Lanthanide continental crust

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. 

Our major purpose is to demonstrate how the rare earths, and the lanthanides in particular, have provided unique insights into geochemistry and cosmochemistry. Many of the examples which we will use will be drawn from those aspects which have received rather less attention in previous reviews. These include the use of the rare earths to elucidate both the evolution of the moon and the composition and evolution of the continental crust of the earth. We will, however, attempt to provide some comment on all areas of geochemistry and cosmochemistry where lanthanides have added significantly to our understanding. 

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... 

Fig. 33. Lanthanide abundance patterns for selected seawater samples (data are from table 19). With the exception of some surface waters, seawater is typically depleted in Ce. Relative to the upper continental crust, seawater is also typically enriched in heavy lanthanides, although considerable variability exists, related both to depth and loeation.

Fig. 34. Lanthanide ibundance pattern for river water (data are from table 19). Note the higher abundances and lack of Ce anomaly, compared to seawater (fig. 33). In general, the pattern is nearly parallel to that of the upper continental crust.

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.)...

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.)...

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. 

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. 

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 uniform lanthanide pattern observed in crustal sediments on the earth, with its regular depletion in Eu, provides not only a key to the problem of sampling the continental crust, but also provides a crucial piece of evidence about crustal evolution. Eu is trivalent at the earth s surface, so that the observed depletion in upper crustal rocks recalls a previous history of the element as a divalent ion under more reducing conditions. It thus provides evidence that the upper crust of the earth resulted from production of granitic melts deep within the crust leading to retention of Eu in the deep crust, with a corresponding depletion at the present surface. 

Ronov et al. also give estimates of the average lanthanide concentrations for the Earth s crust. Those values are very similar to their average for all sediments. Their estimates are probably as good as any in the literature. Such estimates are quite dependent on assumptions about types and compositions of materials in the middle and lower crust, and such assumptions are model dependent. All major segments of the Earth s crust are not similar for example, the composition of the accessible igneous oceanic crust is strikingly different from that of the exposed continental crust its volume is much less than that of continental crust. 

The higher lanthanide concentrations and the light-lanthanide enrichment of average continental material relative to chondrites has been attributed to extraction of these elements from the mantle. A residue is left behind, whose average lanthanide distribution must complement that of the continental crust, assuming that the overall relative lanthanide abundances for the Earth are the same as those in the chondrites. The ocean floor igneous suite appears to be derived from that residue. Are there rocks that can plausibly be considered samples of that residue, and which have distributions deficient in light lanthanides relative to the chondrites Are there rocks that represent primitive terrestrial mantle from which the lanthanides and other incompatible elements have not yet been extracted ... 

Fig. 21.25. Comparison diagram for Precambrian rocks of northeastern Minnesota interpreted by Arth and Hanson (1975) as possibly representing the formation of continental crust from mantle-derived material. Nos. 4 and 5 are early stage tholeiitic basalts, nos. 2, 3, and 6 are intrusive and extrusive granite-related rocks of main, intermediate stages, and no. 1 is a late-stage, intrusive, granite-related rock. Intermediate and late-stage rocks may have derived from material of the approximate composition of tholeiite. but at depths where garnet was a stable residual mineral, accounting for the severe depletion of the granite-related rocks in heavy lanthanides.

It is not clear whether the terrane described by Hanson and co-workers would yield the lanthanide distribution characteristic of continental crust (the NASC distribution). A comprehensive model for the derivation of the crust and that lanthanide distribution remains to be developed. 

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. 

Nd isotopic composition measrrrement of seawater is difficult. Difficulties arise from the need to collect high-precision Nd isotope ratios on samples with only a small amount of Nd (approximately 2 ng per kg of seawater vs. 40 xg Nd per gram of continental crust). Typically, the lanthanides are extracted and pre-concentrated from 10-30 kg samples of seawater, and Nd is separated from its neighbors. The Nd/ Nd ratio is measured by thermal ionization mass spectrometry (e.g., Piepgras et al. 1979, Piepgras and Jacobsen 1988, Bertram and Elderfield 1993). 

Fig. 51. The lanthanide abundance patterns for the aeolian sediment, loess, from China, New Zealand and USA are parallel to that of average upper crust so reflecting the composition of the upper continental crustal material, (Data are from table 31.)...

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 ... 

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... 

While it is clear that granite-like rocks or mixtures of granite-like rocks and basalts can account for the lanthanide distribution of the NASC, the origin of continental material is still obscure. Some granites seem clearly to be products of extreme metamorphism of previously existing sediments. Others seem to be products of partial melting of material deep within the crust, or even the upper mantle, possibly of older volcanics. If volcanic, onto what sort of crust did the precursors to the granites extrude ... 

Acetic acid digestions of suspended particles demonstrate that removal and fractionation of lanthanides in seawater are caused by processes which result in particle surface coatings. In contrast, the detrital phases have dust and crust-like lanthanide compositions indicative of a continental source. 

There is a small and emerging literature on the isotopic systematics of Ce in the oceans. Elderfield (1992), in a short overview of this subject, raises points of concern with respect to interpreting the isotopie data. Recently, Shimizu et al. (1994) presented data on the Ce isotopic composition of the oceans. An earlier paper from the same group reported on the Ce isotopic composition of ferromanganese nodules (Amakawa et al. 1991). The La-Ce isotopic system is based on the decay of La to Ce with a half-life of 2.5 X 10 years. Hence, the application behind the Ce isotopic system is similar to that of the Nd isotopic system. For example, as with Nd and Sr isotopes, continental and ocean crust have different Ce isotopic compositions. In principle, these differences can be exploited as a new indicator of the sources of lanthanides (Ce) to seawater and... 

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