Lanthanide abundances chondrites

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. 

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 best estimate of primordial lanthanide abundances, as discussed in section 4.1, is given by those in the primitive Cl class of carbonaceous chondrites. When normalised to an element such as Si, they closely match the abundances in the sun. 

Cl meteorites contain about one-third water and other volatile elements. For these reasons, we use volatile-free lanthanide abundances for normalisation. Such values are similar to those in H and L ordinary chondrites, used by many workers previously. Accordingly, the use of volatile-free Cl lanthanide abundances for normalisation does not introduce a substantial variation from previous practices, but has a much better cosmochemical justification. The values selected are given in table 4. Since there is no apparent relative fractionation of the lanthanides during planetary accretion processes, these abundances are generally taken to be simply related to bulk earth compositions. Accordingly, their use as normalising abundances for terrestrial samples provides a measure of the degree of fractionation from the primitive terrestrial abundance patterns. 

The latest review of meteorite classifications and properties is given by Wasson (1985). A simplified classification scheme is given in table 8. Bulk lanthanide abundances in chondritic meteorites do not indicate any major cosmochemical fractionation during their formation. Lanthanide patterns in chondrites are relatively uniform, and bulk compositions show no dependence on volatility. No Eu or Yb anomalies are apparent. This information indicates that extreme temperatures were not experienced during their formation. Loss of more volatile elements was common among the H, L, LL, C2, and C3 classes, whereas E chondrites, with fully reduced Fe, have their full complement of volatile elements. 

Fig. IZ Simplified internal structure of the moon, showing the mineralogically zoned source regions from which the mare basalts were derived, the feldspar-rich crust, and KREEP. Lanthanide abundance patterns for these various regions are depicted on the right with the approximate concentrations relative to average chondrites indicated. The lunar interior is undoubtedly more complex both vertically and laterally than depicted.

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... 

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. 

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.

The close similarity of the lanthanide distributions in ocean floor volcanics to that of the chondrites is further evidence that Earth has the same overall average relative lanthanide abundances as the chondrites. Otherwise, the uniformity of these melt products of the mantle found in all the oceans of the world would seem to be fortuitous. The distribution is not unmodified from that of the chondrites, and the lavas are not primitive or first-generation melting products of a primitive terrestrial mantle. Variations in lanthanide concentrations and relative abundances among ocean floor basalts are mainly the result of minor inhomogeneities in the mantle source regions, small differences in conditions of partial melting, and crystal fractionation of the lavas prior to eruption. 

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 ... 

Anodier stone that has found wide cultural use as a carving medium in many early societies is steatite or soapstone, a very soft metamorphic rock related to chlorite and talc. In this laboratory, steatite (actually chlorite) from quarries near Tepe Yahya (Iran) was characterized by observation of the ratios of the relative intensities of basal-plane x-ray diffraction peaks after it was found that NAA-determined trace element concentrations varied wildly within a given specimen. Another technique that has been used involves the determination by NAA of a number of lanthanide elements (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Ho, Er, Tm, Yb and Lu) and the taxonomy of their abundances relative to each other — in other words, true pattern recognition , when plotted as ratios to the levels of the same elements in a standard reference chondrite Although this technique found successful provenience application... 

Various normalising procedures have been adopted to overcome the complexity induced by the Oddo-Harkins effect in comparing data. These have included plotting odd- and even-numbered elements separately, normalising to La, or Yb, and so on. All these methods are of historical interest only, and the Coryell—Masuda (Masuda, 1962, Coryell et al. 1963) method of normalising the lanthanide data is now universally used. This method consists of forming a ratio of one lanthanide pattern to another, commonly a chondritic abundance pattern (fig. 2). This procedure has several advantages ... 

The second source of information about the early solar nebula comes from the primitive meteorites, which provide ages of 4.55 Ae. Although many classes of meteorites show elemental fractionations, the Type 1 carbonaceous chondrites (or Cl, where I = Ivuna, the type example of this class of meteorites) have a composition (excluding the volatile elements by which are meant in this context H, C, N, O and the rare gases) which is close to that of the relative abundances, normalised to Si, derived from the solar spectra. Lanthanide data are given in table 6. A comparison of the solar and meteoritic data is plotted in fig. 3 which shows the close correspondence between the two sets. This similarity is one of the pieces of evidence that we are dealing with the overall composition of the original solar... 

The lanthanide patterns, normalised to the chondritic abundances, are shown in figs. 8 and 9 and the data are listed in tables 10 and 11. Several interesting features emerge. The most informative is the depletion in europium which is discussed in the next section. Secondly, the abundance patterns are broadly parallel to one another, but with some differences in detail. The generally parallel nature of the total patterns (excluding Eu) over more than an order of magnitude is attributed to two causes. 

Sm-Nd isotopic systematics are compatible with Cl chondritic meteorite ratios for the bulk earth. Use of this assumption is a valuable asset in establishing other geochemical abundances. Since both Sm and Nd are relatively refractory, it is a reasonable assumption to take their bulk earth ratios as chondritic. When Nd isotopic ratios are plotted against Sr isotopic ratios (fig. 13), the points from mantle derived samples fall along a narrow zone, the so-called mantle array. The intersection of this zone with the Sm/Nd chondritic ratio provides an estimate of the bulk terrestrial Rb/Sr ratio (=0.03). Since Sr is refractory, the Sr/lanthanide ratios... 

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. 

Of all the materials sampled in the laboratory, the class of meteorites called chondrites is believed to come the closest to retaining the nonvolatile elements of the solar system in their primitive relative abundances. If the processes that formed those meteorites did not appreciably fractionate the nonvolatile elements, then surely they did not separate yttrium and the members of the lanthanide series from each other. Thus, from analyses of chondritic meteorites, the relative elemental abundances of Y and the lanthanides in the solar system are known to a high degree of confidence. 

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. 

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