Osmium chondrites

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. 

Osmium isotopic ratio of the bulk silicate Earth overlaps measurements of ordinary chondrites but is distinct from other chondrite groups. Adapted from Righter et al. (2006). 

Walker, R.J. and J.W, Morgan Rhenium-Osmium Isotope Systematics of Carbonaceous Chondrites, Science, 519 (January 27, 1989). 

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).

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%. 

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... 

The oldest samples that provide direct constraints on the osmium isotopic composition of the upper mantle are rare spinel peridotites contained within the Early Archean Itsaq gneiss complex of southwest Greenland that are interpreted to be —3.81 Ga abyssal peridotites (Friend et al., 2002). The measured and initial compositions determined from low-Re/Os spinel and olivine mineral separates from these peridotites are the most primitive, in the sense of closest to solar-system initial compositions, Os isotopic compositions yet obtained on any terrestrial material (Bennett et al., 2002). This shows that at least some, if not all, of the Early Archean upper mantle was characterized by chondritic Os/ Os isotopic compositions. Osmium isotopic constraints from this time period (—3.8-3.9 Ga) are of particular interest as they provide a rough constraint on the timing of the addition of the late veneer of... 

Walker, R.J. and Morgan, J.W. (1989) Rhenium-osmium isotope systematics of carbonaceous chondrites. Science,... 

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