Lanthanide abundances shales

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. 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 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.

An important observation emerging from estuarine studies is that estuarine reactions not only markedly reduce riverine lanthanide fluxes but also modify the relative abundance of dissolved lanthanides reaching the oceans. The preferential removal of light elements at low salinity creates an effective river composition (that reaching the ocean after modification in estuaries) which is evolved toward that of seawater, which has a heavy-lanthanide enriched shale pattern. This conclusion assumes that no other processes are returning the river-borne lanthanides to the oceans. 

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. 

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). 

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 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... 

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 often quantified by shale-normalized patterns. Normalization to shale represents an abundance relative to that of the upper crust of the continents. A flat shale pattern for river suspended particles would indicate a composition similar to that of averaged continental crust. To study fractionation in rivers, it is also instructive to normalize the dissolved composition to that of suspended particles on the assumption that the particles better represent the solids being weathered in the watershed. 

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