The determinations since 1960 have converged on close agreement between the two primary determination techniques used in direct physical counting experiments—ionization chambers and liquid scintillation counters, and with radioisotope age comparisons using two meteorites.Thus the Sm half-life value of 106 ± 0.8 Byr has now been adopted for standard use by the uniformitarian geological community.Samarium-neodymium (Sm-Nd) dating is a process for determining the ages of rocks based on the radioactive decay of two samarium isotopes (Sm-147 and Sm-146) to two neodymium isotopes (Nd-143 and Nd-142 respectively).Sm-147 has a much longer half-life than Sm-146, but the concentration of both of them varies in different rock types, depending on exactly when the rocks crystallized from the parent magma.In most geological environments, the isotopic composition of Nd varies systematically, though inversely, with the isotopic composition of Sr.Except in very specific environments, Nd will not be as useful as Sr (due to its higher concentration in natural waters) in the interpretation of the weathering process in rocks and minerals.This value is based on the weighted average of four direct counting determinations in the period 1961–1970 and the recalibration in the 1970s of Sm-Nd model ages of two meteorites by forcing them (essentially by circular reasoning) to agree with their Pb-Pb isochron and model ages.
Cratonal source will have a higher In the above evolution curves for Sr, the black line represents the evolution of the whole earth with estimates of the initial value corresponding to BABI.
Furthermore, even this “gold standard” has unresolved uncertainties due to the U decay constants being imprecisely known, and to measured variations of the U ratio in terrestrial rocks and minerals and in meteorites.
Both of these factors are so critical to the U-Pb method, as well as the additional factor of knowing the initial concentrations of the daughter and index isotopes, so it should not be used as a standard to determine other decay constants.
Nd together provides useful insight into geochemical evolution.
Mafic rocks and associated suites of more differentiated rocks can be dated by whole-rock or internal mineral isochrons.