Incorporation Of Rare Earth Elements Research Articles

Cathodoluminescence tools provide clues to depositional history in Miocene and Pliocene mammalian teeth

This paper presents the results of a study which used cathodoluminescence (CL) observations (optical images and SEM spectra) as a tool for assessing diagenetic processes during fossilisation of continental mammalian teeth. The study was performed on fossil teeth of large mammals (Gomphotheriidae, Rhinocerotidae and Hippopotamidae) from Namibia and from Kenya, with ages from the Lower and Middle Miocene to Pliocene. The fossil sites are distinguished by their environmental conditions. They include floodplain deposits, fluvial deposits of the Proto-Orange River and aeolianite deposits in Namibia, and fluvio-lacustrine deposits of different ages in Kenya. Optical CL provides information about the sites' environments and diagenetic processes related to those environments: a blue intrinsic luminescence for the aeolianite deposits and purple to yellowish for floodplain or fluvial deposits. The CL-spectra of these samples show sharp emission bands, which are classically related to late (recent) incorporation of rare-earth elements (REE) substituting in the Ca 2+ sites. High-resolution mapping of the distribution of these REE shows a gradient from the enamel interior (low concentrations) to the outer enamel margin (high concentrations).

Rare-earth elemental systematics of mixed-layered illite–smectite from sedimentary and hydrothermal environments of the Western Carpathians (Slovakia)

Mixed-layered illite–smectite separates of various sizes from bentonites and shales collected in sedimentary and hydrothermal units of the Western Carpathians were analyzed for their major-, trace- and rare-earth elements (REEs). The major elements fairly reflect the changes in the clay mineralogy related to illitization, but a lack of correlation between this chemical composition and the REE contents suggests that their incorporation and fractionation by nucleating and growing illite-type particles is complex during both burial-induced diagenesis and hydrothermal activity. During crystal growth of illite-type fundamental particles, illitization driven by burial-related temperature increase does not fractionate the REEs incorporated in the mineral structures, but it increases their contents, suggesting that the bentonite and shale units of the sedimentary sequence act as closed systems. The amount of smectite layers in the I–S does not relate to the sum of REE or to the La/Lu ratio in the bentonites and shales of the East Slovak Basin sedimentary sequence, outlining that REE incorporation and fractionation concern s the illite layers, not the smectite layers. Alternatively, the hydrothermal system of Dolná Ves acts as an open system with supply of the REEs by external hot fluids, so that the REE contents are not determined by the precipitation of illite. The mixed-layers illite–smectite of the shales yield a narrow span of total REE abundances from 17.3 to 36.5 μg/g, compared to those of the associated bentonites, from 1.20 to 93.7 μg/g, of the same basin. These differences result from varied origins of the original REE-carriers. The Eu anomaly relates to alteration of the authigenic minerals in the sedimentary bentonites acting as a closed system, or to the chemistry of the percolating hot fluids in the Dolná Ves hydrothermal deposit. The data also indicate that no-detectable zircon, apatite, monazite, or any other REE-carrier occurred in the size fractions, at least in the finest, and if any, these minerals had no significant impact on the REE contents and distribution patterns of the varied analyzed size fractions consisting mainly if not exclusively of mixed-layered illite–smectite.

Incorporation of REE into leucophanite: a compositional and structural study

Abstract The crystal structures of nine, and the chemical compositions of ten, natural samples of leucophanite, ideally NaCaBeSi2O6F, were investigated. The analysed samples display a large compositional variation with trace-element abundances >50,000 ppm, primarily due to rare earth elements (REE). Fromthese data, we propose a substitution scheme for the incorporation of REE for Ca, with additional Na substituting for Ca and the generation of vacancies to ensure charge balance. Compositional zonation was observed in some samples; this zonation correlates with variations in cathodoluminescence. The crystal structure of the nine analysed samples could all be refined in space group P212121. We found no evidence for a reduction of symmetry with increased trace-element concentration. Various twin combinations were observed and these seem related to crystallization conditions rather than structural or chemical factors.

An improved method of controlling rare earth incorporation in optical fiber

The process parameters involved at various stages of solution doping method for fabricating rare earth (RE) doped optical fiber have been systematically investigated to optimize the process conditions and achieve better control over RE incorporation. It is observed that the RE concentration, uniformity and the overall fiber quality are strongly influenced by the solution properties and dipping parameters. The soaking solution needs to be judiciously selected depending on the porous layer characteristics to improve the fiber quality and process efficiency. An interesting observation not hitherto reported is the influence of Al ion concentration in the solution on RE incorporation into the core. The investigation helps to obtain the optimum conditions necessary to produce fibers of given specification and achieve greater reproducibility.

Do manganese nodules grow or dissolve after burial? Results from the Central Indian Ocean Basin

Fifty buried manganese nodules at different depth intervals were recovered in 12 sediment cores from the Central Indian Ocean Basin (CIOB). A maximum of 15 buried nodules were encountered in one sediment core (AAS-22/GC-07) and the deepest nodule was recovered at 5.50 m below seafloor in core AAS-04/GC-5A. Approximately 80% of the buried nodules are small in size (∼2 cm diameter) in contrast to the Atlantic Ocean and Peru Basin (Pacific Ocean) where the majority of the buried nodules are large, ∼8 cm and >6 cm, respectively. Buried nodule size decreases with core depth and this distribution appears to be similar to the phenomenon of “Brazil Nut Effect”. Buried nodules exhibit both smooth and rough surface textures and are ellipsoidal, elongated, rounded, sub rounded, irregular and polynucleated. Buried nodules from siliceous ooze are enriched in Mn, Cu, Ni, Zn, Mo, Ga, V and Rb whereas those from red clay are enriched in Fe, Co, Ti, U, Th, Y, Cr, Nb and Rare Earth Elements (REE). Buried nodules from siliceous ooze suggest their formation under hydrogenetic, early digenetic and diagenetic processes whereas those from red clay are of hydrogenetic origin. REE are enriched more than 1.5 times in buried nodules from red clay compared to siliceous ooze. However, the mode of incorporation of REE into buried nodules from both sedimentary environments is by a single authigenic phase consisting of Fe–Ti–P. Shale-normalized REE patterns and Ce anomalies suggest that nodules from siliceous ooze formed under more oxidizing conditions than those from red clay. Nodules buried at depths between 1.5 and 2.5 m are diagenetic (Mn/Fe ratio 10–15), formed in highly oxic environments (large positive Ce anomalies) and record aeolian dust (high Eu anomalies). Chemical composition, surface texture and morphology of buried nodules are similar to those of surface nodules from the same basin. Furthermore, buried nodule compositions do not exhibit any distinct patterns within the core depth, suggesting that buried nodules neither grow nor dissolve after their burial in the sediment column.

Rare Earth Element Behavior in Zircon-Melt Systems

Research Article| February 01, 2007 Rare Earth Element Behavior in Zircon-Melt Systems John M. Hanchar; John M. Hanchar 1Department of Earth Sciences, Memorial University of Newfoundland, Alexander Murray Building, St. John's, NL A1B 3X5 Canada E-mail: johnh@esd.mun.ca Search for other works by this author on: GSW Google Scholar Wim van Westrenen Wim van Westrenen 2Faculty of Earth and Life Sciences, Vrije Universiteit, De Boelelaan 1085 1081 HV Amsterdam, The Netherlands E-mail: wim.van.westrenen@falw.vu.nl Search for other works by this author on: GSW Google Scholar Elements (2007) 3 (1): 37–42. https://doi.org/10.2113/gselements.3.1.37 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation John M. Hanchar, Wim van Westrenen; Rare Earth Element Behavior in Zircon-Melt Systems. Elements 2007;; 3 (1): 37–42. doi: https://doi.org/10.2113/gselements.3.1.37 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyElements Search Advanced Search Abstract Natural zircon crystals incorporate rare earth elements (REE) into their structure at concentrations determined by the pressure, temperature, and composition of their growth environment. In principle, REE concentrations in magmatic zircon crystals can be used to infer their conditions of growth and the composition of the melt from which they grew—provided accurate information is available about the distribution of REE between zircon and melt. Currently available zircon-melt partitioning data show a range in values covering several orders of magnitude for some REE. Further experimental work and studies using carefully selected natural samples are required to fully understand REE incorporation in zircon. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Surface modification of W9Cr4V2Mo high-temperature bearing steel by rare earth ion implantation

Wear and corrosion are the main failure mechanisms of bearings and it is important to prolong their working lifetimes by improving the bearing surface properties. Incorporation of rare-earth elements is one of the viable means in many technologies such as plating, chemical heat treatment, and thermal spraying. In this work, various kinds of rare earth ions including praseodymium, lanthanum and neodymium were implanted into W9Cr4V2Mo high-temperature bearing steel specimens using a metal vapor vacuum arc source. Pin-on-disk wear tests and microhardness and potentiodynamic polarization measurements were employed to evaluate the mechanical properties and chemical stability of the treated specimens. The chemical composition and surface morphology of the implanted layers were characterized by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Our results indicate that the tribological characteristics of the treated samples are affected significantly due to the smoother and harder surface, while the enhancement of the corrosion resistance in the 3.5 wt.% NaCl aqueous solution after ion implantation is not appreciable. The modified surface yields better wear resistance.

Toward the Comparison of Rare Earth Element and Actinide Behavior in Materials: A Computational Study of Ce- and U-Bearing Britholites

We present a computational investigation into the nature of bonds formed by f-elements in materials. The paper presents an example of the incorporation of rare earth elements (REE) and actinides in minerals derived from fluorapatite: Ca(10)(PO(4))(6)F(2). These minerals, called britholites, allow many substitutions on all three Ca, P, and F sites and are considered as potential host phases for radioactive elements separated from nuclear waste. REE and actinides have very similar physical and chemical properties, but REE are not radioactive and much more easily handled. REE are, therefore, very often used as a surrogate for actinides in experimental studies. The representative elements of rare earths and actinides chosen for this first investigation are cerium and uranium, respectively. We have studied all the various configurations of Ca(9)X(PO(4))(6)(-)(y)()(SiO(4))(y)()F(2), where X stands for Ce(3+), Ce(4+), U(3+), and U(4+), and y is equal to 1 and 2 for three-time and four-time charged cations, respectively. Calculations have been performed within the density functional theory (DFT) framework according to the computation scheme determined in a previous study. The analysis of the energies of the various configurations shows that the incorporation of all the cations considered stabilizes the apatitic structure. This stabilization, however, is greater for four-time charged cations than for three-time charged ones, which shows that Ce and U are both preferentially substituted in the +IV oxidation state. In addition, the substitution in one of the two cationic sites of the apatitic structure is always more favorable. Then, the geometry analysis shows a larger decrease in size of this cationic site for U than for Ce, as well as different volume variations for Ce and U substitutions in the two cationic sites. This cannot be explained by steric effects alone. Finally, the electronic density analysis yields three essential results: U and Ce form significantly covalent bonds, U forms bonds more covalent than Ce, and finally four-time charged cations form more covalent bonds than three-time charged ones. The comparison of these results with the formation enthalpies of the various phases shows a positive correlation between the covalence degree of the bonds formed by the f-element and the stability of the structure. In addition, our results prove that Ce- and U-bearing britholites exhibit very similar energetic, structural, and electronic properties. Ce, therefore, appears to be a good simulant for U.

Growth and concentration dependencies of rare-earth doped lithium niobate single crystals

A series of lithium niobate LiNbO 3 (LN) single crystals doped by rare-earth elements (Tm, Er, Dy, Tb, Gd, Pr) were grown under the same conditions by the Czochralski technique in a wide range of dopant concentrations. The distribution coefficients of rare-earth elements in the “crystal—melt” system of LN were determined at the beginning of the crystal growth. Their dependence on the dopant concentration in melt ranging from 0.1 to 3.5 wt% was investigated. Concentration dependences of physicochemical, ferroelectric and structural characteristics of doped LN single crystals were studied using X-ray diffraction, thermal analysis and Raman scattering measurements. A mechanism of rare-earth elements incorporation into the crystal lattice of LN was proposed for a wide range of dopant concentrations.

Characterization of porous core layer for controlling rare earth incorporation in optical fiber

The porous core layer deposited by modified chemical vapour deposition process has been analyzed in terms of thickness, pore size distribution, homogeneity and characteristics of the soot particles to investigate their variation with deposition temperature and input vapour composition. The compositions selected were SiO(2), SiO(2)-GeO(2) and SiO(2)-P(2)O(5). Rare earth ions were incorporated into the deposit by a solution doping technique. The analysis of deposited microstructures was found to provide a quantitative indication about the rare earth incorporation and its variation with respect to process conditions. Thus the characterization provides a method of controlling rare earth doping and ultimate preform/fiber properties.

Effect of cerium, holmium and samarium ions on the thermal and structural properties of the HZSM-12 zeolite

The study of the incorporation of rare earth elements as additives in Y zeolites is a very interesting field of research, mainly by its potential application as additives in catalytic cracking process. In this work was studied the thermal and structural properties of cerium, holmium and samarium supported on HZSM-12 zeolite. The obtained materials were characterized by X-ray diffraction (XRD), infrared spectroscopy (FTIR), nitrogen adsorption, thermogravimetry (TG/DTG), differential scanning calorimetry (DSC) and differential thermal analysis (DTA). TG/DSC/DTA analyses showed that the dehydration temperatures of RE/HZSM-12 zeolites (RE=Ce, Ho, Sm) increase in relation to pure HZSM-12. The acid properties were investigated by pyridine thermo desorption via TG. The results showed two events of mass loss attributed to elimination of pyridine adsorbed on the weak+medium acid sites and on the strong acid sites.

Trace element partitioning and substitution mechanisms in calcium perovskites

We have determined the partition coefficients of a large number of trace elements between CaTiO3 perovskite and anhydrous silicate melts at atmospheric pressure and 3 GPa. Determination of the concentration limits of Henry’s law behaviour in the CaO-Al2O3–SiO2–TiO2 system reveals that the incorporation of rare earth elements (REE) and tetravalent large ion lithophile elements (LILE4+ such as U and Th) at the Ca-site of CaTiO3 perovskite occurs with charge compensation through Ca-vacancy formation rather than by coupled substitution of Al for Ti. When melt composition is varied, we find that partition coefficients for REE and Th are strong functions of the CaO content of the melt. The observed trends are in excellent agreement with those predicted from the Ca-vacancy model. Given that they adopt the same crystal structure and have similar trace element partitioning behaviour, CaTiO3 perovskite and the deep mantle phase CaSiO3 perovskite can be considered analogous to one another. When the analogy is pursued in detail, we find that partitioning into both phases follows the composition-dependence predicted by the Ca-vacancy model. Thus, substitution of REE, U4+ and Th into CaSiO3 in the lower mantle also occurs with Ca-vacancy formation to balance charge. Furthermore when 2+, 3+ and 4+ partition coefficients for both phases are plotted as functions of CaO melt content, the trends for CaSiO3 and CaTiO3 appear to be continuous. This surprising result means that partitioning into Ca-perovskite is independent of pressure and temperature and also of whether or not the host is CaSiO3 or CaTiO3. One implication is that CaSiO3 crystallising from a peridotitic magma ocean may have partition coefficients for Th and U up to about 400. Crystallisation and sequestration of as little as 0.25 volume% of this phase in the lower mantle early in earth history would make a significant contribution to current mantle heat production.

Experimental REE partitioning between calcite and aqueous solution at 25.DEG.C. and 1atm: Constraints on the incorporation of seawater REE into seamount-type limestones

Partitioning experiments of rare earth elements (REEs) between calcite and aqueous (CaCl2 + NaCl) solution at 25°C and 1 atm have been made in order to elucidate the incorporation process of seawater REE into marine limestones. Calcite-supersaturated solution doped with REE was constantly pumped into a reactor, in which calcite seeds and the solution were mixed and CO2 + N2 gas was bubbled to allow calcite overgrowths. Absolute values of REE partition coefficients, Kd(REE) = (XREE/XCa)calcite/([REE]total/[Ca])solution, could not be determined because of very small calcite overgrowths and difficulty in evaluating XCa. The relative values of Kd(REE) like Kd(REE)/Kd(Gd), however, were determined successfully. Kd(REE) given by [REE]total depends on REE(III)-carbonate complexation in solution, and does not correspond to a reaction incorporating a specific dissolved REE species into calcite. Hence, here we used Kd(REE)ω = (XREE/XCa)calcite/([REECO3+]/[Ca])solution, which is given by Kd(REE) and REE(III)-carbonate complexation constants. Kd(REE)ω relates directly to ΔGr. for the reaction incorporating REECO3+(aq) into calcite. The series variation of experimental logKd(REE)ω shows a convex tetrad effect, suggesting larger Racah parameters of REECO3+(aq) relative to REE(III) in calcite. The variation also shows a kink at around Pm, Sm and Eu, possibly reflecting a coordination change of REE(III) in calcite across the series. Yttrium is definitely enriched in solution relative to heavy REEs. Using the relative values of experimental logKd(REE)ω and REE analyses of upper Paleozoic Japanese Ishimaki and Tahara limestones, REE abundance patterns for seawater coexistent with the marine limestones have been calculated provided that pH, ΣCO2 and salinity are the same in the ancient and present times. The calculated patterns are quite similar to those for relatively deep waters at around 400 to 1000 m in the modern Pacific. Ishimaki and Tahara limestones are of the seamount-type. Hence, the calculated seawater REE patterns with large negative Ce anomalies suggest that the incorporation of seawater REE into the limestones occurred in moderately deep water, probably because of subsidence of volcanic seamounts that were capped by Ishimaki and Tahara limestones.

REE and TRU Incorporation into Monazite Structure Ceramics

AbstractOne of the high levels of actinide, and in particular Cm, waste streams at the Russian radiochemical Production Association (PA) Mayak was generated during spent fuel reprocessing. Using oxalate precipitation, the rare earth elements (REE) and transuranic elements (TRU) settled out in the form of oxalate residues. Due to in high REE contents in this residue, the mineral-like matrix based on (REE)PO4 solid solution, with monlclinic monazite structure have been proposed to use as a suitable ceramics form for final actinide immobilization. For this purpose the synthetic REE oxalates were first transformed into REE orthophosphates in a thin-film evaporator (TFE). Then the (REE)PO4 powder was compacted both by either hot uniaxial pressing (HUP) or cold uniaxial pressing followed by sintering (CUP). This ceramic with the monazite structure has a high density and exhibits chemical durability by leaching.

The behaviour of the rare earths during the precipitation of sodium, potassium and lead jarosites

The behaviour of the rare earths during the precipitation of sodium, potassium and lead jarosites from both sulphate and chloride media was investigated. Efforts to synthesize the end-member rare earth analogues of sodium jarosite and potassium jarosite were unsuccessful. The precipitation of sodium jarosite or potassium jarosite in the presence of various concentrations of the rare earths resulted in the incorporation of <0.3 wt.% of the rare earths in the jarosite structure. Comparable levels of rare earth incorporation were noted in both sodium jarosite and potassium jarosite. Increasing solution pH or increasing ferric ion concentration resulted in higher precipitate yields, but had only a minimal effect on the structural incorporation of the rare earths in the jarosite. Sodium jarosite and potassium jarosite precipitated from predominantly chloride media had levels of rare earth incorporation comparable to those of the jarosites made from sulphate solutions. Lead jarosite precipitated from rare earth-containing solutions incorporated <0.2 wt.% of the rare earths. SEM studies showed that the presence of the rare earths had little effect on the morphology of the precipitated sodium, potassium and lead jarosites. Clearly, the trivalent rare earth (RE) elements do not substitute extensively for trivalent iron in the jarosite structure, and this behaviour likely reflects the larger size, different co-ordination and the significantly lower hydrolyzability of the trivalent rare earth ions relative to ferric ions.

Rare earth element and strontium isotopic study of seamount-type limestones in Mesozoic accretionary complex of Southern Chichibu Terrane, central Japan: Implication for incorporation process of seawater REE into limestones.

Ishimaki and Tahara limestones occur as exotic blocks juxtaposed in the Mesozoic (Jurassic) accretionary complex of Southern Chichibu Terrane in eastern Aichi Prefecture, central Japan. They are supposed to be of the seamount-type limestone, since they have no terrigenous materials and are intimately associated with greenstones. REE (rare earth elements) and Sr isotopic studies for the limestones have been made in order to know their geochemical characteristics, ages and origins. Their 87Sr/86Sr ratios, when referred to the seawater 87Sr/86Sr curve and relevant geological data, suggest that Ishimaki and Tahara limestones are the late Permian and the Carboniferous to the Early Permian, respectively. Two greenstone fragments found inside the Ishimaki limestone block and one greenstone sample associated with Tahara limestone block, resemble the Hawaiian alkali basalt in their REE and Y patterns. This is supporting the idea that the limestone blocks may be parts of reef limestones on ancient volcanic seamounts. All the limestone samples, except three unusual Tahara ones, show seawater REE and Y signatures in their chondrite-normalized patterns. Their REE/Ca ratios, however, are 102-104 times as high as those ratios of modern biogenic carbonates like corals and the seawater. Accordingly, seawater REE and Y were incorporated into the limestones, when originally biogenic carbonates transformed into inorganic calcite and its secondary growths occurred in diagenesis in contact with sufficient seawater. This view is favored by the reported REE partition experiment between calcite overgrowths and seawater solution. The seawater Ce anomaly as a function of water depth in the modern ocean is a key to infer the water depth of the REE and Y incorporation. The Ce anomalies given by log(Ce/Ce∗) for about a half of Ishimaki samples and most of Tahara ones are between -0.5 and -0.2, which are compatible with the shallow water origin. Another half of Ishimaki samples, however, have log(Ce/Ce∗) values between -0.7 and -1.0, suggesting moderately deep waters (ca. 500-1000 m deep or more). This may reflect such a situation that water depths of REE incorporation into the seamount-type limestones are generally greater than the depositional water depths of original biogenic carbonates because of the fate of limestone-capped volcanic seamounts decided by the oceanic plate motion.

Isotopic composition of recent shark teeth as a proxy for environmental conditions

The O, C, and Sr isotope compositions of teeth from ten species, belonging to five families, and three orders of sharks were measured to determine the influence of habitat, diet, and possible species-specific fractionation effects on the isotopic composition of biogenic phosphate from fish. The sharks were recently caught in subtropical waters off the KwaZulu-Natal (KZN) coast of South Africa, as well as from cold waters in Prince William Sound (PWS), Alaska, and Victor Bay (VB), Nunavut, Canada. δ 18O values of tooth phosphate (δ 18O P) range from 20.9 to 23.5‰ for the KZN sharks. For most species the range in measured δ 18O P values is about 0.6‰, but it may be as high as 1.1‰ for different teeth from a single shark. Dentine and enameloid within individual teeth have no apparent differences in δ 18O P values. The δ 18O P values of the KZN shark teeth reflect the typical habitat of the studied species, primarily the thermal structure of the water column off KZN at depths between 20 and 280 m. The δ 18O P values of teeth from different Greenland sharks from VB and Pacific sleeper sharks from PWS are very homogeneous, averaging 25.8 and 24.7‰, respectively. These values appear to be in equilibrium with deep (>500 m) ocean waters in each case at temperatures of about −0.3°C or less. There is little discernable evidence for species-specific fractionation effects for the oxygen isotope composition of phosphate in the studied marine fish. The oxygen isotope composition of carbonate in apatite averages about 9.1‰ higher than corresponding δ 18O P values, in agreement with equilibrium fractionation between carbonate and phosphate, but with a large variance (1σ = ±1.5‰). δ 18O C values also vary by up to 1‰ between enameloid and dentine within single teeth, but in a non-systematic way. Differences in δ 13C values between carbonate in enameloid and dentine is also large (up to 8‰) but the δ 13C values vary systematically. Enameloid is always enriched in 13C compared to dentine and the 13C content increases with developmental stage of the teeth. δ 13C values measured for enameloid (1.6 to 4.8‰) appear to approach equilibrium with dissolved inorganic carbon in seawater. In contrast, δ 13C values for dentine range from −6.4 to −2.3‰ for KZN sharks, and −9.0 to −10.8‰ for the cold-water sleeper sharks, and are compatible with a predominantly dietary carbon source. The 87Sr/ 86Sr ratios of teeth from KZN sharks as well as those from PWS and VB are uniform, averaging 0.709167. Sr content varies from 1270 to 2100 ppm, a range that is similar to that in well preserved fossilized teeth. Seawater Sr is thus clearly incorporated in vivo. Concentrations of Sm and Nd are in the ppb range and contrast the ppm range in fossilized teeth, indicating a postmortem incorporation of rare earth elements in apatite of the teeth.

Pseudopotential description of rare earths in oxides: The case ofEr2Si2O7

The applicability of ultrasoft pseudopotentials to the problem of rare-earth incorporation in silicates is investigated using the compound ${\mathrm{Er}}_{2}{\mathrm{Si}}_{2}{\mathrm{O}}_{7}$ as a test case. It is found that density-functional theory within the generalized gradient approximation provides a good description of the structural parameters, when treating the Er $4f$ states as a partially occupied core shell. The density of states and the distribution of electronic charge are analyzed, and it is concluded that the presence of Er tends to increase the covalency of neighboring Si-O bonds.

Trace element distribution in eclogites and their clinopyroxene/garnet pair: a case study from Soazza (Switzerland)

Rare earth element (REE) abundances have been determined in six samples of metabasites from the Pennidic Adula nappe in the area of Soazza (Mesolcina Valley, Switzerland). The samples cover a broad range of mineralogical composition from eclogite to epidote amphibolite through symplectitic garnet amphibolite. The REE patterns do not vary significantly and indicate a common origin of these rocks as “normal” mid-ocean ridge basalts (N-MORB). Two main eclogitic phases, clinopyroxene (cpx) and garnet (grt), have also been analysed for REE and other trace elements (Sc, V, Cr, Co, Ni, Cu, Zn, Sr, Y, Zr). The incorporation of REE and most trace elements in structural sites of cpx and grt is in accordance with that expected mainly from an evaluation of site geometry, ionic radius and charge balance and is therefore dependent on the major-element composition of the host phases. The changes in cpx/grt partition coefficients (D i ) for HREE are particularly associated with the temperature condition of the equilibration of the pair.

Strontium isotopes and rare-earth element geochemistry of hydrothermal carbonate deposits from Lake Tanganyika, East Africa

At Cape Banza (North Tanganyika Lake), fluids and aragonite chimneys have been collected many times since the discovery of this sublacustrine field in 1987. This sampling has been investigated here for the Sr isotopic compositions and the rare-earth element features of the carbonates and a few fluid samples. The 87Sr/ 86Sr ratios of the chimneys indicate that they have precipitated from a mixture of lake water (more than 95%) and hydrothermal fluids. No zoning in the chimneys was detected with our Sr data. For the rare-earth elements, the situation is more complex. The external walls of the chimneys are rare-earth-element-poor (La ≈ 500 ppb, Yb ≈ 200 ppb, La/Yb = 2 to 3.4). Their shale normalised rare-earth element patterns suggest that they are in equilibrium with the inferred carbonate-depositing fluids. The rare-earth element concentrations of the internal walls of the chimneys are significantly light rare earth elements (LREE)-enriched with La contents sometimes up to 5 ppm. We suggest that they contain more vent-fluid rare-earth elements than the external wall samples, possibly adsorbed on the surface of growing crystals or simply hosted by impurities. It was not possible to constrain the nature of these phases, but the variations of the compositions of the internal wall materials of the active chimneys with time, as well as data obtained on an inactive chimney indicate that this rare-earth element excess is mobile. Partition coefficients were calculated between the external wall aragonite and carbonate-depositing fluid. The results are strikingly similar to the values obtained by Sholkovitz and Shen (1995) on coral aragonite, and suggest that there is no significant biologic effect on the incorporation of rare-earth elements into coral aragonite and that the various carbonate complexes involved Me(CO 3 +) complexes are the main LREE carriers in seawater (Cantrell and Byrne, 1987) instead of Me(CO 3) 2 − in Banza fluids) have the same behaviour during aragonite precipitation.