Lanthanide distributions

The lanthanides, distributed widely in low concentrations throughout the earth s cmst (2), are found as mixtures in many massive rock formations, eg, basalts, granites, gneisses, shales, and siUcate rocks, where they are present in quantities of 10—300 ppm. Lanthanides also occur in some 160 discrete minerals, most of them rare, but in which the rare-earth (RE) content, expressed as oxide, can be as high as 60% rare-earth oxide (REO). Lanthanides do not occur in nature in the elemental state and do not occur in minerals as individual elements, but as mixtures. 

Cook (1972) has recognized two different types of phosphorite in northwest Queensland pelletal and non-pelletal. The lanthanide distribution in the former is normal for marine phosphorites, whereas it is depleted in the latter type with the exception of the heavy lanthanides which are relatively more abundant. Pelletal types contain greater concentrations of elements which are known to substitute in the structure of apatite, whereas the other types contain components which probably are of detrital origin or are derived from weathering. 

Neutron activation analyses of sixteen samples of sea xmter (eight in duplicate) taken at six widely spaced stations in the Central Atlantic Ocean between 16° N and Equator (depths below 1000 m,) showed that the lanthanide patterns are relatively conservative characteristics of water masses. The differences in lanthanide distribution and total abundance between different water masses are small but significant. The absolute mass abundances of the lanthanides can be illustrated by the following values for North Atlantic Deep Water ... 

D ecent development of our knowledge of lanthanide distributions in nature encouraged us to believe that variations in ratios of these elements might well characterize individual water masses as summarized by Haskin et al. (5), considerable lanthanide fractionation has occurred in the formation of the earth s crust it might be expected that these fractionations would be reflected in the lanthanide patterns of material eroded from diflFerent regions and supplied to the oceans. Since, on the other hand, the lanthanide patterns of marine shales and sediments (5, 6, 11) do not reflect these regional diflFerences but are essentially uniform on a world-wide basis, sea water should express the diflFerential residues on a... 

Information concealed in data can often be most rapidly comprehended from graphical displays. Owing to the alternation in abundance between adjacent even-Z and odd-Z elements simple plots of the type absolute abundance versus atomic number will often easily obscure small diflFerences in lanthanide distribution patterns. The method often used to remove the even-Z, odd-Z eflFect is to divide one distribution, element by element, by a known distribution, and plot the resulting ratios on a logarithmic scale against a linear scale of atomic number or ionic radius (5). If the two distributions are identical, all the ratios are the same and a horizontal line appears. Trends of diflFerences in the distributions appear as curves or sloped lines. 

To obtain the values of yi the logarithm of the normalized abundance of the i-th element was taken. That is, for a particular sample, the abundance of the i-th element is divided by the abundance of that element in the average lanthanide distribution (Table III), and the logarithm of this ratio is formed to give yi. 

Lanthanide distributions in evaporites have not been studied in any great detail. Some incomplete average analyses of evaporites from Russia were reported by Ronov et al. (1974). These data were characterized by very low concentrations with total lanthanide abundances being less than about 10-15 ppm. 

Only one detailed study has systematically examined the effects of diagenesis on lanthanide distributions. Chaudhuri and Cullers (1979) analysed Miocene/Pliocene sediments from a deep well (1.8-4.8km depth) in the Gulf Coast of Louisiana, which was sampled through the illite/montmorillonite mixed layer of illite transition. Such variability as was seen in absolute lanthanide abundances and in La/Yb ratios was correlated to changes in provenance rather than to diagenetic factors. 

The observations described above have important implications with regard to actinide/lanthanide distributions within the body. It is apparent that in binding to transferrin, the f elements are participating in certain aspects of the iron transport pathways in vivo. However, no plutonium, e.g., is found within red blood cells following incorporation and there is no unequivocal evidence that plutonium and the other actinides or lanthanides are transported into cells via transferrin-receptor-mediated endocytosis (Duffield and Taylor 1986). This, too, is a puzzling aspect of f-element-transferrin chemistry and biochemistry which needs more study. 

The lanthanide distribution ratio was observed to increase with atomic number. 

Following the original suggestion by Winsche that fi.ssion products might be extractable from a liquid U-Bi fuel by molten salts in a manner similar to solvent extraction, experiments were conducted by Barcis using the LiCl-KC l eutectic and lanthanide-bismuth alloys [(>]. If the mechanism was indeed one of liquid-liquid extraction, then the lanthanide distribution should follow a simple distribution law and as such be independent of total concentration. Experimentally, this was not the case, and it was subsequently shown by Wiswall [7,8] and later independently by Cubic-ciotti [9] that the results could be explained by assuming that a chemical reaction had occurred as follows ... 

Data for process design. It will be noted from the relative positions of the lines of Fig. 22-3 that it is not possible to assume, a priori, that the order of the lanthanide distributions will be directly predictable from free energy of formation data. For example, from Table 22-6 the order of decreasing stability of the chlorides is Sm, La, Ce, and Nd, whereas at constant Xmk the experimental order is La, Sm, Nd, and Ce. The difference in order is apparently due to the large variation of the activity coefficients of the lanthanides in bismuth. 

The results of the lanthanide distribution experiments have provided a basis for the design of a countercurrent, salt-metal extraction process [2]. Results of uranium distribution. studies indicate that in small-scale exper-iment.s, a satisfactory separation of lanthanides from uranium may be achie -ed in a single equilibrium contacting stage. The experimental value of the distribution coefficient, Ks, was found to be of the order of 20 to 50, vhcre A s is defined as... 

The lanthanide distribution at the Earth s surface does not match that of the chondrites. It is approximated by the abundances in a composite sample of North American shales (table 21.1) (Haskin et al., 1968). In the shales, the heavier lanthanides (Gd-Lu) and Y are uniformly enriched to about 15 times their chondritic concentrations. The lighter lanthanides are increasingly enriched from Gd ( 20,times the chondritic value) to La (—100 times). The concentration... 

It is clear from table 21.1 and fig. 21.1 that the lanthanide distribution in the composite of North American shales (NASC) is different from that of the chondrites and the sun s atmosphere. Since the differences are a smooth function of atomic number (except for Eu), they are probably a result of processes of internal planetary differentiation. Differential condensation from a gas does not produce such smooth distributions (Boynton, 1975). What, then, does the NASC distribution represent, or tell us about the history of Earth ... 

The uniformity in this distribution is surprising. Most rocks are heterogeneous, complex chemical systems with complicated histories. There was no reason to presume that two samples of rock from the same rock formation, or even adjacent fragments from a single sample would show such similar lanthanide distributions, let alone rocks of different ages, from different places, and with different bulk compositions and mineralogies ... 

The mixing of the two effects is at most slightly modified by any mobilization of lanthanides over long distances. The lanthanide distributions of local regions appeared in the carbonate sediments of those regions. To a first approximation, lanthanide ions enter the aqueous phase during weathering, but only briefly. They are immobilized near their sources. 

The common occurrence of the NASC lanthanide distribution attests to the thoroughness of mixing of lanthanides from different igneous sources over multiple cycles of weathering and sedimentation. It also attests to the uniformity of composition of different continental areas. To what extent does the NASC distribution represent the average for the Earth s crust ... 

The moving ocean floors accumulate sediments, but not in great thicknesses like those near continental margins. Most igneous rocks of the ocean floor are strongly deficient in light lanthanides relative to the NASC but are not depleted in Eu. The ocean floors account for most of the surface area of the Earth. What lanthanide distributions are found in ocean sediments ... 

A main feature of the lanthanide distribution in these materials is a substantial depletion in Ce. Excess Ce is found in authigenic ferromanganese nodules (e.g., Goldberg et al., 1963 Ehrlich, 1968 Glasby, 1972-73). Presumably, the selective uptake of Ce by these common oceanic materials accounts for the relative deficiency of that element in ocean water. Concentrations of lanthanides in most biogenic and authigenic oceanic materials are relatively low, and the proportions of those materials in common ocean sediments are low, so their relative abundance distributions do not appreciably affect the overall abundances for the sediments that contain them. 

Copeland et al. (1971) describe clays from the mid-Atlantic ridge that are derived from local igneous material and whose lanthanide distribution therefore matches that of the ocean floor basalts. The lanthanides must have been released from the minerals of the parent igneous rock into solution, but they did not equilibrate in any general way with ocean water before being captured by the clays. 

As discussed above, most ocean sediments have lanthanide distributions similar to those of the NASC. There is no net selective transport of certain lanthanides relative to others from the terrigenous sources to the oceans. The relative Ce deficiency in most ocean water is apparently a result of selective uptake of that element by authigenic ferromanganese nodules. The reasons for the other differences between the ocean water distribution and that of the NASC are not known. The lanthanide distributions for ocean water and authigenic materials demonstrate clearly that internal fractionation of the lanthanide group does occur in the oceanic environment. The concentrations of the lanthanides in ocean water are so low, however, that no large reservoir of material with a lanthanide distribution complementary to that of ocean water is to be expected. As Wildeman and Haskin (1965) indicated, the total amount of lanthanides in ocean water is less than the amount present in the upper 0.2 mm of ocean floor sediment. Thus, preferential extraction of some lanthanides from a few millimeters of sediment would not measurably alter the distribution in that sediment. 

There are no sharp boundaries separating various lava types from each other except those of definition. There is a more or less continuous graduation from one type to another over a range of silica concentrations extending from <45% to >70% Si02- Associated with the many varieties that result from different source compositions and conditions of origin is a plethora of names that usefully connote compositional and kinship relations only to experienced geologists. The three types described above are sufiicient for this discussion of lanthanide distributions. 

Lanthanide distributions in common lavas range from somewhat depleted to tremendously enriched in light lanthanides, relative to chondrites. Concentrations of heavier lanthanides are not as variable as those of light lanthanides. Positive, negative, or no Eu anomalies may be present. There is no systematic variation in lanthanide distributions with rock type as a whole, but there are somewhat systematic variations among volcanic rocks that are genetically related. It is convenient to consider two classes of lanthanide distributions for... 

Basalts, basaltic andesites, and andesites with this distribution are common in some island arcs (e.g., Jakes and Gill, 1970 Ewart et al., 1973 Taylor et al., 1969). Their presence is believed to result from melting of subducted oceanic crust. By and large, the sediment layers which lie above the ocean floor tholeiites and are derived mainly from continental material are not subducted but piled up against continental margins in some manner that prevents their modifying significantly the trace element and isotopic abundances of oceanic crustal matter in the production of this class of island arc volcanics. Nor does ocean water severely modify the lanthanide distributions in volcanics that are extruded under... 

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