Samarium/neodymium dating

Chesley, J.T., A.N , Halliday, and R.C. Scrivener Samarium-Neodymium Direct Dating of Fluorite Mineralization," Science. 949 (May 17, 1991)... 

Isochron age calculations are commonly made for the Rb-Sr (rubidium-strontium], Sm-Nd (samarium-neodymium], and U-Pb (uranium-lead] radioactive systems. They are most commonly applied to whole-rock systems, that is, a suite of samples thought to have formed at the same time, such as an igneous plu-ton or a suite of lavas. Isochron age calculations may also be made for a suite of minerals in a rock, in which case they date the time at which the minerals lost isotopic contact with each other, that is, became closed systems. This approach can be useful in dating metamorphism. 

The Cocks Formation contains a porphyritic pillow lava that was dated by RoweU et al. (1993) by means of the samarium-neodymium (Sm-Nd) method (Appendix 3.6.6). The rock yielded a Sm-Nd model date of700-800 Ma, which indicates that the flow was extruded during the Neoproterozoic Era The metamorphic grade of the Cocks Formation also increases in a northerly direction. 

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. 

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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