Lanthanides shale

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. 

Shales -lanthanides m [DANTHANIDES] (Vol 14) -lithium m [LITHIUM AND LITHIUM COMPOUNDS] (Vol 15)... 

Lanthanides are constituents of many different minerals in igneous rocks, shale, and silicates however, the two major sources for commercial production are bastnasite (a flu-orocarbonate) and monazite (a phosphate)... 

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... 

When REE fractionation is discussed, it is common to normalize the data to the values in shale which are thought to be representative of the REEs in the upper continental crust. The shale-normalization not only helps to eliminate the well-known distinctive even-odd variation in natural abundance (the Oddo-Har-kins effect) of REEs but also visualizes, to a first approximation, fractionation relative to the continental source. It should be noted, however, that different shale values in the literature have been employed for normalization, together with the ones of the Post-Archean Australian Sedimentary rocks (PAAS) adopted here (Table 1). Thus, caution must be paid on the choice of the shale values if one ought to interpret small anomalies at the strictly trivalent lanthanides such as Gd and Tb. Alternatively, for detailed arguments concerning fractionation between different water masses in the ocean, it has been recommended that the data are normalized relative to the REE values of a distinctive reference water mass, for example, the North Pacific Deep Water (NPDW, Table 1). The NPDW-normalization eliminates the common features of seawater that appeared in the shale-normalized REE pattern and can single out fractionation relative to the REEs in the dissolved end product in the route of the global ocean circulation. 

Fig. 2. (a) Raw lanthanide abundance data for Australian shales and Cl chondritic meteorites, showing the inherently higher concentrations of even-numbered elements (the Oddo—Harkins effect, due to the greater stability of even-numbered nuclides), (b) The lanthanide pattern resulting from normalising the Australian shale abundance data to the Cl chondritic values. This normalisation illustrates both the relative abundance and fractionation of the lanthanides compared to values typical of the primordial solar nebula. (Data are from table 4.) ... 

The second common lanthanide abundance pattern which is uniform and which has widespread geochemical significance, is that observed in most post-Archean sedimentary rocks such as shales. This pattern, as discussed later, is generally taken to represent that of the upper continental crust exposed to weathering and erosion, so that it forms a suitable base for comparison of terrestrial surface processes affecting the lanthanides. Two different sets of shale abundances have been used for normalisation. The first is the North American Shale Composite (NASC, Haskin et... 

Figure 38 shows lanthanide abundance patterns for iron formations and for young iron-rich sediments from Cyprus. These patterns are normalised to the average contemporaneous sediment, reflecting the terrigenous sources of seawater lanthanides [PAAS is used for Post-Archean iron formations average Archean shale... 

Fig. 38. Lanthanide abundance patterns for selected iron formations and iron-rich sedimentary rocks. Data are normalized to average shale values of the same period, Archean iron formation being normalized to average Archean shale (McLennan and Taylor 1984) and the others normalized to PAAS. In detail, iron formations exhibit considerable variability in lanthanide patterns. These samples illustrate the general feature of Eu enrichment, relative to contemporaneous upper continental crust, for Archean and early Proterozoic iron formations. The younger examples display no such Eu enrichment. This feature has been used to suggest that early Precambrian seawater was dominated by a hydrothermal signature, enriched in Eu (see fig. 30). (Data are from table 22.)...

The other major example of a relatively uniform lanthanide abundance pattern, in addition to that observed in chondritic meteorites, is found in most terrigenous sedimentary rocks, notably shales (table 23). This pattern (fig. 39) is characterised by light-lanthanide enrichment, a pronounced depletion in Eu (Eu/Eu = 0.66) and for the heavy lanthanides, abundances parallel to, and about ten times those of... 

Fig. 41. Lanthanide patterns for various grain size fractions of an unconsolidated sediment. Data normalized to the <2 pm fraction, the closest approximation to a shale derived from the same source. (Data are from table 24.)...

Taylor and McLennan (1981) suggested that while lanthanide patterns in finegrained sedimentary rocks were parallel to upper crustal abundances, they probably overestimated the absolute abundances by about 20%. Mass balance calculations involving averages of the various sedimentary rock types (shales, sandstones, carbonates, evaporites) substantiate this adjustment (Taylor and McLennan 1985). 

Haskin and Gehl (1962) were the first to notice unusual lanthanide patterns for Precambrian sedimentary rocks. Haskin et al. (1968) confirmed a relative enrichment of Eu in Precambrian sedimentary rocks compared to the North American Shale Composite (NASC) sample. More detailed studies by Wildeman and Haskin (1973) and by Wildeman and Condie (1973) confirmed Eu enrichment and lower total lanthanides for Precambrian sedimentary rocks. Most of the samples for these latter studies came from Archean terrains. These differences in lanthanide patterns between Archean and post-Archean terrigenous sediments have become a crucial observation for models of the evolution of the continental crust. 

Fig. 44. Lanthanide abundance patterns in Australian shales ranging in geological age from mid-Proterozoic to Triassic. There is no change in the relative abundance patterns over a period of about 1.5 billion years. (See table 26 for sample details.)...

Australian shales dating back to the mid-Proterozoic (data are from table 26). These patterns, which are representative of the data base for the Australian average shale (PAAS), are similar to those of composite shale samples from Europe (ES) and North America (NASC). All these patterns are characterised by light-lanthanide enrichment and relatively flat heavy lanthanides (at about 10 times chondritic), and a rather uniform depletion in Eu (Eu/Eu =0.65). This uniformity both within and between continents is interpreted to represent the lanthanide abundances in the upper continental crust exposed to weathering. 

The lanthanide patterns for many modem sediments are similar to that of the post-Archean shales. The chemical composition of suspended particulate matter in some of the world s major rivers was examined by Martin and Meybeck (1979) (table 27, fig. 45). The patterns are very similar to PAAS although slightly enriched in total lanthanides (about 1.1-1.4 times). The cause of the enrichment is probably related to grain size. 

Coarser-grained sedimentary rocks such as arkoses typically have lanthanide patterns which are parallel to those of shales. Chondrite-normalised plots are given in fig. 46 (table 28). The patterns of these sandstones tend to have lower total abundances than shales, although, like shales, the values are quite variable. A number of authors have noted the lower abundances in coarser-grained sedimentary rocks as compared to shales (Haskin et al. 1966b, Nance and Taylor 1976, Culler et al. 1979). On the other hand, the overall shape of the patterns (Eu/Eu ", LaN/Yb, etc.) is generally similar for such sandstones and shales. 

The lanthanide abundances in quartz-rich sedimentary rocks (quartzites, orthoquartzites, etc.) are typically very low (table 28, fig. 46). The shape of the pattern, however, is similar to that of typical shales. As discussed above, the role of heavy minerals is more important when sizeable clay fractions are absent (Cullers et al. 1979, Taylor et al. 1986, see also below). The most common effect is to cause enrichment of the heavy lanthanides (Gd-Lu). 

Fig. 46. Lanthanide abundance patterns for aikoses and quartzites. Note the overall lower abundances but amilar patterns to typical shales. Quartzites, with very low abundances and high heavy-mineral concentrations, can exhibit some heavy lanthanide enrichment. (Data are from table 28.)...

Fig. 21.1. Concentrations of lanthanides and yttrium in a composite sample of 9 chondritic meteorites (Haskin et al., 1%8) are plotted against lanthanide atomic number in the lowest part of the figure. Relative lanthanide abundances for the solar atmosphere (Ross and Aller, 1976) and lanthanide concentrations for a composite of 40 North American shales (Haskin et al., 1968) are compared with the chondritic abundances in the middle and upper parts of the figure by plotting ratios of their lanthanide concentrations to those of the chondrites. Such comparison diagrams are used throughout this chapter.

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 ... 

Ronov et al. (1972, 1974) broadened the study to include the Russian and Scythian platforms (representative of stable, continental regions), the mio-geosyncline of the high Caucasus mountains, the mountains themselves, and the seaward eugeosyncline. The lanthanide concentrations in clays (shales) increased in a regular manner from the eugeosynclinal sediments to the Russian Platform a similar trend was observed for carbonates (fig. 21.3). The opposite... 

There are substantial difficulties with this explanation. The island arc volcanics in question are relatively deficient in light lanthanides compared with the NASC and the Precambrian sediments. No combination of their distribution and that of Eu-deficient crustal material can produce the NASC-like distribution with increased Eu. Also, the sediments showing Eu anomalies include well differentiated shales, sands, and carbonates, probably not of eugeosynclinal origin. Finally, several of the Precambrian sediments had relative Eu abundances greater than that of the chondrites and, therefore, the island arc basalts. 

Fig. 1. (a) The concentration of dissolved lanthanides in the surface waters of the Sargasso Sea. A composite of data measured by TIMS (Sholkovitz and Schneider 1991) and INAA (De Baar et al. 1983). Note classic sawtooth abundance pattern. Pm does not exist in nature, (b) Shale-normalized pattern of the composite seawater shown in (a) using shale concentrations of table 1. Tb, being inconsistent, probably reflects an incorrect concentration of the seawater. 

Fractionation of the lanthanides is a major theme of the literature and hence this article. Fractionation in natural waters is often quantified by computing shale-normalized ratios... 

Lanthanide distribution models (Elderfield 1988, Byrne and Kim 1993, Erel and Morgan 1991, Erel and Stolper 1993) provide a basis for the comparative shale-normalized lanthanide concentrations (lanthanide fractionations) which are observed in the oceans. Lanthanide fractionation models are formulated in terms of competitive complexation equilibria involving, on one hand, solution complexation, and on the other, surface complexation on marine particles. Following the developments of Elderfield (1988) and Byrne and Kim (1993), shale-normalized lanthanide concentrations (Mj)sn in seawater can be expressed as... 

Fig. 6a. Lanthanide concentrations in seawater normalized to North American Shale Composite (Piepgras and Jacobsen 1992) are shown for average deep water and average surface water.

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