Osmium abundance

Improved analytical capabilities have led to the analysis of several hundred xenoliths for osmium isotopic composition. The compatible nature of osmium during mantle melting means that, unlike incompatible-element-based isotope systems, peridotite residues have much higher osmium contents than mantle melts and thus the system is less readily disturbed by later metasomatism (see Section 2.05.2.5.3). This is clearly shown by rhenium and osmium abundances (Figure 21). The vast majority of rhenium contents of both cratonic and noncratonic peridotite xenoliths are below the PUM value proposed by Morgan et al (1981) and many are P-PGE depleted. This contrasts with almost universal TREE enrichment of whole-rock peridotites. That the Re-Os system is not immune from the effects of metasomatism is illustrated by the consideration of extended PGE patterns (Figure 20 Section 2.05.2.5.3 Pearson et al., 2002, 2004). Dismption of both rhenium and osmium in some mantle environments may have occurred (Chesley et al, 1999), especially where sulhde metasomatism is involved (Alard et al, 2000). However, Pearson et al. (2002, 2004) and Irvine et al (2003) have shown that coupled PGE and Re-Os isotope analyses can effectively assess the level of osmium isotope disturbance in peridotite suites. 

Abundances of lUPAC (the International Union of Pure and Applied Chemistry). Their most recent recommendations are tabulated on the inside front fly sheet. From this it is clear that there is still a wide variation in the reliability of the data. The most accurately quoted value is that for fluorine which is known to better than I part in 38 million the least accurate is for boron (1 part in 1500, i.e. 7 parts in [O ). Apart from boron all values are reliable to better than 5 parts in [O and the majority arc reliable to better than I part in 10. For some elements (such as boron) the rather large uncertainty arises not because of experimental error, since the use of mass-spcctrometric measurements has yielded results of very high precision, but because the natural variation in the relative abundance of the 2 isotopes °B and "B results in a range of values of at least 0.003 about the quoted value of 10.811. By contrast, there is no known variation in isotopic abundances for elements such as selenium and osmium, but calibrated mass-spcctrometric data are not available, and the existence of 6 and 7 stable isotopes respectively for these elements makes high precision difficult to obtain they are thus prime candidates for improvement. 

Ruthenium and osmium are generally found in the metallic state along with the other platinum metals and the coinage metals. The major source of the platinum metals are the nickel-copper sulfide ores found in South Africa and Sudbury (Canada), and in the river sands of the Urals in Russia. They are rare elements, ruthenium particularly so, their estimated abundances in the earth s crustal rocks being but O.OOOl (Ru) and 0.005 (Os) ppm. However, as in Group 7, there is a marked contrast between the abundances of the two heavier elements and that of the first. 

Osmium is the 80th most abundant element on Earth. As a metal, it is not found free in nature and is considered a companion metal with iridium. It is also found mixed with platinum- and nickel-bearing ores. It is recovered by treating the concentrated residue of these ores with aqua regia (a mixture of 75% HCl and 25% HNO). The high cost of refining osmium is made economically feasible by also recovering marketable amounts of platinum and nickel. 

Iridium is the 83rd most abundant element and is found mixed with platinum, osmium, and nickel ores. The minerals containing iridium are found in Russia, South Africa, Canada, and Alaska. 

Osmium occurs in nature, always associated with other platinum group metals. It usually is found in lesser abundance than other noble metals. Its most important mineral is osmiridium (or iridosmine), a naturaUy occurring mineral alloyed with iridium. 

The Re- Os method was first applied to extraterrestrial samples in the early 1960s when Hirt et al. (1963) reported a whole-rock isochron for 14 iron meteorites that gave an age of 4 Ga. Further development of this system was hindered by several technical difficulties. Rhenium and osmium each exist in multiple oxidation states and can form a variety of chemical species, so complete digestion of the samples, which is required to chemically separate rhenium and osmium for mass spectrometry, is difficult. In addition, accurate determination of rhenium abundance and osmium isotopic composition requires spiking the samples with isotopically labeled rhenium and osmium, and equilibration of spikes and samples is challenging. A third problem is that osmium and, particularly, rhenium are very difficult to ionize as positive ions for mass spectrometry. These problems were only gradually overcome. 

Figure 9.49 Comparison of isotope abundances of metallic Os and natural highly enriched 187Os samples (a) results of isotope analysis on metallic osmium (lohnson Matthey Chemicals) (b) isotope analysis on natural 187Os enriched samples due to /3 decay of 187Re.108...

The reason for it is not obvious since gold is not a very rare element on earth, and other metals, for example, platinum, rhodium, osmium, and rhenium, are less abundant and more expensive. Its yellow color cannot be the reason either, since other metals, such as copper, and its alloys as bronze or brass, have different colors from the bright silver of most of the metals. Probably, the reason resides in its noble character. In fact, gold does not tarnish with time, and coins and jewelry remain indefinitely unalterable even after long exposure to extremely aggressive conditions. 

Finally, the isotopic distribution of the parent ion can serve as a fingerprint for a given metal composition. The isotopic distribution of the parent ion of H2FeRuOs2(CO),3 is shown in Fig. 2. The wide isotopic distribution from mle 911 to mle 895 arises because iron, ruthenium, and osmium have, respectively, 4, 9, and 7 naturally occurring isotopes of appreciable abundance. The various combinations, taken together with the appropriate weighting factors, give the calculated distribution shown in Fig. 2. If the metal composition of a particular cluster is uncertain, the experimental isotopic distribution may be compared with that calculated for a number of trial compositions, and thus the correct composition can often be uniquely determined. 

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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