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The main challenge of Sm-Nd dating is accurately determining the ratios between the different isotopes of neodymium, specifically Nd-144: Nd-143 and Nd-144: Nd-142.
Nd-144 is a naturally-occurring isotope of neodymium and so its relative abundance compared with the two isotopes produced by the radioactive decay of samarium can reveal the extent of that decay.
However, because Sm and Nd have very similar chemical properties (unlike Rb and Sr), large ranges of Sm/Nd in whole-rock systems are rare, and in particular, low Sm/Nd ratios near the y axis are very rare.
Therefore, because of the difficulty of obtaining a wide range of Sm/Nd ratios from a single rock body, and because of the greater technical demands of Nd isotope analysis, the Sm Sr isochrons had proved unsatisfactory.
All rocks contain some neodymium-142, but rocks older than 4.2 billion years should contain more of it.