Chondrites ratios

Chondrite-normalized element/silicon ratios measured in chondritic porous IDPs. Cl chondrite ratios are shown by the vertical dashed line. Modified from Schramm et al. (1989). 

Mean compositions of tracks and crater residues produced by Stardust particles, compared to Cl chondrite ratios (horizontal lines), (a) Fe- and Cl chondrite-normalized composition determined by averaging 23 track analyses by SXRM (filled squares), (b) Si- and Cl- chondrite-normalized composition of seven crater residue analyses by SEM-EDX (filled squares), and TOF-SIMS analyses for five craters (open squares). Modified from Flynn et al. (2006). 

Data from Palme and Beer (1993), except rare gases in nL or pL g at (STP) from Anders and Grevesse (1989), elements marked D (Dreibus et al., 1995), J1 (Jochum, 1996), Jl Os is calculated from the average carbonaceous chondrite ratio Ir/Os of 0.0949 from Jochum (1996) J2 (Jochum et ah, 2000) W (Wolf and Palme, 2001) Z (Zhai and Shaw, 1994) elements incompletely condensed in Cl meteorites. Average Cl abundances from Anders and Grevesse (1989), Table 1, columns 6 and 2, except C, N, O and rare gases He, Ne, Ar, Kr, Xe which are only from Orgueil. 

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. 

Hart and Zindler (1986) also based their estimate on chondritic ratios of RLE. They plotted Mg/Al versus Nd/Ca for peridotites and chondritic meteorites. The two refractory elements, neodymium and calcium, approach chondritic ratios with increasing degree of fertility. From the intersection of the chondritic Nd/Ca ratio with observed peridotite ratios. Hart and Zindler (1986) obtained an Mg/Al ratio of 10.6 (Table 2). 

RSEs comprise two groups of metals the HSEs—osmium, rhenium, ruthenium, iridium, platinum, and rhodium with metal/silicate partition coefficients >10" —and the two moderately siderophile elements—molybdenum and tungsten (Table 2). As the major fractions of these elements are in the core of the Earth, it is not possible to establish independently whether the iDulk Earth has chondritic ratios of RLE to RSE, i.e., whether ratios such as Ir/Sc or W/Hf are chondritic in the bulk Earth. Support for the similar behavior of RLE and RSE in chondritic meteorites is provided by Figure 9. The ratio of the RSE, Ir, to the nonrefractory siderophile element, Au, is plotted against the ratio of the RLE, Al, to the nonrefractory lithophile element, Si. Figure 9 demonstrates that RLEs and RSEs are correlated... 

Some HSE ratios in upper mantle rocks often show significant deviations from chondritic ratios. For example, Schmidt et al. (2000) reported a 20-40% enhancement of ruthenium relative to iridium and Cl-chondrites in spinel Iherzolites from the Zabargad island. Data by Pattou et al. (1996) on Pyrenean peridotites, analyses of abyssal peridotites by Snow and Schmidt (1998), and data by Rehkamper et al. (1997) on various mantle rocks suggest that higher than chondritic Ru/lr ratios are widespread and may be characteristic of a larger fraction, if not of the whole of the upper mantle. A parallel enrichment is found for rhodium in Zabargard rocks (Schmidt et al, 2000). There are. 

The concentrations of palladium in upper mantle rocks are also much more variable than those of the refractory siderophiles, with anomalously high Pd/Ir ratios with up to twice the Cl-chondritic ratio (Pattou et al., 1996 Schmidt et al., 2000). These variations exceed those in chondritic meteorites considerably. 

Another constraint on the origin of the xenon now found in the upper mantle is obtained from the composition of fissiogenic Xe. In a closed-system reservoir, plutonium-derived Xe will dominate over uranium-derived Xe (see Section 4.11.3.2). In a system that has been closed throughout solar system history and starting with a chondritic ratio of... 

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