Ocean lanthanides

Lanthanide distribution models (Elderfield 1988, Byrne and Kim 1993, Erel and Morgan 1991, Erel and Stolper 1993) provide a basis for the comparative shale-normalized lanthanide concentrations (lanthanide fractionations) which are observed in the oceans. Lanthanide fractionation models are formulated in terms of competitive complexation equilibria involving, on one hand, solution complexation, and on the other, surface complexation on marine particles. Following the developments of Elderfield (1988) and Byrne and Kim (1993), shale-normalized lanthanide concentrations (Mj)sn in seawater can be expressed as... 

A comparison of profiles from one station in the western North Pacific (Piepgras and Jacobsen 1992) and one station from the South Atlantic Ocean (German et al. 1995) are used below to introduce the major features (fig. 14) of oceanic lanthanide distributions. The following characteristics summarize the major features of lanthanides in the oceans ... 

Rare earths - [LANTHANIDES] (Vol 14) -ocean raw material [OCEAN RAW MATERIALS] (Vol 17)... 

Table 1 A periodic table of the elements in seawater indicating the element number, the dominant inorganic species predicted to be found in the oceans, the range of concentrations observed in the open ocean, and an estimate of the element s mean concentration. The mean concentration is shown on the left-hand column in parenthesis and on the right-hand column as a thick line. Hydrogen, noble gases, lanthanides, and elements after lead are omitted.

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 ... 

D ecent development of our knowledge of lanthanide distributions in nature encouraged us to believe that variations in ratios of these elements might well characterize individual water masses as summarized by Haskin et al. (5), considerable lanthanide fractionation has occurred in the formation of the earth s crust it might be expected that these fractionations would be reflected in the lanthanide patterns of material eroded from diflFerent regions and supplied to the oceans. Since, on the other hand, the lanthanide patterns of marine shales and sediments (5, 6, 11) do not reflect these regional diflFerences but are essentially uniform on a world-wide basis, sea water should express the diflFerential residues on a... 

The total concentrations of the lanthanides in the Indian Ocean (J) are about 100 times greater than in the deep Central Atlantic Ocean, but comparable with that of surface waters of the Pacific Ocean near California (4). 

If one assumes that both cerium and yttrium are normal relative to the other lanthanides in chondrites, the present work shows that for open ocean samples cerium is depleted relative to the lanthanides in sea water by an average factor of approximately 5. On the other hand, yttrium is enriched by an average factor of 2.3. 

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. 

In 1982, the first oceanographically consistent vertical profiles were reported for nine out of ten lanthanides that could be measured by the ID-TIMS method in the North Atlantic. Since then, a significant amount of data on the distribution of REEs have accumulated from various oceanic regions. For example, Figure 2 shows the station locations where the REEs were measured in seawater, together with the... 

Our focus in this review is on the luminescence behavior of solid state lanthanide and transition metal systems over a pressure range extending up to 300 kbar. Since this magnitude of pressure is well beyond everyday experience, it is beneficial to consider how these pressures compare to those encountered in the physical world. Table 1 presents selected examples from a more comprehensive compilation presented by Jayaraman [68]. The pressures in Table 1 range from 10 bar in outer space to 10 bar at the center of the sun. The unit of pressure of relevance to this review is the kbar. From Table 1, we see that 1 kbar corresponds approximately to the pressure at the deepest point in the ocean. A pressure of 50 kbar would result if one were to invert the Eiffel tower and place it on... 

Under oxidising conditions, notably associated with deposition of iron hydroxides in the oceans, cerium is oxidised to Ce, with a consequent reduction in radius of 15% (Shannon 1976). This results in the precipitation of cerium hydroxides and phosphates in manganese nodules and a consequent dramatic depletion of Ce, compared to the other lanthanides, in seawater. This property has been used, for example, to estimate paleoredox conditions in ancient oceans. 

The lanthanides are notably insoluble under conditions existing at the surface of the earth and relatively immobile under most metamorphic conditions. They have extremely short residence times in sea water, less than the mixing time of the oceans, which is about 1000 years. Accordingly, this property is of use in tracing oceanic mixing history. 

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).

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. 

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. 30. Selected lanthanide patterns for hydrothermal solutions from oceanic and continental environments (data are from Michard and Albarede 1986). Oceanic hydrothermal solutions tend to have much higher abundances and are strongly enriched in Eu, suggesting that plagioclase is being preferentially attacked by the hydrothermal solutions. The negative Eu anomaly in the continental hydrothermal solution is quite variable, commonly being even more pronounced. This pattern is probably reOecting the average upper crustal rocks which are being affected by such solutions.

Kerrich and Fryer (1979) also used the fractionation of the lanthanides to constrain the origin of Archean hydrothermal gold deposits in the Canadian Shield. Strong enrichment in Eu occurred in stratiform carbonate beds and in quartz veins, indicative of hydrothermal processes under reducing conditions on the sea floor. The widespread occurrence of such deposits in rocks of Archean age has led to the suggestion that oceanic chemistry in that epoch was dominated by inputs from volcanic sources, rather than by inputs from continental crustal sources, as is presently the case (Fryer et al. 1979). 

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... 

There is a considerable variability in lanthanide patterns in sea water, both regionally and with depth (Elderfield and Greaves 1982, De Baar et al. 1983, 1985a). With the exception of a thin surface zone which may be dominated by aeolian input, there is a general increase of light lanthanides with depth. Total lanthanides are higher in surface waters of the Atlantic Ocean, but lower in deep waters, compared with those of the Pacific (De Baar et al. 1985a). 

Evidence from lanthanide abundances identified the source of metalliferous sediments deposited at mid-ocean ridges. They possessed a significant depletion in Ce, pointing clearly to a seawater source for the lanthanides present in the hydrothermal solutions (Piper and Graf 1974, Fleet 1984). 

Slight but significant variations (Fig. 1) in the electronic configuration of the lanthanide series result in (1) chemical anomalies that result in Ce and Ce" with respective ionic radii of 103.4 and 87 pm (2) chemical anomalies for europium which can exist as the europous cation Eu , and the europic cation Eu, with respective ionic radii of 117 and 95.0 pm. In the near biosphere Eu(II) formation does not occur but in some environments, such as oceans where it is an important aid for understanding oceanic processes [6], oxidation of Ce(III) can occur. 

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