Neodymium, abundance

Silicate is depleted from surface waters by biological processes and remineralized in the deep water. Moreover, silicate tends to accumulate in water masses as they age (cf. Broecker and Peng, 1982). These processes account for both the increasing concentration of silicate with depth, and its increasing concentrations from the North Atlantic to the circum-Antarctic to the Pacific. With a few exceptions (notably the North Atlantic), neodymium abundances show a smooth increase with depth, and in deep water they are highest in the Pacific, lowest in the Atlantic, and intermediate in the Indian (Figure 6). 

Mg, Mn and Ca in garnets with a Jarrell-Ash laser microprobe, using ruby and neodymium lasers have shown that reliable data on the abundance of the different elements can be obtained in a very short time. 

ISOTOPES There are 47 isotopes of neodymium, seven of which are considered stable. Together the stable isotopes make up the total abundance in the Earth s crust. Two of these are radioactive but have such long half-lives that they are considered stable because they still exist on Earth. They are Nd-144 (half-life of 2.29x10+ years) and Nd-150 (half-life of 6.8x10+ years). All the other isotopes are synthetic and have half-lives ranging from 300 nanoseconds to 3.37 days. 

Neodymium is the third most abundant rare-earth element in the Earths crust (24 ppm). It is reactive with moist air and tarnishes in dry air, forming a coating of Nd O, an oxide with a blue tinge that flakes away, leaving bare metal that then will continue to oxidi2e. 

Although neodymium is the 28th most abundant element on Earth, it is third in abundance of all the rare-earths. It is found in monazite, bastnasite, and allanite ores, where it is removed by heating with sulfuric acid (H SO ). Its main ore is monazite sand, which is a mixture of Ce, La, Th, Nd, Y, and small amounts of other rare-earths. Some monazite sands are composed of over 50% rare-earths by weight. Like most rare-earths, neodymium can be separated from other rare-earths by the ion-exchange process. 

Neodymium occurs in nature in the minerals hastnasite, monazite, cerite and allanite. The element always is associated with other rare earths, especially cerium group elements. Its abundance in the earth s crust is about 0.0024%. 

Isotope abundances which are free from all sources of bias are defined as absolute isotope abundances. The absolute isotope composition of elements can be measured by MC-TIMS and MC-ICP-MS via gravimetric synthetic mixtures or standard solutions from highly enriched isotopes, as demonstrated for neodymium,11 erbium13 and samarium,11 13 99 respectively. 

Fig. 1. Dry lake mineral bed near Mountain Pass, California contains over one million pounds of neodymium and nearly one-half million pounds of praseodymium, bo til elements once regarded as rare earths" and of limited scientific curiosity. During recent years, die rare earths have become significant materials in the electronic, chemical, metallurgical, glass, cryogenic, nuclear, and ceramic refractory industries. Lanthanum, another rare-earth element, is more abundant than lead...

Figure 9.3 The relative abundance of the rare earth elements according to Goldschmidt and Thomassen (above) and Ida Noddack (below[57]). The Noddacks found a much higher abundance of neodymium than the Norwegian researchers, but their data were not generally accepted. Both diagrams illustrate the Oddo-Harkins rule very clearly.

Chondritic relative abundances of strongly incompatible RLEs (lanthanum, niobium, tantalum, uranium, thorium) and their ratios to compatible RLEs in the Earth s mantle are more difficult to test. The smooth and complementary patterns of REEs in the continental crust and the residual depleted mantle are consistent with a bulk REE pattern that is flat, i.e., unfractionated when normalized to chondritic abundances. As mentioned earlier, the isotopic compositions of neodymium and hafnium are consistent with chondritic Sm/Nd and Lu/Hf ratios for bulk Earth. Most authors, however, assume that RLEs occur in chondritic relative abundances in the Earth s mantle. However, the uncertainties of RLE ratios in Cl-meteorites do exceed 10% in some cases (see Table 4) and the uncertainties of the corresponding ratios in the Earth are in same range (Jochum et ai, 1989 W eyer et ai, 2002). Minor differences (even in the percent range) in RLE ratios between the Earth and chondritic meteorites cannot be excluded, with the apparent exception of Sm/Nd and Lu/Hf ratios (Blicher-Toft and Albarede, 1997). 

Most apatites are chlorine-rich (2-4.3%) with low F/Cl (0.1 -0.3). High fluorine ( 5%) and low chlorine (—0.25%) have been reported for apatite in spinel Iherzolites from Pacific OIB (Hauri et al., 1993). Extremely high strontium contents, commonly >2X10 ppm and up to 7 wt.% (Ionov et al., 1997 Table 9) are common in mantle apatites meaning that this phase is a major repository for strontium when present in peridotites at abundances of 0.1% or above. Rb/Sr is very low. Apatites have high levels of REE and are LREE-enriched (Table 9). Lanthanum and cerium concentrations can reach >1 wt.% and neodymium concentrations can be above 1,000 ppm. Sm/Nd is below PUM. HFSE are low and so the presence of this phase does not affect bulk rock HFSE chemistry. 

The problem of crustal contamination is particularly acute for low mg continental flood basalts and smaller volume continental tholeiitic basalts, both of which have low trace-element concentrations (see Sections 3.03.3.2.3 and 3.03.3.3). The issue is less critical for many smaller volume continental rocks, such as kimberlites and alkali basalts, which have much higher abundances of many trace elements. As a result of their high strontium and neodymium content, for example, the isotopic compositions of these elements in kimberlites and alkali basalts are relatively insensitive to modification during crustal contamination. Conversely, the osmium and lead concentration of basaltic magmas are so low that these isotope systems are particularly vulnerable to modification by interaction with cmstal rocks (McBride et al, 2001 Chesley et al, 2002) hence these systems provide relatively sensitive indicators of crustal assimilation. 

The abundant crust that was produced in the 1.9-1.7 Ga interval has been the subject of numerous studies, and the evolution of that work illustrates important elements in study of the continental crust. Following the demonstration of a juvenile origin for the —1.8 Ga crustal assemblage in Colorado (DePaolo, 1981), there followed a period in which neodymium isotopic data were gathered for numerous terrains in the northern continents (Figure 4). In North America, studies by Nelson and DePaolo (1984, 1985), Bennett and DePaolo (1987),... 

Isotopic analyses of Altaid-collage rocks are not as abundant as those in the Canadian Cordillera, but have appeared in increasing numbers since about 1996. Kovalenko et al. (1999, 2003) presented neodymium isotopic data on juvenile Altaid rocks in Russia and Mongolia, while Wu et al. (2000), Jahn et al. (2000) and Chen and Jahn (2002) made extensive studies of... 

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