Re- Os system

Figure 4.42. Mo alloys multi-diagram. Notice that along the vertical axis, from Ta to Pt, the sequence of the isothermal sections Ta-W, W-Re, Re-Os, etc. is shown. In the same axis an approximate representation is also suggested for the other binary combinations (Ta-Re Ta-Os, Ta-Ir, etc., W-Os, W-Ir, etc.). A confirmation of the (even partial) validity of this representation is given by the phase sequences observed in the W-Re and Re-Os systems in comparison with that of the W-Os system. (For the symbols see Fig.4.41).

Ravizza G. and Tnrekian K. K. (1989). Application of the Re- Os system to black shale geochronometry. Geochim. Cosmochim. Acta, 53 3257-3262. 

The Re- Os system in cosmochemistry has been primarily used to investigate the crystallization paths and ages of asteroidal cores through measurements of magmatic iron... 

The Re- Os system does not lend itself to the determination of internal isochrons for most meteorites. Chen et al. (1998) produced an internal isochron for the St. Severin (LL6) chondrite using metal separates and whole-rock samples (Fig. 8.21). The slope of the isochron gives a date of 4.60-0.15 Ga (X = 1.666 x 10 11 yr 1), consistent with expectations based on other chronometers, but not precise enough to improve upon other techniques. 

As with the Sm- Nd and Re- Os systems, careful chemistry is required to cleanly separate the parent and daughter elements because mass spectrometry cannot resolve 176Lu from 176Hf. The ion-exchange chemistry is similar to that for samarium-neodymium. In fact, fractions of samarium, neodymium, lutetium and hafnium are often produced in a single procedure. Mass spectrometry is done by ICPMS because this is the only method that effectively ionizes hafnium. 

Because of the numerous secondary effects that modify the whole-rock elemental and isotopic compositions, our discussion below will focus solely on mineral-separate analyses for the Rb-Sr, Sm-Nd, Lu-Hf, and U-Pb isotope systems. In comparison, the Re-Os system is much more robust to the effects of... 

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. 

Data for Sm-Nd, Rb-Sr, and Th-U-Pb isotopic systems are readily affected by contamination from the enclosing lavas and by the secondary enrichment events. Only the Re-Os system seems capable of giving reliable ages for the... 

In contrast to the Pt-Os decay scheme, the Re-Os system gives rise to large variations in Os/ Os of marine deposits and is currently the subject of vigorous investigation. This work is motivated by two fundamentally important attributes of Os geochemistry the relatively short marine residence time of Os, and the record of past variations in the Os/ Os of sea water preserved in marine sediments. Direct analyses of sea water do not yield evidence of any resolvable difference in the... 

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