Post-Archean sedimentary rocks

Nance WB, Taylor SR (1976) Rare earth element patterns and crustal evolution—I. Australian post-Archean sedimentary rocks. Geochim Cosmochim Acta 40 1539-1551 Nishio Y, Nakai S (2002) Accurate and precise lithium isotopic determinations of igneous rock samples using multi-collector inductively coupled plasma mass spectrometry. Anal Chim Acta 456 271-281 Nishio Y, Nakai S, Hirose K, Ishii T, Sano Y (2002) Li isotopic systematics of volcanic rocks in marginal basins. Geochim Cosmochim Acta 66 A556... 

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... 

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

The composition of the Archean upper crust as revealed in the sedimentary record stands in marked contrast to that of the Post-Archean crust. A significant difference is shown by the REE patterns in the Archean sedimentary rocks, which relative to those of the Post-Archean crust, typically show no Eu anomalies and lower enrichment in the LREE. 

In modern environments such as arcs or Pacific deep-sea environments, where volcanic provenances dominate, there is ubiquitous evidence for upper crustal material with negative Eu anomalies. In contrast to the situation in Post-Archean time, the isolation of small Archean cratons enabled the survival of distinct suites of sedimentary rocks with much variation in REE patterns. 

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. 

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. 

Such post-Archean patterns are typically not observed in the Archean (see section 10.3). McLennan et al. (1979b) studied early Proterozoic (about 2.5-2.2 billion years old) sedimentary rocks from the Huronian sequence of Canada. They found a... 

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... 

Figure 10 Comparison of chondrite-normalized REE patterns for the Archean and post-Archean upper continental crust, estimated from the sedimentary rock data. The absence of a negative Eu anomaly in the Archean upper crust indicates that intracrustal differentiation processes were not widespread at that time...

Average chondrite values from Haskin et al. (1%6, 1968). 2. Average North American Shale composite Haskin et al. 1966 (NASC). 3. Post-Archean average Australian Sedimentary rock (PAAS) (Nance and Taylor, 1976). 

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