Sm nd isotopic dating Sextreme dating ep 1
Although rejected or ignored, this 117 ± 2 Byr value for the Sm decay, preference has been given to the half-life value of 106 ± 0.8 Byr determined by forcing the Sm-Nd data to agree with Pb-Pb dates.But many unprovable assumptions are also involved, not the least being that the radioisotope systems closed at the same time and subsequently remained closed.Since both the daughter and index isotopes vary in time, establishing an initial daughter concentration (via the isochron method) becomes problematic because the y-intercept of the linear plot cannot be assumed to be the primordial ratio of Geochronologists solve this problem by using “stony meteorites” to establish a primordial isotope ratio for This method assumes that the terrestrial Nd has evolved in a uniform reservoir whose Sm/Nd ratio is equal to that of a chrondritic meteorite that is further thought to be similar in composition to the current photosphere of the sun minus the volatile elements.Hence, the logic used to estimate the initial value of the daughter isotope concentration springs from the evolutionary models for solar system formation and is, again, quite circular in its essence.
Because the crust is continuously sampling the mantle, the mantle becomes more depleted in Sm-Nd over time.
Elemental investigations have indicated the general immobility of REE on a whole-rock scale during the processes of weathering and low-temperature metamorphism associated with sedimentary rock formation (e.g.
Haskin et al., 1966), and even during high-grade metamorphism (Green et al., 1969).
Neodymium isotopes are a common daughter product of the decay of Samarium, a rare earth element (REE).
REE's are extremely concentrated in melt compared to the concentration of them in mineral form.
Archaen pluton have very similar Nd ratios to the Chondritic Uniform Reservoir (CHUR) (Dickin, 1995).