Rare Earth Element Values Research Articles

Matrix-independent separation of rare-earth elements and yttrium from geological materials using constant calcium content-oxalate precipitation and cation exchange for determination by high-resolution inductively coupled plasma atomic emission spectrometry (ICP-AES)

Inductively coupled plasma atomic emission spectrometry (ICP-AES) is a precise and accurate analytical technique for determination of rare-earth elements (REE) in geologic materials provided the REE are separated from matrix elements. The separation method reported here uses cation exchange following an oxalate precipitation separation. Oxalate precipitation separates REE and Y with Ca from other matrix elements. Calcium oxalate is an effective carrier for REE, but is much more effective for light and middle REE (LREE and MREE) than for heavy REE (HREE). Complete REE recovery (especially of HREE) requires a sufficiently high Ca content in the sample. The method used here is to bring each sample (1 g) up to a Ca content of 107 mg by addition of calcium carbonate prior to sample digestion. Subsequent ion exchange with a mixed resin gives complete recovery of REE and Y (and in some cases Sc) without modification for sample type because the salt load is constant no matter what the starting material. Analysis of nine REE, Y and Sc is carried out using a very high-resolution sequential scanning ICP-AES system. High resolution minimizes RE inter-element interferences, and use of mixed-element standards (mixed in geologic proportions) minimizes corrections, usually to a negligible level. Analyses of U.S.G.S. standard reference materials BHVO- 1, DNC- 1 and TLM- 1 compare well with published values for REE and Y. Recovery of Sc is apparently not always complete.

Geochemistry and sedimentology of a facies transition from limestone to iron-formation deposition in the early Proterozoic Transvaal Supergroup, South Africa

This study deals with the geochemistry and sedimentology of a facies transition from interbedded carbonate-shale to banded iron-formation in the Campbellrand carbonate sequence to the overlying Kuruman Iron Formation of the Transvaal Supergroup in South Africa which is approximately 2.3 Ga old. Four major lithologies are (1) limestone and dolomite, (2) shale and interbedded shale carbonate, (3) siderite-rich banded iron-formation, and (4) iron oxide-rich banded iron-formation. These rocks are unaltered and essentially unmetamorphosed with a maximum metamorphic-diagenetic overprint ranging in temperature from 110 degrees to 170 degrees C and with pressures of not more than 2 kbars.The oldest rocks are limestones and lesser dolomite with abundant cryptalgalaminae and intraclastic textures. Interbedded with the limestones and dolomites are carbonaceous shales, some of which are unusually ferruginous or pyrite rich. These carbonates and shales are overlain by meso- and microbanded siderite-chert iron-formation which grades upward into magnetite-, chert-, and carbonate-rich iron-formation.The averages for major and trace elements and rare earth element (REE) contents of the limestones, dolomites, and shales are distinct from those of the two types of iron-formation. The two most chemically diverse rock types are the shales (enriched in alkalis and most trace elements) and the two iron-formation types (siderite and magnetite rich) which are almost totally depleted in all elements except Si, Fe, Mg, and Ca (and CO 2 ). The high Al 2 O 3 contents of the shales (avg 9.55 wt %) correlate well with their high organic carbon contents (avg 3.91 wt %); the limestones and dolomites have average Al 2 O 3 and organic carbon contents of about 3.0 and 0.7 wt percent, respectively; the two iron-formation types have Al 2 O 3 average values of 0.099 wt percent (siderite rich) and 0.066 wt percent (magnetite rich) and corresponding averages for organic carbon of 0.080 wt percent (siderite rich) and 0.012 wt percent (magnetite rich). The REE are generally concentrated in the shales by a factor of about 10 over the iron-formations. The limestones and dolomites have intermediate values, and the iron-formations are most depleted. The iron-formation REE patterns (in a ratio with North American shale composite (NASC)) have pronounced positive Eu anomalies and slight negative Ce anomalies. Both of these anomalies are absent in the shales, but the limestones and dolomites show slight positive Eu anomalies.The siderite-rich iron-formations consist of chert-siderite-ankerite (or ferroan dolomite) with traces of pyrite and stilpnomelane. The siderite in these commonly microbanded assemblages is very fine grained along well-defined bedding planes. It is concluded, from petrographic study, that siderite is a primary precipitate. The ankerites and ferroan dolomites in these siderite-rich occurrences are commonly euhedral (rhombohedral), much coarser grained, and appear to be of later (diagenetic) origin. The magnetite-rich iron-formations consist of chert-magnetite-ankerite (or ferroan dolomite) + or - siderite + or - hematite + or - stilpnomelane + or - minnesotaite. In these oxide-rich iron-formations the ankerites (and ferroan dolomites) are similarly much coarser grained than the finely banded siderite. On the basis of the geochemical data and a reconstruction of the depositional basin for the carbonate-shale to iron-formation transition, we conclude that the limestone-dolomite-shale lithologies originated in a water column quite distinct from that in which the iron-formations were precipitated. We propose a model with a stratified water column in which the surface waters (during a regressive stage in the depositional basin) were the site of much organic carbon productivity and the locus of cryptalgal limestones and intraclastic limestone deposition; with at somewhat greater depth (below the chemocline) deposition of pyritic carbonaceous shale. Our model depicts the deeper waters (during a transgressive stage of the basin with the Kaapvaal craton more deeply submerged) as the site for iron-formation deposition; these deeper waters were depleted in organic carbon and enriched in dissolved ferrous iron relative to the shallower water mass, with continued availability of oxygen along the chemocline separating the two water masses. The ultimate source of the iron (and probably the SiO 2 ) in the iron-formations appears to be a very dilute hydrothermal input in the deep ocean waters, as concluded from mixing calculations of REE values for modern Atlantic Ocean water and hydrothermal solutions from the East Pacific Rise.

Rare earth elements in the phosphatic-enriched sediment of the Peru shelf

Apatite-enriched materials from the Peru shelf have been analyzed for their major oxide and rare earth element (REE) concentrations. The samples consist of (1) the fine fraction of sediment, mostly clay material, (2) phosphatic pellets and fish debris, which are dispersed throughout the fine-grained sediment, (3) tabular-shaped phosphatic crusts, which occur within the uppermost few centimeters of sediment, and (4) phosphatic nodules, which occur on the seafloor. The bulk REE concentrations of the concretions suggest that these elements are partitioned between the enclosed detrital material and the apatite fraction. Analysis of the fine-grained sediment with which the samples are associated suggested that this detrital fraction in the concretions should have shale REE values; the analysis of the fish debris suggested that the apatite fraction might have seawater values. The seawater contribution of REE's is negligible in the nodules and crust, in which the apatite occurs as a fine-grained interstitial cement. That is, the concentration of REE's and the REE patterns are predominantly a function of the amount of enclosed fine-grained sediment. By contrast, the REE pattern of the pelletal apatite suggests a seawater source and the absolute REE concentrations are relatively high. The REE P 2 O 5 ratios of the apatite fraction of these samples thus vary from approximately zero (in the case of the crust and nodules) to as much as approximately 1.2 × 10 −3 (in the case of the pellets). The range of this ratio suggests that rather subtle variations in the depositional environment might cause a significant variation in the REE content of this authigenic fraction of the sediment. Pelletal glauconite was also recovered from one sediment core. Its REE concentrations closely resemble those of the fish debris.

The need for standardization of normalized multi-element diagrams in geochemistry: a comment.

A potpourri of mutually inconsistent normalized multi-element diagrams (‘spider-diagrams’) has been used in geochemistry, on the basis of different choices of: (i) element order; (ii) normalizing materials (chondrite, ‘primitive’ mantle, MORB, composite shale, etc.); (iii) normalizing values (many incompatible sets are used for conventional chondrite-normalized rare-earth element (REE) plots, which generate ‘enrichments’ varying by as much as 65 % from a single rock, and Eu anomalies of quite divergent strengths). Standardization is urgently required, not least because (1) many published diagrams are unnecessarily difficult to compare; (2) different element orders can drastically affect patterns, or lead to artificial ‘pseudo-anomalies’ lacking petrogenetic significance. One type of chondrite- and MORB-normalized spider-diagram (each based on one set of normalizing values) should suffice to represent most igneous rocks. Arguments favouring the chondrite-normalizing REE values of Nakamura (1974), the chondrite-normalized 19-element plot of Thompson (1982), and the MORB-normalized 15-element plot of Pearce (1982, 1983) are preferred, hopefully as an impetus to wider discussion. Consistently ordered sub-sets can be chosen from either multi-element plot if fewer elements are in fact analysed. A FORTRAN77 program has been written to realize these suggestions.

Analysis of low levels of rare earths by radiochemical neutron activation analysis

A procedure for the radiochemical neutron-activation analysis for the rare earth elements (REE) involves the separation of the REE as a group by rapid ion-exchange methods and determination of yields by reactivation or by energy dispersive X-ray fluorescence (EDXRF) spectrometry. The U. S. Geological Survey (USGS) standard rocks, BCR-1 and AGV-1, were analyzed to determine the precision and accuracy of the method. We found that the precision was ±5–10% on the basis of replicate analysis and that, in general the accuracy was within ±5% of accepted values for most REE. Data for USGS standard rocks BIR-1 (Icelandic basalt) and DNC-1 (North Carolina diabase) are also presented.

Experimental study on rare-earth element partitioning in olivine and clinopyroxene formed at 10 and 20kb for basaltic systems.

Partition coefficients of rare-earth elements (REE), Ba and Sr were precisely determined for crystal glass pairs prepared by partial melting and partial solidification of alkali olivine basalt at 10kb and of picrite basalt at 10 and 20kb. Mutually unfractionated REE partition patterns were observed for olivine-glass pairs prepared by partial melting and partial solidification experiments on alkali olivine basalt. A similar unfractionated REE pattern was observed for (clinopyroxene + olivine)/glass formed by 10kb partial melting of picrite basalt. On the other hand, clinopyroxene was produced in 10kb partial solidification of the same picrite basalt and the clinopyroxene showed fractionated, terrace-shaped REE partition pattern. Partial solidification experiments on picrite basalt produced only clinopyroxene as a mineral coexisting with melt, and the resultant REE partition pattern was a guyot-shaped one. It should be noted that the REE partition pattern (terrace-shaped or guyot-shaped) of clinopyroxene bears on the mutual relationship concerning the relative magnitudes of partition values of Fe and heavy REE. Finally, it is worth while to note that clinopyroxene is prevailingly formed in partial solidification process.

Electrothermal atomisation for atomic-absorption determination of some rare earths in silicate rocks and minerals

The interfering effects due to the composition of the silicate matrix of a common geological sample have been overcome by an ion-exchange separation procedure. The early described cation-exchange device allows a complete separation of the rare earth element group (plus barium, yttrium and scandium) from the major and minor elements of the matrix (i. e. calcium, iron, aluminium, sodium, etc.). Some analytical flameless atomic absorption procedures have been carried out on the eluate fraction: dysprosium, holmium, thulium, ytterbium and europium are determined in international standard rocks and minerals. The REE (rare earth elements) values fall well into the range reported in literature.

Discussion on 'Chemistry, thermal gradients and evolution of the lower continental crust' by J. Tarney & B. F. Windley

Tarney & Windley (1977) questioned the andesite model for crustal growth ( Taylor & White 1966 ; Taylor 1967 ; Jakes & Taylor 1974 ; Taylor 1977 ). Much of their argument is based on rare earth element (REE) data from Archaean granulites, which they claim are representative of the lower continental crust. The most damaging comment against the andesite model is their calculation ( Tarney & Windley 1977 , fig. 1) of the REE pattern in the lower continental crust. Using a value for andesite (representing the bulk crust) and average sedimentary rocks (representing the upper crust)they calculated a lower crustal composition which has a strong positive Eu-anomaly and extremely low LREE (in fact there is no La or Ce!). They stated that their andesite value was taken from Nance & Taylor (1976) ; this is incorrect, since no andesite value is reported in that paper, nor is an andesite value reported in the only other paper by those authors ( Nance & Taylor 1977 ). Tarney & Windley do not tabulate their REE values, but estimates can be made from their chondrite-normalized plots. Their values are compared (Table 1) with the estimates of Taylor (1977 , 1979 ) for the bulk continental crust, based on the andesite model.The upper crustal estimate ( Nance & Taylor 1976 ) and the correct calculation for the lower crust (assuming the upper crust is 1/3 of the whole crust) are also given in Table 1. These values are plotted in Fig. 1. In marked contrast to Tarney & Windley's calculations, the REE pattern for the lower

Abundances of the fourteen rare-earth elements, scandium, and yttrium in meteoritic and terrestrial matter

Abundances and isotopic ratios have been determined for the fourteen rare-earth elements (REE) (Pm excluded) plus Sc and Y by neutron-activation analysis in nineteen meteorites and in three terrestrial specimens. The meteorites investigated included eleven chondrites (1 carbonaceous, 2 pigeonitic, 2 hypersthenic, 3 bronzitic and 3 enstatitic), two calcium-rich and three calcium-poor achondrites, one mesosiderite and two iron meteorites; the terrestrial specimens included a fresh volcanic basalt, and a deep-seated peridotitic and an eclogitic specimen. The absolute abundance of a typical REE (La) in bronzitic and hypersthenic chondrites was found to be 0.34 p.p.m. Relative REE, Sc and Y distributions were identical in the five different chondrite categories. On an absolute basis, the REE, Sc, and Y per 10 6 Si atoms varied monotonically over the chondrite spectrum i.e. from carbonaceous through enstatitic meteorites. Meteoritic isotopic abundances of the sensitive Yb 168/Yb 174 ratio agreed to within ±5 per cent of the terrestrial abundance ratios. Absolute REE abundances in the two calcium-rich achondrites, Nuevo Laredo and Pasamonte, were from 10 to 12 times greater than the average chondritic values. No REE fractionation was found in these achondrites; however, Sc was fractionated by a factor of ~2 relative to the REE. Fractionation of the REE, Sc and Y was found in all three calcium-poor achondrites, Johnstown, Shalka and Norton County, relative to the chondritic meteorites. Absolute REE, Sc and Y abundances varied from ~0.05 to ~0.5 times the comparable values in chondrites. Eu has been depleted in Johnstown and Norton County by ~3—4 times. Th has been depleted in Johnstown to the same degree as La. No REE and Y fractionation was found in the mesosiderite, Estherville; however, Sc has been fractionated. Absolute REE abundances in the two Fe meteorites were ~10 −4 times the REE abundances in chondrites. In one iron, the REE may have been fractionated. Absolute REE. Sc and Y abundances in the uneroded Kilauea Iki-22 basalt, peridotitic and eclogitic specimens were from 30 to 10 times greater than the abundances in chondrites. Compared with chondrites, the light REE have been enriched by factors of up to 5, which agrees with the enrichments of the light REE in terrestrial sediments and igneous rocks, etc. Theoretical calculations of expected REE abundances by Clayton et al. agree well with the present chondritic REE values. For convenience, a description of the detailed procedure for the REE, Sc and Y determinations (now in current use at this Laboratory) has been relegated to the Appendix.