Heavy-lanthanide enrichments

Fig. 42. Lanthanide abundance patterns for accessory minerals. Note in particular the high abundances are light lanthanide enrichment of monazite and al-lanite, and the extreme heavy lanthanide enrichment of zircon. (Data are from table 24.)...

Fig. 46. Lanthanide abundance patterns for aikoses and quartzites. Note the overall lower abundances but amilar patterns to typical shales. Quartzites, with very low abundances and high heavy-mineral concentrations, can exhibit some heavy lanthanide enrichment. (Data are from table 28.)...

Heavy-lanthanide enrichments in seawater are reasonably explained by eqs. (1) and (2) wherein the extent of lanthanide solution complexation increases between La and Lu to a much greater degree than is the case for surface complexation. It is probable that competitive solution/surface complexation exerts the dominant role in heavy-lanthanide enrichments in seawater. However, it should be noted, as well, that lanthanide phosphate co-precipitation also appears to promote heavy-lanthanide enrichments in solution. Although the existence of lanthanide phosphate precipitation in seawater has not been directly demonstrated, such controls are consistent with observed total lanthanide concentrations in some environments (Byrne and Kim 1993, Johannesson et al. 1995) and are consistent, as well, with the general features of lanthanide fractionation in the oceans. 

An important observation emerging from estuarine studies is that estuarine reactions not only markedly reduce riverine lanthanide fluxes but also modify the relative abundance of dissolved lanthanides reaching the oceans. The preferential removal of light elements at low salinity creates an effective river composition (that reaching the ocean after modification in estuaries) which is evolved toward that of seawater, which has a heavy-lanthanide enriched shale pattern. This conclusion assumes that no other processes are returning the river-borne lanthanides to the oceans. 

Masuzawa and Koyama (1989) attributed the positive Ce anomalies of their sediment trap samples (Japan Sea) to a biologically-mediated oxidation process associated with the presence of Mn oxide particles. This process is discussed in detail in the subsection on Ce redox chemistry. The shale-normalized patterns of Masuzawa and Koyama (1989) do not show any consistent form from which to draw conclusions about lanthanide(lll) fractionation. Only their 2750 m sample is slightly light-element enriched the other four samples have flat or heavy-enriched patterns. Sediment trap particles from the eastern equatorial Pacific Ocean (Murphy and Dymond 1984) are strikingly different in that their shale-normalized patterns are like those of seawater heavy-lanthanide enrichment and negative Ce anomalies. 

The lanthanide abundance pattern for Shergotty (table 9, fig. 7) is unique among meteorites and indicates a complex prehistory in the Martian mantle from which they appear to have been derived, as basalts, by partial melting. The enriched pattern of the heavy lanthanides (Gd-Lu) resembles that of pyroxenes (the parent rocks appear to have been pyroxene cumulates). It provides no evidence that garnet was a residual phase in the source from which these basalts were derived, for, if so, the reciprocal pattern would be displayed. Leaching experiments show that most of the lanthanides are contained in accessory phases (whitlockite and apatite) rather than in the major mineral phases. 

In this section, we discuss the question of the bulk planetary abundances of the rare earth elements. Central to the problem of planetary abundance determinations is the assumption that the composition of the original solar nebula, for the non-gaseous elements, is given by the composition of the Cl meteorites. It is accordingly of interest to see what evidence is available from the planets, and how it relates to the primordial nebula values. In the previous section, we have seen that although the moon is enriched in the lanthanides relative to those in the primordial solar nebula by about 2.5 times, the pattern is probably parallel to that of Cl. The evidence for an apparent depletion in the heavy lanthanides is readily explicable as a consequence of early lunar magma ocean crystallisation of phases such as olivine and orthopyroxene, which selectively accept Gd-Lu. 

Garnet has a unique lanthanide abundance pattern among the common rockforming minerals, concentrating the heavy lanthanides (Gd-Lu). Melts which have been in equilibrium with this mineral thus possess eomplementary patterns steeply enriched in the light lanthanides (La-Sm), and depleted in the heavy lanthanides (Gd-Lu). These patterns usually indicate that they have been derived from the mantle, where eclpgite is stable, rather than from the crust. 

Fig. 32. PAAS-normalized lanthanide patterns for early Proterozoic uranium deposits from the Pine Creek Geosyneline, Australia (data are from McLennan and Taylor 1979). Compared to the unmineralized host sedimentary roek, these deposits are extremely enriched in heavy lanthanides and depleted in light lanthanides. The abundances and amount of fractionation is related to the U content, suggesting the lanthanide mobility and U mineralization are directly related, thus helping to constrain the origin of the deposit. The fractionation of Sm and Nd during ore-formation allows dating by the Sm-Nd method.

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. 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.

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. 

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). 

The cause of very strong relative enrichment in light lanthanides for volcanic liquids, as will be discussed later, is often ascribed to the presence of the mineral garnet in the source region. Garnet has a high affinity for heavy lanthanides but a... 

Fra.ctiona.1 Precipituition. A preliminary enrichment of certain lanthanides can be carried out by selective precipitation of the hydroxides or double salts. The lighter lanthanides (La, Ce, Pr, Nd, Sm) do not easily form soluble double sulfates, whereas those of the heavier lanthanides (Ho, Er, Tm, Yb, Lu) and yttrium are soluble. Generally, the use of this method has been confined to cmde separation of the rare-earth mixture into three groups light, medium, and heavy. 

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... 

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