Chondrites refractory 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. 

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... 

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. 

Figure 7 shows the abundances of the four refractory lithophile elements—aluminum, calcium, scandium, and vanadium—in several groups of undilferentiated meteorites, the Earth s upper mantle and the Sun. The RLE abundances are divided by magnesium and this ratio is then normalized to the same ratio in Cl-chondrites. These (RLE/Mg)N ratios are plotted in Figure 7 (see also Figure 1). The level of refractory element abundances in bulk chondritic meteorites varies by less than a factor of 2. Carbonaceous chondrites have either Cl-chondritic or higher Al/Mg ratios (and other RLE/Mg ratios), while rumurutiites (highly oxidized chondritic meteorites), ordinary chondrites, acapulcoites, and enstatite chondrites are depleted in refractory elements. The (RLE/Mg)N ratio in the mantle of the Earth is within the range of carbonaceous chondrites.

The Lu- Hf isotopic system (half-life —37 Gyr) is, in many ways, chemically similar to Sm- Nd. In both isotopic schemes the parent and daughter elements are refractory lithophile elements, such that their relative abundances in the Earth were probably not modified during accretion, nor did they participate in core formation. Thus, as for the Sm-Nd system, the compositions of chondritic meteorites can, in principle, be used to establish bulk silicate Earth isotopic compositions and Lu/Hf ratios directly. The potential, therefore, exists for establishing a precise isotopic baseline to use for recognizing fine-scale deviations in isotopic compositions, which can then be used to reveal... 

The minor amount of manganese in the core reflects the volatility model assumed for this element. O Neill and Palme (1997) argue that manganese and sodium have similar volatilities based on the limited variation in Mn/Na ratios in chondrites (see also Chapter 2.01). However, a plot of Na/Ti versus Mn/Na in chondrites (Figure 10) shows that indeed Mn/Na varies as a function of volatility this illustration monitors volatility by comparing titanium, a refractory lithophile element, with sodium, a moderately... 

The Moon also has an unusually low iron content. Typical inner solar system objects contain about 30% iron - Cl chondrites, for example contain about 36% iron by mass -whereas the Moon contains only 13 wt%. Some of this iron is probably located in the lunar core, although this will only account for a small amount of iron, since the lunar core is small (3-4% of its mass) compared to that of the Earth. In contrast the more refractory lithophile elements such as Al are more abundant on the Moon given the thick aluminous lunar crust (Taylor, 1987). 

A chondritic model for the Earth s mantle is usually based upon the composition of Cl chondrites, adjusted for the loss of volatile elements and for the separation of the siderophile elements into the core. This leads to a mantle composition which is enriched in refractory lithophile elements by about 1.5 times the Cl chondrite value. There are however difficulties with the chondritic model because Cl chondrites and the Earth have different Mg/Si ratios (Fig. 3.9), as was discussed in Chapter 2 (Section 2.4.4). Some authors believe that this difference is original and dates from processes within the solar nebula indicating different evolutionary histories between the two. If this is true then Cl chondrites are not an appropriate starting composition for the composition of the bulk Earth and alternative models have to be considered. 

The U (uranium)-Th (thorium)-Pb (lead) isotopic system represents three independent decay schemes and is a powerful but complex tool with which to unravel the history of the Earth s mantle (Text box 3.2). During planetary accretion U and Th are refractory, lithophile elements and will reside in the mantle. Pb on the other hand is a volatile and chalcophile/ siderophile element and may in part, be stored in the core. Initial U and Th concentrations are derived from chondritic meteorites, and initial Pb isotope compositions are taken from the iron-sulfide troilite phase in the Canyon Diablo meteorite. The initial bulk Earth U/Th ratio was 4.0 0.2 (Rocholl Jochum, 1993). 

The HfrW ratio of a bulk planetary mantle must be inferred by comparing the W concentrations with another element that behaves similarly during silicate melting (i.e., has a similar incompatibility), tends to stay in the mantle and whose abundance relative to Hf is known. The latter two conditions are met by refractory lithophile elements (RLE) because their relative abundances in bulk planetary mantles are chondritic (see above). This is because they are neither fractionated by core formation (because they are lithophile) nor by volatile element depletion (because they are refractory). The Hf/W ratio of a bulk planetary mantle can thus be calculated as follows ... 

As refractory lithophile elements, the REE play an important role in constraining the overall composition and history of the silicate fraction of planets, which for the terrestrial planets is also termed their primitive mantle (equivalent to the present-day crust plus mantle). Since there is no evidence for significant planetary-scale fractionation of refractory elements during the assembly and differentiation of planetary bodies, it is widely accepted that the primitive mantles of terrestrial planets and moon possess chondritic proportions of the REE. As such, the absolute concentrations of REE (and other refractory elements) in primitive mantles provide an important constraint on the proportions of volatile elements to refractory elements and on the oxidation state (i.e., metal/silicate ratio) of the body. To date, the only major planetary bodies for which REE data are directly available are the Earth, Moon, and Mars, and Taylor and McLennan" recently reviewed these data. 

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... 

Updated solar photospheric abundances are compared with meteoritic abundances. It is shown that only one group of chondritic meteorites, the Cl chondrites, matches solar abundances in refractory lithophile, siderophile, and volatile elements. All other chondritic meteorites differ from Cl chondrites. The agreement between solar and Cl abundances for all elements heavier than oxygen and excluding rare gases has constantly improved since Goldschmidt (1938) published his first comprehensive table of cosmic abundances. 

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).

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. 

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