribution to aerosols.     

Chemical and strontium isotopic characteristics of shallow groundwater in the Ordos Desert Plateau,North China: Implications for the dissolved Sr source and water–rock interactions

In this study, the chemical and Sr isotopic compositions of shallow groundwater and rainwater in the Ordos Desert Plateau, North China, and river water from the nearby Yellow River, are investigated to determine the dissolved Sr source and water–rock interactions, and quantify the relative Sr contribution from each end-member. Three groundwater systems have been identified, namely, GWS-1, GWS-2 and GWS-3 according to the watershed distribution in the Ordos Desert Plateau. Ca²⁺ and Mg²⁺ are the most dominant cations in GWS-1, while Na⁺ is dominant in GWS-3. In addition, there is more SO₄²⁻ and less Cl⁻ in GWS-1 than in GWS-3. The shallow groundwater in GWS-2 seems to be geochemically between that in GWS-1 and GWS-3. The ⁸⁷Sr/⁸⁶Sr ratios of the shallow groundwater are high in GWS-1 and GWS-2 and are low in GWS-3. By geochemically comparing the nearby Yellow River, local precipitation and deep groundwater, the shallow groundwater is recharged only by local precipitation. The ionic and isotopic ratios indicate that carbonate dissolution is an important process controlling the chemistry of the shallow groundwater. The intensity of the water–rock interactions varies among the three groundwater systems and even within each groundwater system. Three end-members controlling the groundwater chemistry are isotopically identified: (1) precipitation infiltration, (2) carbonate dissolution and (3) silicate weathering. The relative Sr contributions of the three end-members show that precipitation infiltration and carbonate dissolution are the primary sources of the shallow groundwater Sr in GWS-3 whereas only carbonate dissolution is responsible for the shallow groundwater Sr in GWS-1 and GWS-2. Silicate weathering seems insignificant towards the shallow groundwater's chemistry in the Ordos Desert Plateau. This study is helpful for understanding groundwater chemistry and managing water resources.     

Rare earth element behavior and Pb,Sr, Nd isotope systematics in a heavy metal contaminated soil

The aim of this study is to characterize the processes and phases which control migration and retention of rare earth elements (REE) in a heavy metal contaminated soil. In addition to concentration data, we used Pb, Sr and Nd isotopic compositions in order to distinguish between natural and anthropogenic trace metals and to characterize the phases leached away during the sequential extraction procedure.The samples were sequentially extracted in 3 steps with 1 N acetic acid (HAc), 1 N HCl and 1 N HNO₃. The Pb isotope data showed that anthropogenic Pb had mainly been retained in the uppermost 10 cm by the organic matter of the topsoil. The⁸⁷Sr/⁸⁶Sr ratios of the HAc extracts are almost constant and indicate that soil carbonate is derived from regionally outcropping carbonate-rich sediments. Most HCl and HNO₃ extracts have more radiogenic Sr isotopic compositions, but it is unclear whether this reflects a growing influence of anthropogenic or silicate-derived Sr.The depth distribution of the REE is mainly controlled by two different parameters: soil pH for the HAc extractable REE and FeMn oxides for the REE in the HCl and HNO₃ extracts. A part of the HNO₃ extractable REE was also bound to the organic matter of the topsoil. The REE concentrations in the HAc extractable phase increase with depth and increasing soil pH, which indicates that they are derived from the surface and hence are of anthropogenic origin. This is confirmed by¹⁴³Nd/¹⁴⁴Nd isotope ratios which show a mixing between a natural end-member at the top and an anthropogenic end-member at the base of the profile. We assume that the anthropogenic REE were transported in dissolved form as carbonate complexes and then precipitated during downward migration as soil pH increased.     

The energetics of natural garnet solid solution

Approximate mixing properties of the end-member components of the quarternary garnet solid solution, (Fe,Mg,Ca,Mn)₃Al₂Si₃O₁₂, have been derived through theoretical analysis of observational data, combined with certain experimental results and crystal chemical considerations. The results suggest that the mixing of pyrope with grossularite, spessartite, and almandine would involve significant positive excess free energies of mixing leading to the critical mixing temperatures of 694±55, 535±140, and 479±63 °C respectively. Spessartite would mix with almandine nearly ideally, and with grossularite with small positive deviation from ideality. The quarternary solution reduces essentially to a ternary mixture of pyrope, grossularite, and almandine + spessartite. The solid solubility relation, and tie line coordinates in this ternary system has been calculated as a function of temperature; the solid solution is found to be intrinsically stable for practically all ternary compositions at 600 °C.     

Microscale carbon isotope variability in ALH84001 carbonates and a discussion of possible formation environments

The carbonates in martian meteorite ALH84001 preserve a record of aqueous processes on Mars at 3.9 Ga, and have been suggested to contain signatures of ancient martian life. The conditions of the carbonate formation environment are critical for understanding possible evidence for life on Mars, the history of water on Mars, and the evolution of the martian atmosphere. Despite numerous studies of petrographic relationships, microscale oxygen isotope compositions, microscale chemical compositions, and other minerals associated with the carbonates, formation models remain relatively unconstrained. Microscale carbon isotope analyses of ALH84001 carbonates reveal variable δ¹³C values ranging from +27 to +64‰. The isotopic compositions are correlated with chemical composition and extent of crystallization such that the Mg-poor, early-formed carbonates are relatively ¹³C depleted and the Mg-rich, later forming carbonates, are ¹³C enriched. These data are inconsistent with many of the previously proposed environments for carbonate formation, and a new set of hypotheses are proposed. Specifically, two new models that account for the data involve low temperature (<100°C) aqueous processes: (1) the carbonates formed during mixing of two fluids derived from separate chemical and isotopic reservoirs; or (2) the carbonates formed from high pH fluids that are exposed to a CO₂-rich atmosphere and precipitate carbonate, similar to high pH springs on Earth.     

Relative contributions of silicate and carbonate rocks to riverine Sr fluxes in the headwaters of the Ganges

Exhumation of the Himalayan-Tibetan orogen is implicated in the marked rise in seawater ⁸⁷Sr/⁸⁶Sr ratios since 40 Ma. However both silicate and carbonate rocks in the Himalaya have elevated ⁸⁷Sr/⁸⁶Sr ratios and there is disagreement as to how much of the ⁸⁷Sr flux is derived from silicate weathering. Most previous studies have used element ratios from bedrock to constrain the proportions of silicate- and carbonate-derived Sr in river waters. Here we use arrays of water compositions sampled from the head waters of the Ganges in the Indian and Nepalese Himalaya to constrain the end-member element ratios. The compositions of tributaries draining catchments restricted to a limited range of geological units can be described by two-component mixing of silicate and carbonate-derived components and lie on a plane in multicomponent composition space. Key elemental ratios of the carbonate and silicate components are determined by the intersection of the tributary mixing plane with the planes Na = 0 for carbonate and constant Ca/Na for silicate. The fractions of Sr derived from silicate and carbonate sources are then calculated by mass-balance in Sr-Ca-Mg-Na composition space. Comparison of end-member compositions with bedrock implies that secondary calcite deposition may be important in some catchments and that dissolution of low-Mg trace calcite in silicate rocks may explain discrepancies in Sr-Ca-Na-Mg covariation. Alternatively, composition-dependent precipitation or incongruent dissolution reactions may rotate mixing trends on cation-ratio diagrams. However the calculations are not sensitive to transformations of the compositions by incongruent dissolution or precipitation processes provided that the transformed silicate and carbonate component vectors are constrained. Silicates are calculated to provide ∼50% of the dissolved Sr flux from the head waters of the Ganges assuming that discrepancies between Ca-Mg-Na covariation and the silicate rock compositions arise from addition of trace calcite. If the Ca-Mg-Na mixing plane is rotated by composition-dependent secondary calcite deposition, this estimate would be increased. Moreover, when ⁸⁷Sr/⁸⁶Sr ratios of the Sr inputs are considered, silicate Sr is responsible for 70 ± 16% (1σ) of the ⁸⁷Sr flux forcing changes in seawater Sr-isotopic composition. Since earlier studies predict that silicate weathering generates as little as 20% of the total Sr flux in Himalayan river systems, this study demonstrates that the significance of silicate weathering can be greatly underestimated if the processes that decouple the water cation ratios from those of the source rocks are not properly evaluated.     

The stabilities of gibbsite,boehmite, aluminous goethites and aluminous hematites in bauxites,ferricretes and laterites as a function of water activity,temperature and particle size

Stability relationships between Al-goethite, Al-hematite, boehmite and gibbsite are presented in terms of water activity [H₂O], temperature (T), grain size and bulk-composition in the system Fe₂O₃-Al₂O₃-H₂O at a total pressure of 1 bar.Al-goethite and Al-hematite are treated as ideal solid solutions, the former of the end-members FeOOH (goethite) and AlOOH (diaspore) and the latter of the end-members Fe₂O₃ (hematite) and Al₂O₃ (corundum). Using log K_sp provided by the literature for the various phases involved, the common associations observed in laterites, bauxites and ferricretes do not have stability fields over geologically reasonable intervals of [H₂O] and T. Consequently a new internally consistent set of log K_sp values is proposed and used, and allows such associations to have actual stability fields in the appropriate diagrams. The new log K_sp values used in the calculations are such that the solubilities of the end members are greater than those commonly listed. This is in agreement with natural observations which show that such minerals, are generally poorly crystallized and of very small size. The assumption of an ideal solid solution in aluminous goethite and aluminous hematite combined with the new log K_sp values leads to prediction of composition limits for these two minerals which agree well with observed values. The fact that an ideal solid solution must extend continuously from one end-member to the other is masked by the appearance of other stability fields (e.g. gibbsite or boehmite) which cross and overlap a part of the solid solution stability fields of AlOOH-FeOOH and Fe₂O₃-Al₂O₃.     

Salinity of the Archaean oceans from analysis of fluid inclusions in quartz

Fluids trapped in inclusions in well-characterized Archaean hydrothermal quartz crystals were analyzed by the extended argon–argon method, which permits the simultaneous measurement of chlorine and potassium concentrations. Argon and nitrogen isotopic compositions of the trapped fluids were also determined by static mass spectrometry. Fluids were extracted by stepwise crushing of quartz samples from North Pole (NW Australia) and Barberton (South Africa) 3.5–3.0-Ga-old greenstone belts. The data indicate that fluids are a mixture of a low salinity end-member, regarded as the Archaean oceanic water, and several hydrothermal end-members rich in Cl, K, N, and radiogenic parentless ⁴⁰Ar. The low Cl–K end-member suggests that the salinity of the Archaean oceans was comparable to the modern one, and that the potassium content of the Archaean oceans was lower than at present by about 40%. A constant salinity of the oceans through time has important implications for the stabilization of the continental crust and for the habitability of the ancient Earth.     

用生物标志物定量计算混合原油油源的数学模型

多烃源层叠合盆地混源油油源的定量计算是十分困难的问题.典型原油的人工混合配比实验显示,当两个生物标志物浓度不同的原油相混合时,各类生物标志物的比值参数随端元原油混入量呈非线性变化,以生物标志物比值参数和简单的二元线性关系方程定量计算混源油的混合比例将导致错误的结论.三个或者三个以上原油相混合时,各类生物标志物比值参数的变化更加复杂,各端元原油的贡献更加难以判识.但是,人工混合模拟实验表明混合油中生物标志物绝对含量与端元油的混入量呈线性关系,数学推导证明了这种线性关系,由此推导出相应的数学计算模型,其中:二元混合时比值参数与混入量呈双曲线关系,三元混合呈双曲面关系,四元及其以上的多元混合呈多维曲面,可以矩阵的方式定量计算各端元油的比例.依据这些数学模型,应用生物标志物的绝对含量和(或)生物标志物比值参数均可以定量计算出混源原油中各类原油的贡献比例.数学模型比通常的人工模拟实验方法更加经济、方便、精确和可靠.     

The recharge of glacial meltwater and its influence on the geochemical evolution of groundwater in the Ordovician-Cambrian aquifer system,northern part of the Baltic Artesian Basin

The geochemical evolution of groundwater in the Ordovician-Cambrian aquifer system in the northern part of the Baltic Artesian Basin (BAB) illustrates how continental glaciations have influenced groundwater systems in proglacial areas. The aquifer system contains water that has originated from various end-members: recent meteoric water, glacial meltwater and relict Na-Cl brine. The saline formation water that occupied the aquifer system prior to the glacial meltwater intrusion has been diluted by meltwaters of advancing-retreating ice sheets. The diversity in the origin of groundwater in the aquifer system is illustrated by a wide variety in δ¹⁸O values that range from −11‰ to −22.5‰. These values are mostly depleted with respect to values found in modern precipitation in the area. The chemical and isotopic composition of groundwater has been influenced by mixing between waters originating from different end-members. In addition, the freshening of a previously saline water aquifer due to glacial meltwater intrusion has initiated various types of water-rock interaction (e.g. ion exchange, carbonate mineral dissolution).     

Selected organic matter source indicators in the Orinoco,Nile and Changjiang deltas

Carbon isotopic composition, carbon to nitrogen ratios, carbon preference (n-C₂₃ to n-C₃₂), and microscopic examination of selected samples were compared and contrasted as organic matter source indicators. The Orinoco, Nile, and Changjiang deltas were sampled and analyzed. The δ ¹³C of organic matter increased with distance offshore at all three locations. Organic carbon content decreased seaward on the Orinoco and Changjiang deltas but increased on the Nile delta. C/N ratios decreased regularly with distance offshore on the Orinoco delta, ranged from 10 to 25, and correlated positively with δ ¹³C_OM. Estimations of the relative input of marine versus terrestrial organic matter to the sediments was dependent on the parameter analyzed. These discrepancies were primarily due to the definition of end-member compositions and the fraction of the total organic matter that is measured by a given parameter. Multiple-source indicators must be measured and integrated and end-members must be properly defined in order to obtain realistic input estimates.     

Experimental determination of synthetic NdPO4 monazite end-member solubility in water from 21°C to 300°C: implications for rare earth element mobility in crustal fluids

The solubility of synthetic NdPO₄ monazite end-member has been determined experimentally from 21 to 300°C in aqueous solutions at pH = 2, and at 21°C and pH = 2 for GdPO₄. Measurements were performed in batch reactors, with regular solution sampling for pH measurement, rare earths and phosphorous analysis by inductively coupled plasma mass spectrometry (ICP-MS) coupled with a desolvation system. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were employed to check that no reprecipitation of secondary phases occurred and that the mineral surfaces remained those of a monazite. Coupled with speciation calculations, measured solution compositions permitted the determination of NdPO₄ and GdPO₄ solubility products which are in general agreement with previous experimental determination on rhabdophane at 25°C, but showing that monazite is more than two orders of magnitude less soluble than inferred on the basis of previous thermodynamic estimates. The temperature evolution from 21 to 300°C of the equilibrium constant (K) of the NdPO₄ monazite end-member dissolution reaction given by: