Rhenium/osmium dating

Since negative thermal ion mass spectrometry (N-TIMS) for rhenium/osmium dating (see O Sect. 17.18) was developed about 1990, not too much work has been published. This is probably due to the high rhenium blanks from the current generation of clean platinum filaments and to problems in achieving isotopic exchange and equilibrium between sample and spike for osmium. Another drawback is the non-homogeneity of samples. Because of this, rhenium and osmium concentrations may vary hy up to 40% in the same sample. 

Hirschmann M. (1991). Thermodynamics of multicomponent olivines and the solution properties of (Ni,Mg,Ee)2Si04 and (Ca,Mg,Fe)2Si04 olivines. Amer. Mineral, 76 1232-1248 Hirt B., Herr W, and Hoffmeister W. (1963). Age determinations by the rhenium-osmium method. In Radioactive Dating, International Atomic Energy Agency, Vienna. 

We were quite elated, and it appeared that it was a rich field. Now, fifty years later, I must say that it wasn t as rich as we thought. But we have over the years discovered half a dozen natural radioactive elements, and two of these, the samarium-147 with its decay to neodymium-143 and rhenium-187 with its decay to osmium-187, prove to be of use in Nuclear Dating. The importance of rhenium is that it is iron soluble while the other radioactivities are insoluble in metallic iron. In fact, the best half life we have for rhenium-187 was obtained by measuring the osmium-187 to rhenium-187 ratio in iron meteorites which had been dated by other methods. This work was started many years ago by Dr. Herr and others in Germany. The half life is 43,000,000,000 years. 

Figure 11.28 shows the rhenium and osmium isotopic compositions of black shales and sulfide ores from the Yukon Territory (Horan et al., 1994). The black shale and sulfide layers are approximately isochronous. The superimposed reference isochrons bracket the depositional age of the enclosing shales. One reference line represents the minimum age (367 Ma) with an initial ( Os/ Os)q ratio of one, consistent with the mantle isotopic composition at that age (see later). The other reference isochron is drawn for a maximum age of 380 Ma, with ( 870s/ 860s)o = 12 (the maximum value measured in terrigenous sediments). Further examples of application of Re-Os dating of sediments can be found in Ravizza and Turekian (1989). 

The decays of " Sm (Samarium) to " Nd (Neodymium), Lu (Lutetium) to Hf (Hafnium), and Re (Rhenium) to Os (Osmium) all have long parental half-lives and very low natiual abimdances of the parent isotopes which means that the radiogenic daughter isotopes accumulate very slowly. Before about 1980 these decay schemes were of little value as radiometric dating methods. However, improvements in analytical techniques have permitted geochronologists to apply these three dating methods to certain types of problems. 

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