Lanthanides absolute abundances

The lanthanide elements were originally known as the Rare Earths from their occurrence in oxide (or, in old usage, earth) mixtures. They are not rare elements, and the absolute abundances in the lithosphere are relatively high. 

The results of the analyses of sixteen sea water samples (eight in duplicate) taken on cruise No. 20 of R.V. ATLANTIS-II, Woods Hole Oceanographic Institution, are presented in this paper. Sampling positions and characterization of samples are given in Table I. The results are summarized in Table II. Although scandium is not a lanthanide, the absolute abundance of this element is also given in the table. However,... 

Information concealed in data can often be most rapidly comprehended from graphical displays. Owing to the alternation in abundance between adjacent even-Z and odd-Z elements simple plots of the type absolute abundance versus atomic number will often easily obscure small diflFerences in lanthanide distribution patterns. The method often used to remove the even-Z, odd-Z eflFect is to divide one distribution, element by element, by a known distribution, and plot the resulting ratios on a logarithmic scale against a linear scale of atomic number or ionic radius (5). If the two distributions are identical, all the ratios are the same and a horizontal line appears. Trends of diflFerences in the distributions appear as curves or sloped lines. 

Because the total lanthanide abundances of these eleven samples are different, the average distribution was found by taking the arithmetic mean of the logarithm of the absolute abundances of each element. The antilogarithms of these results are given in Table III. 

The fact that the lanthanides in marine phases reflect quite closely their pattern in seawater (but not the absolute abundances) produces a possible method of examining secular variations of lanthanide patterns in seawater. This aspect has not been investigated in any detail, with the exception of some studies of lanthanide patterns in apatite (Wright-Clark et al. 1984) and in iron formations (see section 8.4). 

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). 

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. 

Neutron activation analyses of sixteen samples of sea xmter (eight in duplicate) taken at six widely spaced stations in the Central Atlantic Ocean between 16° N and Equator (depths below 1000 m,) showed that the lanthanide patterns are relatively conservative characteristics of water masses. The differences in lanthanide distribution and total abundance between different water masses are small but significant. The absolute mass abundances of the lanthanides can be illustrated by the following values for North Atlantic Deep Water ... 

As the first step in the analysis of our data, we divided each of the twenty-four distributions, element by element, by the average absolute mass abundance of the lanthanides in twenty chondrites (5) and plotted the ratios on a logarithmic scale against a linear scale of atomic number. A visual inspection of the normalized patterns indicated that at least eleven distributions, apart from diflFerences in total lanthanide abundance, were identical nearly all the other patterns diflFered only slightly from the eleven samples mentioned. The normalized pattern for one sample is shown in Figure 2. Obviously, the distributions of lanthanides in sea water are quite different from the pattern in chondrites. 

Only one detailed study has systematically examined the effects of diagenesis on lanthanide distributions. Chaudhuri and Cullers (1979) analysed Miocene/Pliocene sediments from a deep well (1.8-4.8km depth) in the Gulf Coast of Louisiana, which was sampled through the illite/montmorillonite mixed layer of illite transition. Such variability as was seen in absolute lanthanide abundances and in La/Yb ratios was correlated to changes in provenance rather than to diagenetic factors. 

This article has two broad divisions, reflecting the research directions of its authors. One division covers elemental distributions and physical forms of lanthanides in rivers, estuaries and oceans. The emphasis here is on lanthanide abundances in space and time and the general processes responsible for absolute and relative lanthanide concentrations. The second division centers on the aquatic chemistry of lanthanides. This section s emphasis is physical chemistry and covers solution complexation, surface complexation (sorption), and the coupling which exists between physical chemistry and lanthanide distributions. 

In summary, the lanthanides undergo an array of reactions in estuaries which affect both their absolute and relative lanthanide abundances. Hence, estuaries are excellent natural laboratories to test and study the influence of aquatic geochemical processes on lanthanide fractionation. To what extent does fractionation in estuaries control the composition of seawater Is the release of dissolved lanthanides from estuarine shelf sediments a quantitatively important process with respect to the ocean budget and composition Answers to these and other related questions will require more detailed study and modeling. 

The increase in concentration between oxic and anoxic waters is accompanied by large scale fractionation of the trivalent-only lanthanides. Although these elements have no redox chemistry of their own, their absolute and relative abundance are altered by variations in redox conditions, in particular the cycling of iron and manganese. 

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