Behavior Of REE Research Articles

Minor Phases as Carriers of Trace Elements in Non-Modal Crystal–Liquid Separation Processes II: Illustrations and Bearing on Behaviour of REE, U, Th and the PGE in Igneous Processes

Minor phases which strongly concentrate selected trace elements, termed here ‘carrier-phases’, release relatively large amounts of those elements to the liquid phase when they are eliminated during partial melting and glean relatively large amounts of those elements when they first appear during progressive crystallization. It is characteristic of such relationships that concentrations of the selected trace elements in the bulk residues of partial melting will rise to a peak somewhat before the last of the carrier-phase is eliminated during progressive melting. In the liquids produced during equilibrium partial melting a corresponding peak in the concentration of the trace element occurs at the point where the carrier-phase is eliminated; the corresponding peak in trace element concentration in the liquids produced by accumulated perfect fractional melting is found somewhat above that point. These peaks become more sharply accentuated as the distribution coefficient of the trace element into the carrier-phase increases. The highest trace element concentration in a partial melt liquid product is found in the small drop of liquid produced during perfect fractional melting at the point where the carrier-phase is eliminated. Still higher concentrations may be found in the first cumulates containing the carrier-phase which precipitate during perfect fractional crystallization but the corresponding liquids do not contain exceptionally high concentrations. Under favourable conditions a large proportion of the available mass of a trace element in a magmatic system may be transferred from the solid to the liquid phases or vice versa with only a small change in the mass fraction of liquid in and energy content of the system. Within that range, separation of otherwise very similarly behaved trace elements becomes possible. Further complexities arise and the opportunities for separation increase when two carrier-phases compete with differing success for the same group of trace elements.

REE fractionation by accessory minerals in epidote-bearing metaluminous granitoids from the Sierras Pampeanas, Argentina

AbstractA study of the distribution of REE in epidote-bearing metaluminous granitoids from Sierra de Chepes, Sierras Pampeanas, Argentina, reveals that a large proportion of the REE reside in the accessory minerals (allanite, epidote, titanite, apatite and zircon), and therefore these minerals control the behaviour of REE in granitic magmas. Well-developed chemical zonation in titanite indicates that the REE content decreases in the melt during crystallization of this mineral. The textural and chemical characteristics of euhedral epidote suggest a magmatic origin, and in that case it may have played an important role in the fractionation of the REE. The amount of silica and any other geochemical parameter indicative of fractionation progress in the dominant granodioritic-tonalitic facies (gtf) do not correlate with observed variations in the REE patterns. When many accessory minerals are involved, as in the gtf, the differentiated melts (e.g. aplites) are REE poor. Thus, the presence/absence of accessory minerals in granitoids can be indicative of the generation of differentiated melt enriched or poor in REE and other trace elements. This may have an economic significance, as it may allow us to predict the probable geochemistry of the differentiated melts (i.e. those that tend to develop mineralization) from the textural analysis of the ‘regional’ granitic rock.Finally, the type and abundance of accessory minerals in the granitic suite can also help us to define the geotectonic environment where magmas were generated.

REE mobilization during small-scale high-pressure fluid–rock interaction and zoisite/fluid partitioning of La to Eu

A zoisite–quartz segregation and its adjacent host metabasite from the Tauern Window (Austria) have been investigated in detail to study the geochemical behaviour of REE during high-pressure fluid–rock interaction and the role of zoisite. The segregation formed at T = 500 to 550°C/ P ≫ 0.6 GPa because of decompressional dehydration of the host metabasite. During the formation of the segregation the REE La to Tb and Dy to Lu were geochemically decoupled. In the metabasite La to Tb are incorporated in allanite and were leached from the host rock by the fluid phase. This decoupling and the leaching of La to Tb significantly alter the REE patterns of the host rock in the immediate vicinity of the segregation from LREE enriched to LREE depleted. Based on a mass-balance calculation for La, Ce, Nd, Sm, and Eu the composition of the fluid from which the segregation was precipitated is calculated. The fluid is LREE enriched with (La/Sm) N = 3.7. Individual REE contents range from Eu = 2.4 ± 1.1 ppm up to Ce = 90 ± 20 ppm, ΣREE = 190 ± 50 ppm. Calculated partition coefficients D REE zoisite/fluid are generally <1 and increase from D La zoisite/fluid = 0.08 ± 0.02 to D Eu zoisite/fluid = 0.8 ± 0.4, indicating a stronger preference of zoisite for MREE than for LREE. To describe the fractionation of La relative to Sm between zoisite and fluid, the La-Sm exchange coefficient between zoisite and fluid is calculated. The derived exchange coefficient K D(La-Sm) zoisite/fluid = 0.2 ± 0.1 suggests that a fluid in equilibrium with zoisite will have a three- to ten-times higher La/Sm ratio than coexisting zoisite.

The behavior of Al, Mn, Ba, Sr, REE and Th isotopes during in vitro degradation of large marine particles

Abstract The extent and the time constant of dissolution of a set of inorganic tracers during the decomposition of large marine particles are estimated through in vitro experiments. Large marine particles were collected with in situ pumps at 30 m and 200 m in the Ligurian Sea at the end of summer. They were subsequently incubated under laboratory conditions with their own bacterial assemblage for 20 days in batches under oxic conditions in the dark. Some samples were initially sterilized in order to observe possible differences between biotic and abiotic samples. Particulate (>0.2 μm) and dissolved ( 2 respiration. Speciation calculations suggest that this pH shift leads to a decrease of the complexation of dissolved REE by carbonate ions. Th isotope data are consistent with an irreversible dissolution of Th and they do not support a rapid particle–solution chemical equilibrium.

Major Element, REE, and Other Trace Element Behavior in Amphibolite Weathering under Semiarid Conditions in Southern India.

A body of komatiitic amphibolite, an enclave within the Archean high-grade orthogneisses in southern India, shows mild chemical weathering under semiarid conditions. Along fractures, chemical weathering has advanced (Chemical Index of Alteration &sqbl0;CIA&sqbr0;=53; CIA of fresh rock approximately 26) to the extent that secondary Mg-Fe-Al clay minerals have formed and the rock has turned brownish red, soft, and fine grained. The weathering process has resulted in the mobilization and redistribution of the so-called immobile elements Fe, Al, Ti, and REE effected by the nature of secondary mineral formation (talc vs. aluminous clay minerals) and also possibly by soil microbes. In the initial stages of secondary mineral formation, there is a small loss of Fe, Al, and REE (noticeably Eu). However, in the fracture zone as well as in the incipiently altered zone, there is significant REE enrichment, probably affected by a different precipitation mechanism. Mobilized REE may have come from a minor alteration of clinopyroxene.

Ore-Forming Fluids of Vein-Type Fluorite Deposits of the Cerro Aspero Batholith, Southern Cordoba Province, Argentina

Vein-type fluorite deposits in the southern part of the Sierras Pampeanas, Córdoba Province, Argentina, occur mainly hosted by calc-alkaline porphyritic biotite granites, which belong to the Paleozoic, post-tectonic Cerro Aspero batholith. The fluorite veins, of Cretaceous age, occupy steeply dipping, strike-slip regional fault zones, and are composed of fluorite and chalcedony, locally with subordinate amounts of pyrite and, in some cases, coffinite and pitchblende. These veins show typical open-space-filling textures and are closely related to pervasive silicic and argillic alteration of the host granite. Three successive stages of mineralization were distinguished on the basis of vein chronology, REE data, and fluid-inclusion study in fluorite ores. These stages generally display slightly fractionated REE patterns (La/Yb = 1.4 to 14), with REE behavior given by a relatively stronger LREE fractionation with respect to HREE. The REE composition of the fluids responsible for fluorite deposition was largely controlled by differential mobility of the REE during the silicic or argillic alteration of the host granite. Preferential leaching of HREE over LREE occurred during both alteration types, but in the argillic alteration the LREE were practically not removed. The total homogenization of primary-like aqueous inclusions took place invariably in the liquid phase at temperatures ranging from 187°C to 103°C, with concentrations of values around 160°C, 136°C, and 116°C (stages I, II, and III, respectively), defining a clear trend of fluid cooling. This cooling is accompanied by large changes in the fO2 of the fluid, from oxidizing to reducing, as inferred from the Eu/Eu∗ ratios and the mineral assemblage (pyrite, pitchblende, and coffinite). The three stages of fluorite deposition exhibit temperatures of ice melting within the interval from −0.3°C to +0.4° C, indicating that the mineralizing fluids were exclusively aqueous and highly dilute. No evidence of fluid mixing or boiling was found. The fluid-inclusion data suggest that the proposed three stages of mineralization probably were the result of a single hydrothermal event, and strongly support a single, uniform fluid reservoir for the ore-forming solutions; evidently, the latter were heated meteoric waters rather than fluids generated in deep-seated environments within the crust.

Element geochemistry of weathering profile of dolomitite and its implications for the average chemical composition of the upper-continental crust

Geochemical behavior of chemical elements is studied in a dolomitite weathering profile in upland of karst terrain in northern Guizhou. Two stages can be recognized during the process of in situ weathering of dolomitite: the stage of sedentary accumulation of leaching residue of dolomitite and the stage of chemical weathering evolution of sedentary soil. Ni, Cr, Mo, W and Ti are the least mobile elements with reference to Al. The geochemical behavior of REE is similar to that observed in weathering of other types of rocks. Fractionation of REE is noticed during weathering, and the two layers of REE enrichments are thought to result from downward movement of the weathering front in response to changes in the environment. It is considered that the chemistry of the upper part of the profile, which was more intensively weathered, is representative of the mobile components of the upper curst at the time the dolomitite was formed, while the less weathered lower profile is chemically representative of the immobile constitution. Like glacial till and loess, the "insoluble" materials in carbonate rocks originating from chemical sedimentation may also provide valuable information about the average chemical composition of the upper continental crust.

Geochemistry Of The GuaxupÉ Granulites, Minas Gerais, Brazil

Geochemical data for granulite terrain are presented in the northernmost portion of the Guaxupé Massif, at Mantiqueira Province, SE Brazil. Several types of granulites are recognized in the area: basic, intermediate and acid granulite. Major and trace elements (including REE) point to only one magma source for these granulites generated at different times. Geochemical data point to plagioclase and apatite fractionation as the responsable by the REE behaviour in intermediate and more basic rocks. Overall composition of the Guaxupé Granulites is similar to average described worldwide in the literature to the lower crust.

Distribution pattern of rare earth elements in fern: implication for intake of fresh silicate particles by plants.

All the lanthanide elements (REE) in fern (Matteuccia) and in soil were determined with inductively coupled plasma mass spectrometer (ICP-MS) in order to discuss REE behavior in fern. The fern sample was divided into three parts: root, stem, and leaf; the soil of the same site was also sampled and the soil sample was divided into two parts: HCl and HNO3 soluble part (soil[HCl]) and HF soluble part (soil[HF]). REE in each part was determined by ICP-MS after solvent extraction separation. The overall variation of the REE pattern of the root does not resemble that of the whole soil, but that of the HF soluble part. A tetrad effect variation (W-type) was found in the REE patterns of root, stem and leaf; more conspicuous tetrad effect variation was observed in HREE region than in LREE region, and was so in stem and leaf than in root. Negative anomalies of Ce were observed in the REE patterns of root, stem and leaf, with bigger anomalies in stem and leaf than in root. The results of this study suggest that REE in fern has come from silicates of soil and has once been in dissolved state.

Trace-element fractionation by impact-induced volatilization; SIMS study of lunar HASP samples

Impact is a dominant process acting on the surfaces of planetary bodies and is the major process that modifies the physical and chemical nature of planetary materials on atmosphere-free bodies once volcanic activity has ceased. Elements with relatively high volatilities are readily mobilized during impact. HASP (high alumina, silica poor) compositions are thought to be generated by impact-induced volatilization processes on the moon. The high Al/Si ratio of HASP is believed to be the result of preferential loss of SiO 2 relative to Al 2 O 3 . Here we report SIMS analyses for HASP glass beads and devitrified glasses from the Apollo 14 regolith to assess the behavior of Li, Be, B, REE, Sr, Y, and Zr during impact events. We compare the behavior of these trace elements to the behavior of major and minor elements analyzed by the electron microprobe. The HASP samples we studied fall into two compositional groups with one group apparently derived from KREEPy lunar lithologies and the other from anorthosite. The KREEPy HASP beads have an estimated mass loss of 28% compared to their KREEPy protolith. We estimate that 44% of SiO 2 and 35% of FeO were lost during impact-induced volatilization. The mass loss estimate for the anorthositic protolith to produce anorthositic HASP is more uncertain because of the uncertainty of the protolith composition. However, an estimate of approximately 25% appears reasonable with 38% loss of silica. Clearly, impact processes, in the early stages of planetary evolution, are important in the modification of geochemical signatures, and ordinarily geochemically coherent elements may be decoupled because of different volatilities.

Residence of REE, Y, Th and U in Granites and Crustal Protoliths; Implications for the Chemistry of Crustal Melts

A systematic study with laser ablation—ICP-MS, scanning electron microscopy and electron microprobe revealed that ∼70–95 wt% of REE (except Eu), Y, Th and U in granite rocks and crustal protoliths reside within REEYThU-rich accessories whose nature, composition and associations change with the rock aluminosity. The accessory assemblage of peraluminous granites, migmatites and high-grade rocks is composed of monazite, xenotime (in low-Ca varieties), apatite, zircon, Thorthosilicate, uraninite and betafite-pyrochlore. Metaluminous granites have allanite, sphene, apatite, zircon, monazite and Thorthosilicaie. Peralkaline granites have aeschinite, fergusonite, samarskite, bastnaesite, fluocerite, allanite, sphene, zircon, monazite, xenotime and Th-orthosilicate. Granulite-grade garnets are enriched in Nd and Sm by no less than one order of magnitude with respect to amphibolite-grade garnets. Granulitegrade feldspars are also enriched in LREE with respect to amphibolite-grade feldspars. Accessories cause non-Henrian behaviour of REE, Y, Th and U during melt—solid partitioning. Because elevated fractions of monazite, xenotime and zircon in common migmatites are included within major minerals, their behaviour during anatexis is controlled by that of their host. Settling curves calculated for a convecting magma show that accessories are too small to settle appreciably, being separated from the melt as inclusions within larger minerals. Biotite has the greatest tendency to include accessories, thereby indirectly controlling the geochemistry of REE, Y, Th and U. We conclude that REE, Y, Th and U are unsuitable for petrogenetical modelling of granitoids through equilibrium-based trace-element fractionation equations.

Mobility of major, minor and some redox-sensitive trace elements and rare-earth elements during weathering of four granitoids in central Portugal

Element mobility was studied in four different granitoids in central Portugal. Samples of fresh and weathered rock parts were collected from each type of granitoid. An increase in the degree of weathering is accompanied by reductions in Na 2O, CaO and SiO 2 (plagioclase), P 2O 5 (apatite), and in most cases also by reduction in K 2O (biotite, potash feldspar). Losses as well as gains are observed for MgO. Relative changes in the Al 2O 3 contents are attributed to physical transport or accumulation of fine weathering products. Mass-balance calculations indicate that kaolinite and in some cases also gibbsite are the dominant weathering products; this is in agreement with X-ray diffraction analyses. Manganese is found to decrease but not Fe. There is a high correlation between V and Fe; this is preserved during weathering. There were both increases and decreases in the trace elements Cr, As and Sb; U had become slightly enriched in the weathered parts. No systematic trends in REE behaviour were observed. Differences in the behaviour of these elements and in their ratios can be explained by a combination of factors such as differential dissolution of the host minerals, redox conditions and redox kinetics, complexation by organic and inorganic ligands, and sorption. The interpretation of the results is complicated by apparent inhomogeneities in the granites on the mesoscale of an outcrop.

Ion microprobe investigation of exsolved pyroxenes in cumulate encrites: Determination of selected trace-element partition coefficients

We have examined the partitioning of selected minor and trace elements in exsolved pyroxenes from cumulate eucrites by secondary ion mass spectrometry (SIMS). Solid-solid partition coefficients (D's) for Al, Cr, Ti, seven REEs, Sr, Y, and Zr have been determined between low Ca- and high-Ca pyroxenes from Moore County, Binda, Serra de Magé, and Moama. Equilibrium temperatures of the co-existing pyroxenes were calculated to constrain the relevant temperatures for the partition coefficients. Trace-element abundance patterns are similar for pyroxenes from the four cumulate eucrites and thus indicate similar distribution behaviors of the trace-elements during exsolution. There is a significant HREE enrichment over LREEs in the low-Ca pyroxenes and abundances of the REEs in augite are greater than those found in the low-Ca pyroxene hosts. This REE behavior reflects the pyroxene crystal structures. The calculated solid-solid partition coefficients are found to vary systematically as expected from pyroxene crystal chemical arguments. Calculated subsolidus equilibration temperatures are within the expected range produced by pyroxene solvus temperature-composition relations, and consistent with petrographic observations of the cumulates. The trace-element patterns and abundances indicate that three of the four cumulates examined (Binda, Moama, Serra de Magé) were formed from the same melt or similar melts crystallizing under similar conditions in their parent body. Moore County has much higher trace-element concentrations and this suggests that it either formed from a more evolved melt than the other three cumulates and crystallized later in the fractionation sequence, or alternatively, it was derived from a different melt system.

Peridotite xenoliths in alkali basalts from the Sikhote-Alin, southeastern Siberia, Russia: trace-element signatures of mantle beneath a convergent continental margin

Abundant spinel peridotite xenoliths occur in late Cenozoic alkali basaltic rocks in the Sikhote-Alin region at the Pacific margin of the Asian continent. Major- and trace-element compositions of representative peridotite xenolith are documented for four occurrences located in different structural units of the continental margin. In each locality, the majority of xenoliths have distinctive microstructures, modal and chemical compositions that are typical for a given xenolith suite. Significant textural and compositional differences between the four xenolith suites suggest that the upper mantle beneath the Sikhote-Alin consists of distinct domains with contrasting composition. The inferred large-scale mantle heterogeneities may be due to juxtaposition of lithospheric blocks of different provenance during accretion of the Sikhote-Alin to the Asian continent. Trace-element patterns of the xenoliths and their minerals obtained ICP-MS technique provide evidence of depletion and enrichment events and indicate contrasting behaviour of REE, HFSE and other incompatible trace elements. The HFSE behave non-concordantly, in particular, some xenoliths have highly fractionated Zr/Hf, Ti/Zr, Nb/Ta, La/Nb and U/Th ratios relative to their values in the primitive mantle. The fractionated compositions may be related to the interaction of evolved subduction-related fluids and melts with lithospheric mantle at the Mesozoic-early Cenozoic active continental margin or to metasomatism during later continental rifting.

Behavior of rare earth elements in a paleoweathering profile on granodiorite in the Front Range, Colorado, USA

A Paleoweathering profile on the Boulder granodiorite in northern Colorado provides an opportunity to trace the behavior of REEs from parent rock, through a weathering profile, into unconformably overlying Permian sediments. With progressive upward weathering of the granodiorite, Na 2O, CaO, SiO 2, Ta Hf , Co Th , Cr Sc , Cr Th , Zr Hf , La Sc , Zr Y , and La Th decrease; Al 2O 3 and Fe 2O 3T increase; and TiO 2, MgO, K 2O, P 2O 5, Rb, Zr, Sc, Cr, Co Hf, Nb, Ta, Y, Th, U, REE, Ti Nb , and Zr Nb increase to maximum values and then either level off or decrease. LREE enrichment is less in the weathering profile than in the parent granodiorite and although the parent does not have an Eu anomaly (or only a slight positive anomaly), all samples from the weathering profile and overlying sediments have significant negative Eu anomalies. This observation is especially important in that it shows conclusively that a negative Eu anomaly can be produced during chemical weathering of granitoids. We suggest these Eu anomalies are due to relative enrichment of the other REEs and partial loss of Eu during the breakdown of plagioclase. The Boulder weathering profile also has a very minor negative Ce anomaly that is within error of a Ce anomaly in the parent. In the unweathered parent, >50% of the REE are contained in sphene, and in the case of La, also in allanite. From 10–20% of the REE are contained in apatite and biotite (± hornblende), and from 7–10% of the HREEs are in zircon. With exception of Eu, for which feldspars contribute about 8%, negligible amounts of REEs occur in the feldspars. In weathered samples, >75% of the REEs are contained in clay minerals. The crossover between sphene and clay control of REEs occurs over a distance of 1 m near the contact with fresh rock. Except for their small negative Eu anomalies, the clay minerals have REE patterns very similar to those of the parent rock. Isocon plots suggest apparent enrichments of many elements in the Boulder weathering profile result from losses of Na, Ca, and Si during plagioclase weathering. In addition, variable amounts of Sr, Eu, Ta, Nb, P, and Ba were lost during weathering. Although Th U , Zr Y , Th Sc , Zr Hf , Lu Hf , and Ti Zr may have been transferred relatively unchanged from granodiorite parent to the bulk weathering profile, most other element ratios and REE distributions were significantly changed during weathering. This observation implies that caution needs to be exercised when using REE patterns and element ratios to trace sediment provenance. The fact that most element ratios and REE distributions also differ between Fountain sediments and the bulk weathering profile may be related to one or a combination of four factors, listed in order of probable decreasing importance: contribution of other sources to the Fountain sediments, sorting of minerals during sediment deposition, remobilization of elements during diagenesis, and leaching of elements by water flow through the upper meter of the weathering profile.

Distribution of REE and other trace elements in the Hetai gold deposit of South China: implications for evolution of an auriferous shear zone

The Hetai gold deposit is contained within a shear zone which is hosted by metavolcano-sedimentary units of the Late Proterozoic Yunkai Group. The shear zone, striking northeast and dipping northwest, is over 3 km long and has a thickness of about 20–50 m. The ore occurs in the shear zone as a series of lenticular and vein-type bodies. The auriferous shear zone has undergone three stages of evolution: the early stage was characterized by ductile deformation when auriferous mylonite containing less than 1 ppm Au was formed; the intermediate stage took place in brittle-ductile environments during which the lenticular ores with about 10–100 oom Au were formed; the last stage was developed during brittle deformation resulting in emplacement of auriferous quartz veins with grades of 1–5 ppm Au. REE pattern analysis demonstrates that no significant modification of lanthanide distribution occurred during the ductile deformation stage of the shear zone evolution, which resulted in the formation of mylonite and ultramylonite, although important mineralogical and chemical changes of the original metamorphic rocks have been observed. In contrast, extensive variations of REE patterns occurred in the hydrothermally infiltrated rocks which are products of the brittle-ductile and the brittle deformation stages of the shear zone evolution. This confirms that REE behavior during dynamometamorphism seems to be similar to that during metamorphism and hydrothermal alteration, which is determined both by fluid/rock ratios and by REE patterns and abundances in reacting fluids and rocks.

Especiación de tierras raras en las soluciones ácidas y neutras del sistema de drenaje del Arroyo del Val (Zaragoza)

Special geochemical characters of the Arroyo del Val drainage basin allow to study different aspects related to REE behaviour. The hydrologic system contains a main stream (Arroyo del Val) of acidic waters naturally generated (sulfate rich solutions, with high levels of Al, Fe and heavy metals, and with pH values between 3 and 4.5) and several calcium bicarbonate (sulfate) tributaries (with pH values as high as 7.5, measurable alkalinity and striking phosphate concentrations). Mixing of the acidic stream with neutral tributaries yields spectacular sulfate-iron-aluminum rich flocculants that precipitate at the confluences, along the principal stream. The concentration of REEs was measured for acidic water samples col1ected along the headwaters zone of the Arroyo del Val stream and for neutral water samples fram two tributaries. The REE content of acidic waters are enriched by 10 2 relative to neutral ones and the shale-normalized REE patterns for these acidic freshwaters are enriched in the inter• mediate REEs (IREE, Eu-Tb). Speciation calculations were performed on both water groups. Dominant forms of REEs in the acidic waters include free metal species (Ln 3+ ≈ 35%) and mainly sulfate complexes ((LnSO + 4 ≈ 60% and minor Ln(SO 4 ) - 2 ≈ 5%). REE sulfate complexation don't fractionate the REEs in solution because there is not an appreciable variation in REE sulfate complexation constants across the lanthanide serie. So, REE convex patterns in these acidic waters must be related to interaction of the acid waters with clay minerals and oxyhydroxide coatings in the paleozoic materials of drainage basin and/or with the col1oid pool of those waters. In the neutral freshwaters of tributaries dissolved REEs are complexed with carbonate and phosphate ions. REE carbonate and phosphate complexation constants across the lanthanide serie increase by several orders of magnitude from La to Lu. And so, phosphate and carbonate complexes account for 80-90 % of LREE and for 100 % of HREE. LnPO 0 4 and Ln(PO 4 ) 3- 2 complexes are quite significant compared to carbonate complexes, mainly in the speciation of HREE. These results il1ustrate the importance of phosphate ion concentration and REE phosphate complexation reactions in REEs speciation calculations.

Geochemical and mineralogical behavior of REE, Th and U in the Akongo lateritic profile (SW Cameroon)

The REE-U-Th geochemistry and mineralogy have been studied in a lateritic profile derived from a syenite (Akongo, SW Cameroon). Mass balance calculations have been done (1) in the syenite to determine the most important REE-U-Th bearing minerals and (2) in the soil profile to understand the movement of these elements. Thorium has been used as an immobile element for these calculations. The weathering of accessory minerals rich in REE and Th of the syenite (allanite, apatite, titanite and epidote) has been especially studied in the saprolite. Several REE-Th secondary minerals (florencite, rhabdophane, cerianite and thorianite), important to explain the fractionation of REE in supergene environments, have been found.

REE Behaviour Across the Cretaceous-Tertiary Boundary

The rare-earth elements (/LEE) do not fractionate during sedimentary processes, and their secondary remobilisation is not significant, therefore they provide information on the source area, since the sedimentary particles conserve the chemical record of the source (e.g. McLennan, 1989; Condie, 1991). In the Cretaceous-Tertiary boundary (KTB) they can provide information on the source area and depositional conditions. We have studied different sequences that belong to the Betic Cordilleras (Spain): 'Agost' (A) and 'Caravaca' (CA), and Basque Cantabrian basin (Spain and France): 'Sopelana' (SO), 'Monte Urko' (URT), 'Zumaya' (ZU), 'Hendaye' (HY) and 'Biarritz' (B).

REE Geochemistry in Nodule Formation Processes in Surface Sediments of Guatemala Deep, Pacific Ocean

The problem of nodule formation can be connected to a considerable extent with postdeposional phase transformations in sediments. This information may be obtained from REE. Previous studies have shown that REE scavenged by hydrogenous Fe oxyhydroxides have positive Ce anomaly and more abundance in LREE. On the contrary hydrothermal Fe oxyhydroxides uptake REE from seawater with negative Ce anomaly and depletion in LREE. On the base of this difference in REE patterns the fate of Fe oxyhydroxides in the sediments with hydrothermal input during diagenesis can be considered. This study describes REE behaviour in sediments, nodules and micronodules (MN) of Guatemala Deep.