Lanthanide abundances patterns

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 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. 

Fig. 9. Lanthanide abundance patterns for high-Ti, high-K and high-Al lunar basalts, again showing the general depletion in Eu, which reflects the pattern in the source regions at depths of 150-400km within the moon. (Data are from table 11.)...

Fig. 10. The wide variety of lanthanide abundance patterns observed in lunar highlands samples ranging from extreme enrichment in Eu in feldspathic rocks (anorthosites, 61016, 15455) to massive depletion of Eu in KREEP (65015), which represents the final residual liquid from the crystallisation of the magma ocean. Sample 68415 is a granulitic breccia close in composition to that of the average highland crust. (Data are from table 12).

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.

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. 23. Lanthanide abundance patterns for Mid-Ocean Ridge Basalts (MORE) and Ocean-Island Basalts (OIB) (data are from table 16). Note the depletion in the light lanthanides (La-Sm) in MORE derived from a depleted mtmtle source, and the enriehment of light lanthanides in OIB. Nd isotopic evidence indicates that the source of both these rock types was characterized by long term depletion of Nd relative to Sm, indicating the observed light lanthanide enrichment in OIB is a recent event.

Fig. 27, Lanthanide abundance patterns for typical Na-rich plutonic rocks (tonalites and trondhjemites) The enrichment of Eu in the trondhjemite is a not uncommon feature of the more differentiated types.

Fig. 28. Typical lanthanide abundance patterns for K-rich granites and rhyolites (data are from table 18). Note the development of extreme depletion in Eu in the more highly fractionated granites.

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. 36. Lanthanide abundance patterns of selected marine sediments. Most phases which have precipitated from seawater show the characteristic depletion in Ce. (Data are from table 21.)...

Figure 38 shows lanthanide abundance patterns for iron formations and for young iron-rich sediments from Cyprus. These patterns are normalised to the average contemporaneous sediment, reflecting the terrigenous sources of seawater lanthanides [PAAS is used for Post-Archean iron formations average Archean shale... 

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.)...

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... 

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.)...

Variations both or grain size and mineral density result in separation of minerals and rock fragments during aqueous and aeolian transport of sedimentary material. Such transport may affect lanthanide abundance patterns in the resulting sedimentary rock because of the widely variable patterns in the constituent minerals. The two most important effects are... 

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.)...

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

Fig. 44. Lanthanide abundance patterns in Australian shales ranging in geological age from mid-Proterozoic to Triassic. There is no change in the relative abundance patterns over a period of about 1.5 billion years. (See table 26 for sample details.)...

Fig. 45. Lanthanide abundance patterns in particulate matter from rivers. Most samples have a pattern similar to PAAS. (Data are from table 27.)...

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.)...

Fig. 48. Lanthanide abundance patterns for Phanerozoic greywackes from recycled orogen (MK84, MK86) and continental block (P39803, MK97) provenances. These varieties of greywackes exhibit distinct negative Eu anomalies. (Data are from table 29.)...

Fig. 50. Both steep (KH21, YKl) and flatter (C-3, 8781) lanthanide abundance patterns are observed in Archean sedimentary rocks, and reflect derivation from felsic and basic igneous rocks, and lend weight to the suggestion that a tamodal mixing model best explains the provenance of Archean sedimentary rocks in greenstone belts. (Data are from table 30.)...

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 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... 

Fig. 52. Lanthanide abundance patterns for the post-Archean upper, lower and bulk erusts (data from table 32). The upper erust is depleted in Eu, and the lower crust enriched in Eu relative to the bulk crust.

Fig. 53. Lanthanide abundance patterns for the Archean upper and bulk crusts. In contrast to the post-Archean upper crust, there is no depletion in Eu, suggesting that K-rich granites were only a minor component in the Archean upper crust, compared with their later dominance. (Data are from table 3Z)...

The lanthanide abundance patterns provide unequivocal evidence of a terrestrial sedimentary parent material for tektites. This conclusion is reinforced by the Sm-Nd isotopic systematics (Shaw and Wasserburg 1982) which demonstrate an origin for the various tektite groups from terrestrial crustal source material. Many other isotopic and chemical (e.g. a negative correlation between Si02 and K2O) parameters reinforce this conclusion and all the evidence points unequivocally to an origin for tektites by meteoritic, cometary or asteroidal impact on terrestrial sedimentary rocks. 

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. 

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