Chondrites siderophile elements

Two different kinds of metals are found in chondrites. Small nuggets composed of highly refractory siderophile elements (iridium, osmium, ruthenium, molybdenum, tungsten, rhenium) occur within CAIs. These refractory alloys are predicted to condense at temperatures above 1600 from a gas of solar composition. Except for tungsten, they are also the expected residues of CAI oxidation. 

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). 

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). 

Chondrite-normalized abundances of siderophile elements in the Earth s mantle. The measured concentrations do not match those expected from low-pressure metal-silicate partition coefficients determined by experiments. Modified from Tolstikhin and Kramers (2008). 

Table 10.1 Elemental abundances for several objects in the Solar System, normalized to Si and Cl chondrites. Following Halliday et al. 2001, Fodders 2003. Siderophile elements are indicated by. ...

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... 

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

The gold content of type 1 carbonaceous chondrites is 0,15 ppm.81 Anders et al.6 assumed C 1 chondrites (the most primitive class of meteorites) to be the dominant contributing component and, after correction of the small amounts of siderophile elements found in the igneous lunar rocks, calculated from the data on Au given in Table 2 the meteoritic component as shown in Table 9. 

For the siderophile elements, the metal particles contribute between 50 and 90% to the bulk composition of the soil samples. As carbonaceous chondrites do not contain metal, reduction and equilibration is required to explain the high contribution of the metal particles. 

Figure 24 Concentration profiles of siderophile elements in a radially zoned Fe,Ni grain in the CBb chondrite, QUE 94411 (a) electron microprobe data (b) and (c) trace element data from laser ablation ICPMS (Campbell et ai, 2001). The nickel, cobalt, and chromium profiles can be matched by nonequilibrium nebular condensation assuming an enhanced dust-gas ratio of —36 X solar, partial condensation of chromium into silicates, and isolation of 4% of condensates per degree of cooling (Petaev etal, 2001). Concentrations of the refractory siderophile elements, osmium, iridium, platinum, ruthenium, and rhodium, are enriched at the center of the grain by factors of 2.5-3 relative to edge concentrations, which are near Cl levels after normalization to iron (reproduced by permission of University of Arizona on behalf of The Meteoritical Society from Meteorit. Planet. ScL, 2002, 37, pp. 1451-1490).

Campbell A. J., Humayun M., and Weisberg M. K. (2(X)2) Siderophile element constraints on the formation of metal in the metal-rich chondrites Bencubbin, Weatherford, and Gujba. Geochim. Cosmochim. Acta 66, 647-660. 

Silicon-bearing kamacite is a minor component of most aubrites, and contains 3.7-6.8 wt.% Ni and 0.1-2.4 wt.% Si. Metal nodules from several aubrites contain approximately chondritic abundances of refractory to moderately volatile siderophile elements (Casanova et al., 1993). 

Aubrites are depleted in siderophile elements relative to chondritic abundances. Excluding Shallowater, bulk rocks have Cl-normalized Fe/Si ratios of 0.006-0.055. Shallowater, with a higher modal metal content (3.7% versus <0.7% Watters and Prinz, 1979), has a Cl-normalized Fe/Si of 0.33 (Easton, 1985). Trace siderophile elements are also depleted for example, excluding dark samples, iridium varies from —10 X Cl to 10 X Cl. Dark clasts from Khor... 

Itqiy is distinct from chondritic meteorites in bulk composition. Aluminum, FREE, europium, sodium, potassium, vanadium, chromium, and manganese are aU depleted. Itqiy has La/Yb of 0. lOxCI, and Eu/Sm of 0.16 X Cl. Refractory siderophile elements are enriched —2-3 X Cl, while moderately volatile siderophile elements are at roughly Cl abundances. The bulk rock Mg/Si and Fe/Si ratios are greater than those of EH or EL chondrites. 

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. 

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).

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. 

Figure 7 Siderophile element concentrations in averaged medium-Ti mare and terrestrial basalts, normalized to Cl chondrites. The elements are plotted in order of Cl-depletion factors in average medium-Ti mare basalt, but for some elements volatility may account in large part for the depletion. Data are from same sources as for Figure 6. To avoid an over-complex diagram, individual terrestrial compositions are not plotted, but all show similar patterns at the scale of this diagram the most noteworthy exceptions being low iridium (9 X 10 times Cl) and nickel (10 times Cl) in BCR-1, and relatively low osmium and iridium (virtually identical to mare basalts) and antimony (0.11 times Cl)...

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%. 

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