Platinum chondrites

Chen, J. H., Papanastassiou, D.A. and Wasserburg, G. J. (1998) Re-Os systematics in chondrites and the fractionation of the platinum group elements in the early solar system. Geochimica et Cosmochimica Acta, 62, 3379—3392. 

In a limestone deposit at Kinnekulle, Sweden, a remarkable collection of more than 40 highly altered meteorites (Fig. 9.18) totaling 7.7 kg have been collected during routine guarrying operations (Schmitz etal., 1997,2001,2003). The 3.2-m-thick limestone layer in which they are found was deposited over -1.75 Myr in the mid-Ordovician. Despite being almost completely replaced by calcite, barite, and phyllosilicates, the meteorites are easily identified by their chondritic texture. Their identification as meteorites is confirmed by measurements of platinum-group elements. The chemical characteristics of relict spinels indicate that they are either L or LL chondrites. 

Jochum K. P. (1996) Rhodium and other platinum-group elements in carbonaceous chondrites. Geochim. Cosmochim. Acta 60, 3353-3357. 

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).

Noble (platinum-group) metal Primary phase Common trace phase in CAIs from many chondrite types. 

Holzheid A., Sylvester P., O Neill H., Ruble D. C., and Palme H. (2000) Evidence for a late chondritic veneer in the Earth s mantle from high-pressure partitioning of palladium and platinum. Nature 406, 396-399. 

Rehkamper M., HaUiday A. N., Alt J., Fitton J. G., Zipfel J., and Takazawa E. (1999a) Non-chondritic platinum group element ratios in abyssal peridotites petrogenetic signature of melt percolation Earth Planet. Set Lett. 172, 65-81. 

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... 

Pattou L., Lorand J. P., and Gros M. (1996) Non-chondritic platinum group element ratios in the Earth s mantle. Nature 379, 712-715. 

The platinum group elements may be presented in the same way as are the REE and incompatible elements and normalized to either chondrite meteorites or to the primitive mantle.. . v... 

Figure 4.27 Chondrite-normalized platinum group element plot. The data are for komamte-massive sulphide ores from Alexo Mine, northen Ontario (from Barnes and Naldrett, 1986). The chondrite -normalizing values are given in Tabic 4.11, column 2 and are taken from Naldrett and Duke (1980).

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