Lanthanide abundances sedimentary rocks

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. 

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... 

Lanthanide abundances in natural waters are extremely low (table 19, fig. 33). This observation is well illustrated by Haskin et al. (1966b), who calculated that the entire mass of lanthanides in the oceans is equivalent to that in about a 0.2 mm thickness of sediment of the same areal extent. The lanthanide patterns of normal ocean waters are significantly enriched in the heavy lanthanides relative to the light lanthanides, when compared to terrigenous sedimentary rocks. Ocean waters are relatively depleted in Ce a reflection of preferential incorporation of this element in... 

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. 39. The uniform lanthanide abundance patterns observed in terrigenous sedimentary rocks from widely separated geographical regions (PAAS, Australia ES, Europe NASC, North America and the wind-derived sediment, loess). This illustrates the general uniformity in the composition of the upper continental crust. (Data are from table 23.)...

Variations both or grain size and mineral density result in separation of minerals and rock fragments during aqueous and aeolian transport of sedimentary material. Such transport may affect lanthanide abundance patterns in the resulting sedimentary rock because of the widely variable patterns in the constituent minerals. The two most important effects are... 

The potential of heavy minerals to distort lanthanide patterns in sedimentary rocks is well recognised (see table 25 and fig. 42 for some typical lanthanide abundances and patterns in common heavy, or accessory minerals). An example from Archean metaquartzites from the Western Gneiss Terrain, Australia is instructive (Taylor et al. 1986). Enrichments of light lanthanides were observed in... 

Fig. 43. Enrichment of light lanthanides in Arehean quartz-rich sedimentary rocks (MN, FN and MN K) from Western Australia, due to high abundance of monazite. The other sample (MN 45) has much lower silica content and does not exhibit any enrichment in light lanthanides. (Data are from Taylor et al, 1986.)...

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). 

The most remarkable feature of the lanthanide abundance patterns in post-Archean sedimentary rocks is their uniformity. Figure 44 shows patterns for... 

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. 50. Both steep (KH21, YKl) and flatter (C-3, 8781) lanthanide abundance patterns are observed in Archean sedimentary rocks, and reflect derivation from felsic and basic igneous rocks, and lend weight to the suggestion that a tamodal mixing model best explains the provenance of Archean sedimentary rocks in greenstone belts. (Data are from table 30.)...

The lanthanide patterns for loesses from widely scattered localities (North America, Europe, China) show very uniform abundances (Taylor et al. 1983), with Eu depletions essentially equivalent to those observed in clastic sedimentary rocks (table 31, fig. 51). The lanthanide data for the New Zealand loesses show slightly lower Eu depletion, reflecting their derivation from greywackes. The uniformity of the lanthanide patterns for the widely scattered loess deposits and their similarity to PAAS, ES and NASC indicates that loess is providing the same information on the composition of the upper crust as that provided by clastic sediments. In addition to providing the information that upper crustal abundances are uniform on a worldwide scale, the loess data provide another significant piece of information concerning crustal evolution. 

Lanthanide abundances for the present upper continental crust are given in table 32 and are derived from the sedimentary rock data. Lanthanide abundances for the Archean upper crust can also be estimated from the Archean sedimentary rock data, although such estimates are less securely based, on account of the inherently greater variability in lanthanide patterns found in most Archean examples. The most recent estimate from Taylor and McLennan (1985) is also given in table 32. 

The change in lanthanide abundance patterns between Archean and post-Archean terrigenous sedimentary rocks has provided a major clue to the overall evolution of the continental crust. Any crust existing before 3.8 Ae was probably destroyed by the... 

The lanthanide abundance patterns provide unequivocal evidence of a terrestrial sedimentary parent material for tektites. This conclusion is reinforced by the Sm-Nd isotopic systematics (Shaw and Wasserburg 1982) which demonstrate an origin for the various tektite groups from terrestrial crustal source material. Many other isotopic and chemical (e.g. a negative correlation between Si02 and K2O) parameters reinforce this conclusion and all the evidence points unequivocally to an origin for tektites by meteoritic, cometary or asteroidal impact on terrestrial sedimentary rocks. 

Fig. 55. (a) Lanthanide abundance patterns for tektites. Note that they parallel those of common sedimentary rocks and the upper crustal pattern consistent with derivation from such material. (Data are from table 33.) (b) Lanthanide abundance patterns for glass derived by melting of subgreywacke by meterorite impact at Henbury, Australia. No significant change in relative or absolute abundances has occurred during the melting process. 

In the discussion of sedimentary rocks it was concluded that the relative lanthanide abundances in those rocks represented the average distributions of their igneous precursors. What are the distributions in igneous rocks Igneous rocks are those that form from high-temperature melts. 

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