Lithophile element

In summary, a key aspect to the utility of U-series isotopes in the study of arc lavas is that whereas Th and Pa are observed and predicted to behave as relatively immobile high field strength elements (HFSE), Ra and (under oxidizing conditions) U behave like large ion lithophile elements (LILE) and are significantly mobilized in aqueous fluids. Fluid-wedge interaction will only serve to increase these fractionations. Just how robust the experimental partition coefficients are remains to be established by future experiments. 

Abstract strong commodity prices in the last few years have led to a remarkable renaissance of uranium exploration in Labrador, focused in a complex and geologically diverse region known as the Central Mineral Belt (CMB). Potentially economic epigenetic U deposits are mostly hosted by supracrustal rocks of Paleoproterozoic and Mesoproterozoic age, and are difficult to place in the traditional pantheon of uranium deposit types. Recent exploration work implies that structural controls are important in some examples, but the relationships between mineralization and deformation remain far from clear. Geochronological data imply at least three periods of uranium mineralization between 1900 and 1650 Ma. It seems likely that the Labrador CMB represents a region in which U (and other lithophile elements) were repeatedly and sequentially concentrated by hydrothermal processes. The current exploration boom lends impetus for systematic research studies that may ultimately lead to refined genetic models that may be applicable elsewhere. 

Uranium mineralization in the CMB is characterized by a diversity of style and host rocks, and it is naive to suppose that a single genetic model can explain all its variations. Like many metallogenic provinces in which incompatible (or lithophile ) elements are important, the CMB of Labrador seems to represent an area in which U has been repeatedly and perhaps sequentially concentrated, and... 

Schauble (2004) applied the theory of stable isotope fractionation to nontradi-tional isotope systems. He pointed out that, differences in coordination numbers among coexisting phases control isotope fractionation of cations. The hghter isotope preferentially occupies the higher coordinated site. Thus, differences in isotope composition of lithophile elements such as Mg, Ca, and Li are likely to reflect changes in coordination numbers... 

Iron oxides present in coal are generally stable for the relatively short period of time that they are exposed to combustion temperatures. Therefore, siderophile elements (e.g., Ni, Co, Mo, Pt, Pd, Au) that are incorporated within iron oxides are also expected to remain stable, and escape any significant thermal transformation reactions (Bums 2003). Similarly, lithophile elements (e.g., Ba, B, Cr, Mn, Sr, V) that are initially found in association with silicates and aluminosilicates in coal are expected to be incorporated within the glassy fraction of coal ash upon thermal transformation of their parent minerals (Bums 2003). 

The aubrites are the most reduced achondrites (Keil et al., 1989). Their silicates are essentially free of iron, and they contain minor metallic iron. A variety of unusual sulfides of calcium, chromium, manganese, titanium, and sodium - all usually lithophile elements -occur in aubrites. These unusual sulfides also characterize the highly reduced enstatite chondrites, which may have been precursors for these rocks. 

Elemental abundances in CR2 chondrites normalized to the Cl composition and plotted in order of decreasing volatility from left to right. Lithophile elements are shown with open circles, siderophile elements with black circles, and chalcophile elements with gray circles. CR2 data from Kallemeyn etal. (1994). 

Kallemeyn, G. W., Rubin, A. E., Wang, D. and Wasson, J. T. (1989) Ordinary chondrites bulk composition, classification, lithophile-element fractionations, and composition-petrographic type relationships. Geochimica et Cosmochimica Acta, 53, 2747-2767. 

As we learned earlier in Chapters 4 and 7, chondritic abundances can vary. Figures 11.9a and 11.9c show variations in lithophile elements, and Figures 11.9b and 11.9d illustrate variations in siderophile and chalcophile elements (all normalized to Cl chondrite abundances, and plotted in order of increasing volatility from left to right in each diagram) for the major classes of anhydrous meteorites. As is apparent in these... 

Compositional variations among chondrites, (a) Lithophile and (b) siderophile and chalcophile elements in ordinary (H, L, LL), enstatite (EH, EL), R, and chondrites. In (c) and (d), the same data are shown for anhydrous carbonaceous chondrite groups. Elements are plotted from left to right in order of increasing volatility. Lithophile elements are normalized to Cl chondrites and Mg, siderophile and chalcophile elements are normalized to Cl chondrites. Modified from Krot et al. (2003). 

Figure 12.17a shows lithophile element abundances, and Figure 12.17b shows sid-erophile and chalcophile element abundances in CM chondrites, normalized to Cl chondrites. Illustrated for comparison are the abundances in CO chondrites, which are the anhydrous carbonaceous chondrite group most closely allied to CM chondrites. As in other chondrites, the greatest differences are in volatile elements. The volatile and moderately volatile elements in CM chondrites are present at 50-60% of the abundances of the refractory elements. The volatile elements are primarily located in the matrix, and the matrix comprises 50-60% of CM chondrites. This implies that the matrix has essentially Cl abundances of all elements, while the chondrules and refractory inclusions have Cl relative abundances of refractory elements but are highly depleted in the volatile elements. The sloping transition in the region of moderately volatile elements indicates... 

Another example of an impact with cosmochemical consequences may be Mercury. The abnormally massive core of that planet may result from a large collision that stripped off mantle material and left a planet with non-chondritic abundances of siderophile versus lithophile elements. 

The lithophile elements are those that generally occur in silicate phases and include among others Si, Al, Ti, K, Na, Zr, Be, and Y. These would be expected to occur in coals in some combination with the silicate minerals kaolinite, illite, other clay minerals, quartz, and stable heavy detrital minerals. 

The incompatibility of certain trace elements with the solid phase results from two factors. First, the large-ion lithophile elements (LILE) such as Ba, Cs, Rb, and Sr have large ionic radii. The LILE are too large for the available ionic sites in the solid and they tend to remain in the liquid phase. A second cause of... 

Cs are more depleted in western coals. Silicon is also depleted in coal, probably because of the presence of clay minerals. Most lithophile elements (i.e., those normally associated with the earth s crust) have EF values near one, but it is interesting that the rare earth elements show slightly, but consistently higher enrichments in eastern coal. The apparent depletion of Ta is probably not real, but an artifact resulting from Wedepohl s use of too large a crustal abundance for it (14). 

Lithophilic elements such as Si and Ca that tend to be associated with rock materials on Earth. 

For example, potassium varied by a factor of almost 10. Various authors found millimeter-sized fragments of this foreign material which represents the second component in the Apollo 12 soil samples28. As the foreign component was rich in potassium, rare earth elements (REE) and phosphorus, the acronym KREEP was devised. Later many other elements were found to be enriched in KREEP, for example, U, Th, Hf, Zr, Nb, and Ta. All these elements have in common large crystal radii (large-ion lithophile elements, LIL elements). The rock type from which the KREEP fragments were derived of is norite. 

Examples of both types are shown in Figs. 13, 14, and 15. Clearly the correlation of MnO with FeO, found in the lunar samples by Laul et al. 100 is due to the fact that Mn++ (R = 0.80 A) can easily replace Fe++ (R = 0.74 A) in the two most abundant Mg, Fe silicates, pyroxene and olivine. The correlation of the LIL elements (large-ion lithophile elements) first observed in KREEP is of the second type. 

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