Rhenium 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. 

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... 

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... 

It is clear that the Earth s mantle has at least two Os-isotopic reservoirs - a plume-related isotopically enriched reservoir and a chondritic upper mantle reservoir. Both have long histories (Fig. 3.32). The variations in composition within the upper mantle reservoir reflect Re-depletion and enrichment related to melt extraction. The isotopically enriched plume reservoir represents chemically isolated, rhenium-enriched, recycled oceanic lithosphere. There is some evidence to suggest that this enriched reservoir may have been in existence since the early Archaean (Walker Nisbet, 2002) and was the source of some Archaean komatiites and the 3.81 Ga Itsaq Gneiss chromitites. If this is true, then basaltic crust was being created and recycled even before 4.0 Ga. Estimates of the present size of this high Re/Os basaltic reservoir vary from 5% to >10% of the whole mantle (Bennett et al., 2002 Walker et al., 2002). 

Figure 9.7 Schematic illustration of the concept of mantle extraction ages (Tma) and rhenium depletion ages (Trd) for a residue of partial melting, of which the present day Os/ Os ratio is indicated by the circle on the y-axis. The mantle extraction age uses the measured Re/ Os ratio to calculate the age at which the Os/ Os ratio of the sampl was the same as that of the chondritic mantle. The rhenium depletion age assumes that all rhenium was removed during the partial melting event, and that any rhenium present in the sample now was added recently.

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

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