Chondrites lithophile elements

Figure 4 Cl-normalized lithophile element ratios in acapulcoite-lodranite clan (ale), winonaite-IAB-iron silicate inclusion clan (wic) and some unique meteorites showing the effects of melting in the silicate-oxide system. Residual silicate source regions will have low Na/Sc and Sm/Sc, while mafic melts will have the opposite characteristics. Many acapulcoite-lodranite clan and winonaite-IAB-iron silicate inclusion clan meteorites have essentially chondritic lithophile element ratios. Data from sources listed in the text, except for Divnoe (Petaev et al., 1994 Weigel et al., 1997), and Enon (Kallemeyn and Wasson, 1985).

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. 

In Figure 3, aluminum is representative of refractory elements in general and the Al/Si ratios indicate the size of the refractory component relative to the major fraction of the meteorite. It is clear from this figure that the Al/Si ratio of Cl meteorites agrees best with the solar ratio, although the ratios in CM (Type 2 carbonaceous chondrites) and even OC (ordinary chondrites) are almost within the error bar of the solar ratio. The errors of the meteorite ratios are below 10%, in many cases below 5%. A very similar pattern as for aluminum would be obtained for other refractory elements (calcium, titanium, scandium, REEs, etc.), as ratios among refractory elements in meteorites are constant in all classes of chondritic meteorites, at least within —5-10%. The average Sun/CI meteorite ratio of 19 refractory lithophile elements (Al, Ca, Ti, V, Sr, Y, Zr, Nb, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Er, Lu, see Table 2) is... 

Elemental abundance patterns for ordinary, Rumuruti-like (R), and Kakangari-like (K) chondrites are fairly flat and enriched relative to Cl for lithophile and refractory lithophile elements. Enstatite chondrites have the lowest abundance of refractory lithophile elements. 

Figure 3 Mean abundances of lithophile elements normalized to Cl chondrites and silicon arranged in order of increasing volatility in seven chondrite groups (Wasson and Kallemeyn, 1988). Refractories (elements condensing above V) are uniformly enriched in CO, CM, and CV chondrites and depleted in H, L,and EH chondrites. Moderately volatile elements, which condense below magnesium and silicon, are all depleted relative to Cl chondrites. These fractionations are related in poorly understood ways to the formation of CAIs and chondrules (reproduced by permission of The Royal Society from Phil. Trans. Roy. Soc. London, 1988, A325, p. 539).

Figure 23 Bulk concentrations of lithophile elements normalized to Cl chondrites and silicon in skeletal-olivine and cryptocrystalline chondmles including cryptocrystalline inclusions in metallic Fe,Ni in two CBb chondrites (HH 237 and QUE 94411). Shaded regions show compositional range of chondmles in other carbonaceous chondrites, excluding the CH group chondmles that are closely related to CB chondmles. The wide range of refractory abundances in CBb chondmles ((0.02-3) X Cl levels) appears to reflect fractional condensation with skeletal-ohvine chondmles condensing at higher temperatures than cryptocrystalline chondmles. Both types are more depleted in moderately volatile elements than other carbonaceous chondrites (after Krot et ah, 2002a).

The most remarkable facts about the enstatite chondrite CAIs are that (i) they exist at all, and (ii) they are not particularly remarkable as CAIs go. If enstatite chondrite CAIs originated by condensation under the highly reducing conditions considered necessary for forming the characteristic enstatite chondrite mineralogy, refractory lithophile elements like calcium and aluminum are expected to be largely locked up in sulfides, carbides, and nitrides (e.g., Lodders and Fegley, 1995). Phases such as melilite, hibonite, and spinel should not be present. The fact that... 

Aubrites contain an unusual suite of sulfides, many containing normally lithophile elements as their major cations. Oldhamite, a minor phase in aubrites, typically occurs as grains in the matrix, but also in sulfide-rich clasts. Oldhamite is the major REE carrier in aubrites, with abundances typically 100-1,000 X Cl chondrites. REE patterns in oldhamite grains are highly variable. 

Few comprehensive bulk compositional analyses are available for silicate inclusions from HE irons, and many of them are of small samples. Netschaevo silicates have magnesium-normalized abundances of refractory, moderately volatile, and volatile lithophile elements within the ranges of ordinary chondrites. The nickel-normalized abundances of refractory and moderately volatile siderophile elements are also similar to those of ordinary chondrites. The silicates have siderophi-le/Mg ratios of (1.9-2.2)xCI chondrites, however. The silicate inclusion in Watson has Cl-normalized element/Mg ratios of —0.86 for most refractory and moderately volatile lithophile elements (Figure 2). Siderophile elements are depleted, and show increasing abundance with increasing volatility (Olsen et al., 1994) Os/Mg = 0.028 X Cl and Sb/Mg = 0.066 X Cl. 

In addition to making comparisons with chondrites, the bulk composition of the Earth also has been defined in terms of a model mixture of highly reduced, refractory material combined with a much smaller proportion of a more oxidized volatile-rich component (Wanke, 1981). These models follow on from the ideas behind earlier heterogeneous accretion models. According to these models, the Earth was formed from two components. Component A was highly reduced and free of all elements with equal or higher volatility than sodium. All other elements were in Cl relative abundance. The iron and siderophile elements were in metallic form, as was part of the silicon. Component B was oxidized and contained all elements, including those more volatile than sodium in Cl relative abundance. Iron and all siderophile and lithophile elements were mainly in the form of oxides. 

The refractory component comprises the elements with the highest condensation temperatures. There are two groups of refractory elements the refractory lithophile elements (RLEs)—aluminum, calcium, titanium, beryllium, scandium, vanadium, strontium, yttrium, zirconium, niobium, barium, REE, hafnium, tantalum, thorium, uranium, plutonium—and the refractory siderophile elements (RSEs)—molybdenum, ruthenium, rhodium, tungsten, rhenium, iridium, platinum, osmium. The refractory component accounts for —5% of the total condensible matter. Variations in refractory element abundances of bulk meteorites reflect the incorporation of variable fractions of a refractory aluminum, calcium-rich component. Ratios among refractory lithophile elements are constant in all types of chondritic meteorites, at least to within —5%. 

The two elements calcium and aluminum are RLEs. The assumption is usually made that aU RLEs are present in the primitive mantle of the Earth in chondritic proportions. Chondritic (undifferentiated) meteorites show significant variations in the absolute abundances of refractory elements but have, with few exceptions discussed below, the same relative abundances of lithophile and siderophile refractory elements. By analogy, the Earth s mantle abundances of refractory lithophile elements are assumed to occur in chondritic relative proportions in the primitive mantle, which is thus characterized by a single RLE/Mg ratio. This ratio is often normalized to the Cl-chondrite ratio and the resulting ratio, written as (RLE/Mg)N, is a measure of the concentration level of the refractory component in the Earth. A single factor of (RLE/Mg) valid for all RLEs is a basic assumption in this procedure and will be calculated from mass balance considerations. 

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