Iridium chondrites

The historical background is presented for the asteroid-impact theory that is based on the iridium anomaly found in rocks frm the Cretaceous-Tertiary boundary. Recent measurements of Ir, Pt, and Au abundances from such rocks in Denmark have shown that the element abundance ratios are different from mantle-derived sources and agree with values for chondritic meteorites within one standard deviation of the measurement errors (7-10%). Rare-earth patterns for these rocks are... 

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. 

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... 

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).

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... 

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)...

Ringwood et al. (1987) advocated a very different interpretation of the high Ni/Ir ratios in Apollo 16 impactites. These authors assumed that the meteoritic components have chondritic Ni/Ir, implying by mass balance (i.e., stripping away the meteoritic component based on iridium)... 

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%. 

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 small amount of the late veneer (<1% chondritic material) would not have had a measurable effect on the abundances of other elements besides HSEs, except for some chalco-phile elements, most importantly sulfur, selenium and tellurium (Figure 15(c)). The amount of sulfur presently in the Earth s mantle (200 ppm. Table 4) corresponds to only 0.37% of a nominal CI-component, while the iridium content suggests a CTcomponent of 0.67%. O Neill (1991) has, therefore, suggested that the late veneer was compositionally similar to H-chondrites which contain only 2% S (Wasson and Kallemeyn, 1988). Because H-chondrites have higher iridium (780 ppb) than Cl-chondrites, the required... 

H-chondrite fraction would only be 0.41% based on iridium. This would correspond to 82 ppm S delivered by the late veneer to the mantle. In this case the Earth s mantle should have combined —120 ppm S before the advent of the late veneer. If core formation in the Earth (or in differentiated planetesimals that accreted to form the Earth) occurred while the silicate portion was molten or partially molten, some sulfur must have been retained in this melt (O NeiU, 1991). 

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