Refractory elements chondrites

Siderophile elements Alkaline elements Elements highly depleted in normal chondrites Refractory elements Other elements... 

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,... 

Two types of models have been proposed that use this general picture as the basis for understanding volatile depletions in chondrites. Yin (2005) proposed that the volatile element depletions in the chondrites reflect the extent to which these elements were sited in refractory dust in the interstellar medium. Observations show that in the warm interstellar medium, the most refractory elements are almost entirely in the dust, while volatile elements are almost entirely in the gas phase. Moderately volatile elements are partitioned between the two phases. The pattern for the dust is similar to that observed in bulk chondrites. In the Sun s parent molecular cloud, the volatile and moderately volatile elements condensed onto the dust grains in ices. Within the solar system, the ices evaporated putting the volatile elements back into the gas phase, which was separated from the dust. Thus, in Yin s model, the chondrites inherited their compositions from the interstellar medium. A slightly different model proposes that the fractionated compositions were produced in the solar nebula by... 

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). 

There is evidence from chondrites that the solar nebula was well mixed between 0.1 and 10 AU during its first several million years of the evolution, as shown by the homogeneity in concentrations of many isotopes of refractory elements (Boss 2004 Chapter 9). This is likely caused by the evaporation and recondensation of solids in the very hot inner nebula, followed by outward transport due to turbulent diffusion and angular momentum removal. Materials out of which terrestrial planets and asteroids are built have been heated to temperatures above 1300 K and are thus depleted in volatile elements. The inner solar nebula, with some exceptions, does not retain memories of the pristine interstellar medium (ISM) chemical composition (Palme 2001 Trieloff Palme 2006). 

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... 

A lower K/U ratio for the Earth and the basaltic achondrites as compared to the chondritic (solar) ratio was emphasized more than ten years ago by Gast115 116 and Wanke117). Since the first lunar samples became available it has been suggested, especially by Gast30,118 that the Moon has a higher concentration of the refractory elements Ca, Al, REE, etc. 

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... 

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).

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. 

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).

Two kinds of metal are found in chondrites grains composed of refractory elements (iridium, osmium, ruthenium, molybdenum, tungsten, and rhenium), which condense along with the refractory oxides above —1,600 K at 10 atm, and grains composed predominantly of iron, cobalt, and nickel, which condense with forster-ite and enstatite at —1,350-1,450 K. The former are associated with CAIs (Palme and Wlotzka 1976) and the latter with chondrules, typically type I or FeO-poor chondrules (B J 1998, pp. 244-278). Unfortunately, few chondrites preserve a good record of the formation history... 

The bulk trace element abundance patterns in CAIs are generally agreed to reflect element volatility, with the most refractory elements enriched relative to solar (Cl chondrite) abundances, and volatile elements depleted. 

The refractory condensate model has fallen out of favor, including with Lewis (1988). Nevertheless, it is a useful end-member case. Goettel (1988) calculated the composition of the silicate portion of an ultrarefractory Mercury (Table 2, column 2). This model composition contains no FeO or volatiles, and has large concentrations of the refractory elements—aluminum, calcium, and magnesium. We calculated the thorium and uranium contents of such refractory condensates by assuming chondritic Al/Th and Al/U ratios. A surface of this composition will contain many of the phases in calcium-aluminum-rich inclusions (CAls), such as forsterite, anorthite, spinel, perovskite, hibonite, and melilite. 

Figure 6 Volatile/refractory element ratio-ratio plots for chondrites and the silicate Earth. The correlations for carbonaceous chondrites can be used to define the composition of the Earth, the Rb/Sr ratio of which is well known, because the strontium isotopic composition of the BSE represents the time-integrated Rb/Sr. The BSE inventories of volatile siderophile elements carbon, sulfur, and lead are depleted by more than one order of magnitude because of core formation. The values for Theia are time-integrated compositions, assuming time-integrated Rb/Sr deduced from the strontium isotopic composition of the Moon (Figure 8) can be used to calculate other chemical compositions from the correlations in carbonaceous chondrites (Halliday and Porcelli, 2001). Other data are from Newsom (1995).

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%. 

In Figure 1 aluminum is representative of the refractory component. AU types of carbonaceous chondrites are enriched in refractory elements, whereas ordinary and enstatite chondrites are depleted. Variations in Mg/Si (Figure 1) are smaller and may be the result of preferred accumulation or loss of olivine as discussed... 

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