Refractory elements elemental abundances

Some carbonaceous chondrites are rich in carbon (Cl and CM chondrites have 1.5-6% carbon), but others are not. Carbonaceous chondrites are now defined on the basis of their refractory elemental abundances, which equal or exceed those in Cl chondrites. Carbonaceous chondrites are derived from very diverse asteroids, which probably formed in very different locations. The parent bodies of Cl and CM chondrites are highly altered, yet the parent bodies of CH and CB chondrites are less altered than all other chondrite bodies. Young et al (1999) infer from oxygen isotopic compositional data that Cl, CM, and CV chondrites could have been derived from different zones in a single, aqueously altered body. However, bulk chemical differences between these groups indicate fractionation during nebular processes, not aqueous alteration (see below), and the components in CM and CV chondrites are quite different. 

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%. 

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.

Chondrite classes are also distinguished by their abundances of both volatile and refractory elements (3). For volatile elements the variation among groups results from incomplete condensation of these elements into soHd grains that accrete to form meteorite parent bodies. Volatile elements such as C,... 

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... 

Comparison of the abundances of volatile and refractory elements, normalized to Cl chondrites, for the Moon and the Earth. After Taylor et al. (2006a). 

The elemental abundance of the lunar mare rocks as compared to that of carbonaceous chondrites vary up to 6 orders of magnitude (Fig. 3a). The strongly siderophile elements and the very volatile elements are highly depleted, while the refractory elements Al, Ca, Ti, REE, Th, U. etc. are enriched. Hence, it is rather difficult to explain the fractionation of the lunar mare basalts by... 

Refractory elements, i.e. REE, Be, Y, Zr, Hf, Nb, and Ta, are present on the Moon in their solar (C 1) abundance ratios. Only W is considerably depleted relative to the other refractories, indicating the presence on the Moon of metallic iron, which is responsible for the depletion of this rather siderophile element. 

The values derived from these correlations for the elemental abundance ratios should be representative for the whole Moon. On this basis it is not possible to distinguish between depletion of the more volatile elements (K, Rb, and Cs) or enrichment of the refractories. As we will see later, both processes have occurred. 

Figure 4 Abundances of refractory and moderately volatile elements in various groups of carbonaceous chondrites, normalized to Cl and Mg. Refractory elements increase from Cl to CV3 chondrites while Mg/Si ratios are constant in all groups of carbonaceous chondrites. Although the elements Cr, Fe, and P are significantly less depleted than Mn and Zn, they show a similar behavior, suggesting volatility related depletions of Cr, Fe, and P in carbonaceous chondrites of higher metamorphic grades (source Wolf and Palme, 2001).

Figure 8 Abundances of elements along the line of sight towards Oph Ophiuchus), a moderately reddened star that is frequently used as standard for depletion studies. The ratios of Oph abundances to the solar abundances are plotted against condensation temperatures. The abundances of many of the highly volatile and moderately volatile elements up to condensation temperatures of around 900 K are, within a factor of 2, the same in the ISM and in the Sun. At higher condensation temperatures a clear trend of increasing depletions with increasing condensation temperatures is seen. It is usually assumed that the missing refractory elements are in grains (source Savage and...

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. 

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