Biomedical rare-earth magnesium alloy:Current status and future prospects

Biomedical magnesium(Mg)alloys have garnered significant attention because of their unique biodegradability,favorable biocompatibility,and suitable mechanical properties.The incorporation of rare earth(RE)elements,with their distinct physical and chemical properties,has greatly contributed to enhancing the mechanical performance,degradation behavior,and biological performance of biomedical Mg alloys.Currently,a series of RE-Mg alloys are being designed and investigated for orthopedic implants and cardiovascular stents,achieving substantial and encouraging research progress.In this work,a comprehensive summary of the state-of-the-art in biomedical RE-Mg alloys is provided.The physiological effects and design standards of RE elements in biomedical Mg alloys are discussed.Particularly,the degradation behavior and mechanical properties,including their underlying action are studied in-depth.Furthermore,the preparation techniques and current application status of RE-Mg alloys are reviewed.Finally,we address the ongoing challenges and propose future prospects to guide the development of high-performance biomedical Mg-RE alloys.

Effects of Rare-earth and Microalloying Elements on the Microstructure Characteristics of Hypereutectoid Rails

We performed thermal simulation experiments of double-pass deformation of hypereutectoid rails with different microalloying elements at a cooling rate of 1℃/s and deformation of 80%to explore the influence of rare-earth and microalloying elements on the structure of hypereutectoid rails and optimize the composition design of hypereutectoid rails.Scanning electron microscopy,transmission electron microscopy,X-ray diffraction,and other characterization techniques were employed to quantitatively analyzed the effects of different microalloying elements,including rare-earth elements,on pearlite lamellar spacing,cementite characteristics,and dislocation density.It was found that the lamellar spacing was reduced by adding various microalloying elements.Cementite lamellar thickness decreased with the refinement of pearlite lamellar spacing while the cementite content per unit volume increased.Local cementite spheroidization,dispersed in the ferrite matrix in granular form and thus playing the role of dispersion strengthening,was observed upon adding cerium(Ce).The contributions of dislocation density to the alloy strength of four steel sheet samples with and without the addition of nickel,Ce,and Ce–copper(Cu)composite were 26,27,32,and 37 MPa,respectively,indicating that the Ce–Cu composite had the highest dislocation strengthening effect.The Ce–Cu composite has played a meaningful role in the cementite characteristics and dislocation strengthening,which provides a theoretical basis for optimizing the composition design of hypereutectoid rails in actual production conditions.

Geochemical characterization of rare earth elements in sediment profiles from the Oualida lagoon(Morocco)

The present work assesses the temporal distribution pattern and geochemical changes of rare earth elements and Yttrium, Scandium, Thorium, and Uranium delivery into the Oualidia lagoon. Two sediment cores were retrieved from the Oualidia lagoon and analyzed using neutron activation analysis. The results indicated that heavy rare earth elements are slightly enriched the sediment cores over light rare earth elements. The highest values of REEs were recorded in the top layers of the cores and depleted with depth, suggesting a possible change in factors controlling their accumulation, including mechanical, chemical, and environmental parameters such as weathering intensity, grain size, and Fe-Mn oxides. The sediments display positive Ce anomalies, which are probably related to the submarine weathering process and detrital input. Noting also the variation of hydrodynamics conditions and confinement of the upstream part of the lagoon played a key role in changing the sediment origins.Thus, further investigation of REEs origin in the Oualidia lagoon sediment is required to identify their sources,provenances, and the factors controlling their spatial and vertical distributions. However, these results provide baseline data of occurring changes in REEs geochemical composition and constitute a typical study case to understand the link between sedimentary and geochemistry processes in a lagoonal ecosystem.

Geochemistry of the Rare Earth Elements (REE) Distribution in Terengganu Coastal Waters: A Study Case from Redang Island Marine Sediment

A study on the rare earth elements (REEs) of Redang Island marine sediments was conducted in August 2011 during the premonsoon season. Bottom sediments were collected from 27 sampling stations on board UMT Discovery II vessel using a Smith McIntyre grab and analyzed for rare earth elements (REEs) using inductively coupled plasma mass spectrometry (ICP-MS). Results showed that the REEs patterns in sediments reflected the source rock patterns with an overall order of abundance such as: light rare earth elements (LREE) > middle rare earth element (MREE) > high rare earth elements (HREE). The chondrite normalized patterns of REEs showed enrichment of LREEs over HREEs with La/Yb of 13.6 while the chondrite normalized value showed low (Gd/Yb)N ratios (2.12) which was illustrated in the flat HREE pattern. The chondrite normalized REE patterns of sediments showed the enrichment of LREE and depletion of HREE. From our findings, REEs concentrations in the study area were found to be of geogenic origins and the distributions were not influenced by anthropogenic sources.

Geochemical Characteristics of the Trace and Rare Earth Elements in Reef Carbonates from the Xisha Islands(South China Sea): Implications for Sediment Provenance and Paleoenvironment

Based on the concentrations of the trace elements,rare earth elements(REE),and Sr isotopic compositions in reef carbonates from the well‘Xike-1’reef core of the Xisha Islands,the constraints on sediment provenance and paleoenvironment were defined.Variations of the terrigenous input into the paleoseawater were traced in detail and the paleoenvironment and sediment provenance were further investigated.The results show that the HREE/LREE values in the reef carbonates are negatively associated with their Th and Al concentrations;however,their Al and Th concentrations show positive correlation.The lowest 87 Sr/86 Sr values in the reef carbonates generally coincide with the lowest values of Al,Th concentrations and the highest values of HREE/LREE.These data indicate that the HREE/LREE,Al concentrations,and Th concentrations of the reef carbonates are useful indexes for evaluating the influence of the terrigenous inputs on the seawater composition in the study area.From top to bottom,the changing process of the HREE/LREE values and Al,Th concentrations can be divided into 6 intervals;they are H1(0–89.30 m,about 0–0.11 Myr),L1(89.30–198.30 m,about 0.11–2.2 Myr),H2(198.30–374.95 m,about 2.2–5.3 Myr),D(374.95–758.40 m,about 5.3–13.6 Myr),L2(758.40–976.86 m,about 13.6–15.5 Myr),and H3(976.86–1200.00 m,about 15.5–21.5 Myr).Moreover,the changing trend of the HREE/LREE values coincides with that of the seawater δ^13C values recorded by benthonic foraminiferal skeletons from the drill core of ODP site 1148 in the South China Sea(SCS),but not with that of the seawaterδ18O values.The high uplifting rates of the Qinghai-Tibet Plateau coincide with the high Th and Al concentrations and the low HREE/LREE values in the reef carbonates.These data indicate that the main factors controlling the changes of terrigenous flux in the SCS are the tectonic activities associated with Qinghai-Tibet Plateau uplifting and the variations of uplifting rates rather than paleoclimatic changes.

Enhancement Effects of Three Rare Earth Elements on the Growth of Chaetoceros Mulleri

Enhancement effects of rare earth elements on the growth of Chaetoceros mulleri is studied in this paper. The results show that all of the light, middle and heavy rare earth elements have similar enhancement effect on the growth of Chaetoceros mulleri, with the beneficial concentrations of La, Gd and Yb being 7.28～87.40 μ mol/L, 6.36～57.23 μ mol/L and 5.78～17.34 μ mol/L. The optimum concentrations of La, Gd and Yb are 7.28 ～50.98 μ mol/L,31.80～44.52 μ m ol/L and 5.78～17.34 1μ mol/L, respectively. When the concentrations of La, Gd and Yb are 7.28～87.40 μ mol/L, 6.36～57.23 μ mol/L and 5.78～ 17,34 μ mol/L, the concentrations of chlorophyll have increased by 9.3～47.0%, 33.4～44.3%, and 36.5～40.3%, respectively as compared with the control group. The mechanism of enhancement of rare earth elements on the growth ot Chaetoceros mulleri is also discussed in this paper.

Correlations between Light Rare Earth Elements in Soil and Navel Orange Tree System in Gannan Area

The total content of light rare earth elements( LREEs) in the soil of navel orange orchards of Gannan area is greater than that of heavy rare earth elements( HREEs). Appropriate content of LREE can not only promote the growth of navel oranges,and it is also conducive to human health. On the basis of exploring the correlations between the content of LREE in the soil of navel orange orchards of Gannan area and the contents of LREE in navel orange leaves and fruit,the influence mechanism of LREEs on the quality of navel oranges was revealed. In this study,with two Newhall navel orange orchards with different soil LREEs background levels in Xinfeng County as the research object,the changes in the content of LREE( lanthanum,La; cerium,Ce; praseodymium,Pr; neodymium,Nd) in leaves and fruit of navel orange at different growth stages were analyzed using Inductively Coupled Plasma-Mass Spectrometry( ICP-MS),and the correlations between the content of LREE in the soil,navel orange leaves and navel orange fruit were studied. The results showed that the contents of the four kinds of LREEs in the soil ranked as Ce > La > Nd > Pr,and there were significant differences among them( P < 0. 01). Navel orange leaves and fruit have selective and heterogenic absorption for LREEs. At different growth stages,La showed the highest accumulation amount in the leaves and fruit of navel orange; and the content of LREE in the leaves of navel orange increased first and then decreased,while that in the fruit of navel orange showed continuous decrease. During the migration of LREEs from soil to leaves to fruit,the content of LREE decreased rapidly as the migration distance increased. The accumulation amount of LREE in navel orange was positively correlated with the content of LREE in the soil. The correlation between the content of LREE in the leaves and fruit of navel orange was greatest. Among the four kinds of LREEs,the correlation of La was greatest,followed by Ce,indicating that the accumulation amount of LREE in the navel orange body was affected by the element types.

Distribution of Rare Earth Elements in Early Permian Reef-Island Ocean Sediments in Eastern Kunlun

Abstract: Based on the study of stratigraphy and fossils, the Early Permian ocean in eastern Kunlun is recognized as a kind of reef-island ocean, in which there exist many different kinds of sediment, including patch carbonate platform, reef facies, transitional facies and deep basin sediments. It has been found that the total contents of REEs increase gradually from carbonate platform facies to deep basin facies. Meanwhile, sediments of different facies have different REE distribution patterns and different Ce anomalies. Most of the sediments of patch carbonate platform facies or reef facies are characterized by extremely negative Ce anomalies or moderately negative Ce anomalies (Ce/Ce*=0.33 to 0.55), and medium or thin-bedded limestones of transitional facies by moderately negative Ce anomalies (Ce/Ce*=0.49 to 0.60). However, sediments of deep basin facies show weak or no negative Ce anomalies (Ce/Ce*=0.69 to 1.47), among which the value of Ce/Ce* in the radiolarian chert is 1.47.

Rare Earth Elements and Yttrium(REY) Geochemistry of Reefal Limestones in the Ordovician, Tarim Basin, NW China and their Paleoenvironment Implications

This study examines the rare earth elements and yttrium(REY)concentrations of twenty-five samples from the reef outcrop exposed along the Lianglitage Mountain in the Ordovician,Tarim Basin in China.The concentration analysis provides constraints on the paleoenvironment during reef deposition.Based on the detailed sedimentology and petrographic work,we divide the reef facies into four sub-facies:the base facies,reef-core facies,reef-flank facies,and sealing facies.The geochemical data(such as major and trace elements,carbon and oxygen isotopes,and REYs)are further used to study the coeval seawater characteristics as well as potential diagenesis overprints.The result indicated that the diagenesis has little effect on the REY patterns of the reefal limestones.The REY concentrations of the reefal limestones are overall low(ranging from 3.69 to 19.60 ppm,arithmetic mean=10.22 ppm,SD=5.4).The PAAS-normalized REY patterns are consistently flat compared to the typical well-oxidized,shallow marine water patterns.However,the light REE(LREE)depletions,positive La anomalies,negative Ce anomalies and positive Y anomalies,suggest that these reefal limestones are likely an indicative of contemporaneous seawater REY signals.The seawater-like Y/Ho ratios(average at 37.51)further support that REY signals in these limestones are likely a reflection of seawater with little diagenetic modifications.The low Y/Ho ratios presented only in the reef-flank facies and sealing facies are likely a suggestion of detrital contamination.Hence,this study confirms that REY patterns of the limestones at the base facies and reef-core facies can record ancient seawater information,and reefs can be used as a potential geochemical proxy for paleoenvironment studies throughout the Earth’s history.

Investigation of Environmental and Biological Effects of Rare Earth Elements (REEs) with a Special Focus on Industrial and Mining Pollutions in Iran:A Review

The present article is a review study on the types of rare earth elements(REEs),environmental and biological effects as well as the sources of emission of these elements as pollution in nature.The purpose of this study is to provide a vision in environmental planning and control of pollution caused by REEs.The evaluation of rare earth elements was studied in human life and its environmental and biological effects,which have particular importance and are entering the life cycle through industrial and mining pollution sources.Since mining activities intensify the dispersion of these elements in the environment and the existence of industrial factories located around urban drainage system plays a unique role in creating and spreading pollution caused by rare earth elements;As a result,two case studies were conducted on two mining and industrial areas.The first case is the Choghart mine in Yazd province as an example of mining pollution,and the second case study is performed on the Kor river as an example of industrial pollution which is caused by industrial activities around it,Then the results are well explained to show both two environments of litho and hydro.Due to this fact that produced environmental pollution can cause exchange pollutant compounds with the surrounding environment besides its long-lasting destructive effects;It can cause irreversible biological effects on living organisms.By targeting this evaluation,several techniques can be proposed to prevent the entry and dispersal of rare earth elements from pollution sources besides methods to reduce the damage of these elements to the ecosystem.

Fractionation characteristics of rare earth elements(REEs) linked with secondary Fe, Mn, and Al minerals in soils

Soil secondary minerals are important scavengers of rare earth elements(REEs) in soils and thus affect geochemical behavior and occurrence of REEs. The fractionation of REEs is a common geochemical phenomenon in soils but has received little attention, especially fractionation induced by secondary minerals. In this study, REEs(La to Lu and Y) associated with soil-abundant secondary minerals Fe-, Al-, and Mn-oxides in 196 soil samples were investigated to explore the fractionation and anomalies of REEs related to the minerals. The results show right-inclined chondrite-normalized REE patterns for La–Lu in soils subjected to total soil digestion and partial soil extraction. Light REEs(LREEs) enrichment features were negatively correlated with a Eu anomaly and positively correlated with a Ce anomaly. The fractionation between LREEs and heavy REEs(HREEs) was attributed to the high adsorption affinity of LREEs to secondary minerals and the preferred activation/leaching of HREEs.The substantial fractions of REEs in soils extracted byoxalate and Dithionite-Citrate-Bicarbonate buffer solutions were labile(10 %–30 %), which were similar to the mass fraction of Fe(10 %–20 %). Furthermore, Eu was found to be more mobile than the other REEs in the soils, whereas Ce was less mobile. These results add to our understanding of the distribution and geochemical behavior of REEs in soils, and also help to deduce the conditions of soil formation from REE fractionation.

Influence of rare earth Ce on hot deformation behavior of as-cast Mn18Cr18N high nitrogen austenitic stainless steel

The hot deformation behavior of Mn18Cr18N and Mn18Cr18N+Ce high nitrogen austenitic stainless steels at 1173-1473 K and 0.01-1 s^(-1) were investigated by thermal compression tests.The influence mechanism of Ce on the hot deformation behavior was analyzed by Ce-containing inclusions and segregation of Ce.The results show that after the addition of Ce,large,angular,hard,and brittle inclusions(TiN-Al_(2)O_(3),TiN,and Al_(2)O_(3)) can be modified to fine and dispersed Ce-containing inclusions(Ce-Al-O-S and TiN-Ce-Al-O-S).During the solidification,Ce-containing inclusions can be used as heterogeneous nucleation particles to refine as-cast grains.During the hot deformation,Ce-containing inclusions can pin dislocation movement and grain boundary migration,induce dynamic recrystallization(DRX)nucleation,and avoid the formation and propagation of micro cracks and gaps.In addition,during the solidification,Ce atoms enrich at the front of solid-li-quid interface,resulting in composition supercooling and refining the secondary dendrites.Similarly,during the hot deformation,Ce atoms tend to segregate at the boundaries of DRX grains,inhibiting the growth of grains.Under the synergistic effect of Ce-containing inclusions and Ce segregation,although the hot deformation resistance and hot deformation activation energy are improved,DRX is more likely to occur and the size of DRX grains is significantly refined,and the problem of hot deformation cracking can be alleviated.Finally,the microhardness of the samples was measured.The results show that compared with as-cast samples,the microhardness of hot-deformed samples increases signific-antly,and with the increase of DRX degree,the microhardness decreases continuously.In addition,Ce can affect the microhardness of Mn18Cr18N steel by affecting as-cast and hot deformation microstructures.

Trace Elements and Rare Earth Elements of Sulfide Minerals in the Tianqiao Pb-Zn Ore Deposit,Guizhou Province,China

Trace elements and rare earth elements（REE） of the sulfide minerals were determined by inductively-coupled plasma mass spectrometry.The results indicate that V,Cu,Sn,Ga,Cd,In,and Se are concentrated in sphalerite,Sb,As,Ge,and Tl are concentrated in galena,and almost all trace elements in pyrite are low.The Ga and Cd contents in the light-yellow sphalerites are higher than that in the brown and the black sphalerites.The contents of Ge,Tl,In,and Se in brown sphalerites are higher than that in light-yellow sphalerites and black sphalerites.It shows that REE concentrations are higher in pyrite than in sphalerite,and galena.In sphalerites,the REE concentration decreases from light-yellow sphalerites,brown sphalerites,to black sphalerites.The ratios of Ga/In are more than 10, and Co/Ni are less than 1 in the studied sphalerites and pyrites,respectively,indicating that the genesis of the Tianqiao Pb-Zn ore deposit might belong to sedimentary-reformed genesis associated with hydrothermal genesis.The relationship between LnGa and LnIn in sphalerite,and between LnBi and LnSb in galena,indicates that the Tianqiao Pb-Zn ore deposit might belong to sedimentary-reformed genesis.Based on the chondrite-normalized REE patterns,δEu is a negative anomaly（0.13-0.88）,andδCe does not show obvious anomaly（0.88-1.31）;all the samples have low total REE concentrations（〈3 ppm） and a wide range of light rare earth element/high rare earth element ratios（1.12-12.35）.These results indicate that the ore-forming fluids occur under a reducing environment.Comparison REE compositions and parameters of sphalerites,galenas,pyrites,ores,altered dolostone rocks,strata carbonates,and the pyrite from Lower Carboniferous Datang Formation showed that the ore-forming fluids might come from polycomponent systems,that is,different chronostratigraphic units could make an important contribution to the ore-forming fluids.Combined with the tectonic setting and previous isotopic geochemistry evidence,we conclude that the ore-deposit genesis is hydrothermal,sedimentary reformed,with multisources characteristics of ore-forming fluids.

Anomalous Concentrations of Rare Metal Elements, Rare-scattered(Dispersed) Elements and Rare Earth Elements in the Coal from Iqe Coalfield, Qinghai Province, China

Total of 23 bench samples were taken from the No. 7 Coal of Iqe Coalfield, Qinghai Province, China, following Chinese Standard Method GB/T 482-2008 （2008）. These samples were analyzed by powder X-ray diffraction （XRD）, inductively coupled plasma mass spectrometry （ICP- MS） and X-ray fluorescence （XRF）. The results indicate that the No. 7 Coal belongs to a low rank （Ro.ran =0.659%） and high-ash coal （40.54%）. Compared to common Chinese and world low-rank coals, the lqe coal contains anomalous concentrations of rare metal elements, rare-scattered （dispersed） elements and rare earth elements. The highest contents of Rb, Cs, Ga and REY reach to 180, 26, 37, and 397 ppm, respectively. Their average contents of these elements are 10.9, 15, 4.8 and 3.5 times higher than those of world coals, respectively. Minerals in the coal include kaolinite, quartz, muscovite, siderite, and traces of rutile, and brookite. Kaolinite could be main host minerals of Rb, Cs, Ga and REY. The anomalous rare element Rb and Cs accumulation in the Iqe coal is related to both organic and inorganic matter. The REY concentrations may be related to circulation of thermal solutions, contained or sorbed by clayey particles, and organic matter as well.

A new type of rare earth elements deposit in weathering crust of Permian basalt in western Guizhou, NW China

A new type of rare earth elements （REEs） deposit was discovered from the gaolinite mudstone in the weathering crust of Permian basalt, Bijie region, western Guizhou, China. It contained ∑RE2O3 0.065%-1.086%. This type of REEs deposit was widely distributed with steady horizon and thickness of 3-4 m. The ore-bearing weathering crust （kaolinite） of the three discovered REEs deposits belonged to the third episode of the Emeishan basalt eruption. The new type of REEs deposit was suggested that basalt （tuff） weathering could lead to the enrichment of the rare earth elements. Therefore, it is of important economic significance to explore REEs deposits in the weathering crust of basalt （tuffs） in Yunnan, Guizhou, and Sichuan Provinces.

A Survey of 16 Rare Earth Elements in the Major Foods in China

Objective The aim of this survey was to investigate the level of contamination of the most consumed foods in China with 26 rare earth elements （REEs）, and to provide the basic data for establishing and revising food safety standards for REEs. Methods Sixteen REEs in foods were measured by inductively coupled plasma-mass spectrometry （ICP-MS） in the labs of the Centers for Disease Control and Prevention of four provinces and two municipalities, during 2009-2020. Results 2 231 samples were analyzed and 29 221 concentration data of 16 REEs were collected. The REEs levels in the investigated foods varied significantly. The concentrations of cerium （Ce）, dysprosium （Dy）, yttrium （Y）, lanthanum （La）, and neodymium （Nd） were relatively high, while the remaining eleven REEs were at low levels. The mean values of total rare earth element oxides （REOs） in cereals, fresh vegetables, fresh aquatic products, fresh meats and eggs varied from 0.052 mg/kg to 0.337 mg/kg. Conclusion 16 REEs in the major foods were at very low contamination levels in the investigated regions.

Rare Earth Elements Geochemistry of Laowan Gold Deposit in Henan Province: Trace to Source of Ore-Forming Materials

The compositions of REE in quartz and pyrite from the main stage of the Laowan gold deposit in Henan Province and that in quartz from Laowan granite were determined by inductively coupled plasma-mass-spectrometry （ICP-MS） to trace the source of ore-forming materials. Meanwhile, the REE compositions of the deposit ore, granite and metamorphic wall rock were also considered for comparative studies in detail. The range of ∑REE of quartz and pyrite from the deposit ores is 4.18 × 10^-6- 30.91 × 10^-6, the average of ∑REE is 13.39 × 10^-6, and the average of ∑REE of quartz in the Laowan granite is 6.68 × 10^-6. There is no distinct difference of REE parameters between the deposit ore quartz and granite quartz. The quartz in gold deposit has the same REE particular parameters as quartzes from Laowan granite, such as δEu, δCe, （La/Yb）N and （La/Sm）N, partition degree of LREE to HREE, especially, the chondrite-normalized REE patterns, but no similarity to those from metamorphic wall rock, which shows that ore-forming hydrothermal fluid is mainly the fluid coming from the Laowan granite magma, rather than metamorphic fluid. Meanwhile, comparison studies on REE features between minerals from the deposit ores and related geological bodies in the deposit show that REE characteristics of minerals can serve as an indicator of ore-forming fluid properties and sources, while the REE characteristics of the bulk samples （such as deposit ores, granites and wall rocks） can not trace the source of the ore-forming materials exactly.

Effect of rare earth elements addition on thermal fatigue behaviors of AZ91 magnesium alloy

Influences of rare earth （RE） elements addition on thermal fatigue behaviors of AZ91 alloy were studied. Repeated heating and cooling cycles were applied on the samples at 170 and 210℃ to develop thermal fatigue cracks. Crack growth mechanisms and microstructural influences were investigated by optical and scanning electron microscopy （SEM） as well as energy dispersive X-ray spectroscopy （EDS）. Thermal fatigue behaviors were observed to improve successively by addition of the RE up to 2wt.%. This improvement was attributed to the consummation of aluminum in melt by precipitation of the needle shaped AII1RE3 phases. This process was attributed to the reduction of MglTAl12 phase volume fraction and consequent decrease of the brittle Mg/MglTAl12 interface which was the main reason for weak thermal properties of the alloy at rather high temperatures. Further additions of RE, however, reduced the thermal shock resistance of the samples by increasing the mean length of the brittle needle shaped phases.

From mantle to critical zone:A review of large and giant sized deposits of the rare earth elements

The rare earth elements are unusual when defining giant-sized ore deposits,as resources are often quoted as total rare earth oxide,but the importance of a deposit may be related to the grade for individual,or a limited group of the elements.Taking the total REE resource,only one currently known deposit（Bayan Obo） would class as giant（〉1.7×10^7 tonnes contained metal）,but a range of others classify as large（〉1.7×10^6 tonnes）.With the exception of unclassified resource estimates from the Olympic Dam 10 CG deposit,all of these deposits are related to alkaline igneous activity- either carbonatites or agpaitic nepheline syenites.The total resource in these deposits must relate to the scale of the primary igneous source,but the grade is a complex function of igneous source,magmatic crystallisation,hydrothermal modification and supergene enrichment during weathering.Isotopic data suggest that the sources conducive to the formation of large REE deposits are developed in subcontinental lithospheric mantle,enriched in trace elements either by plume activity,or by previous subduction.The reactivation of such enriched mantle domains in relatively restricted geographical areas may have played a role in the formation of some of the largest deposits（e.g.Bayan Obo）.Hydrothermal activity involving fluids from magmatic to meteoric sources may result in the redistribution of the REE and increases in grade,depending on primary mineralogy and the availability of ligands.Weathering and supergene enrichment of carbonatite has played a role in the formation of the highest grade deposits at Mount Weld（Australia） and Tomtor（Russia）.For the individual REE with the current highest economic value（Nd and the HREE）,the boundaries for the large and giant size classes are two orders of magnitude lower,and deposits enriched in these metals（agpaitic systems,ion absorption deposits） may have significant economic impact in the near future.

A review of fractionations of rare earth elements in plants

Studies were carried out on several aspects of rare earth elements （REEs）, such as the theory and practice of their applications in agriculture, their geochemical behaviors in natural and agricultural ecosystems, the mechanisms for the increase of crop yield using REE fertilizer, and their toxicology. However, limited knowledge was available for the transfer processes and the features and mechanisms of distribution and fractionations of REEs inside plants. The characteristics of REE fractionations in plants can be used to ＂trace＂ the pathway of REE transportation from soils （solution） to plants. A better understanding of the mechanisms of REE fractionations was helpful to investigate the controlling factors, including both the internal and the external ones. The characteristics and mechanisms of REE fractionations in plants and their significance were reviewed. Furthermore, the prospect for these fields was discussed, in hope of providing a new way in studying the bioavailability of REEs and heavy metals.