Sm nd dating garnet
Samarium–neodymium dating is a radiometric dating method useful for determining the ages of rocks and meteorites, based on radioactive decay of a long-lived samarium (Sm) isotope to a radiogenic neodymium (Nd) isotope.
Neodymium isotope ratios together with samarium-neodymium ratios are used to provide information on the source of igneous melts, as well as to provide age information.
It is sometimes assumed that at the moment when crustal material is formed from the mantle the neodymium isotope ratio depends only on the time when this event occurred, but thereafter it evolves in a way that depends on the new ratio of samarium to neodymium in the crustal material, which will be different from the ratio in the mantle material.
Samarium–neodymium dating allows us to determine when the crustal material was formed.
In addition, epsilon units will normalize the initial ratios to CHUR, thus eliminating any effects caused by various analytical mass fractionation correction methods applied.
Samarium is accommodated more easily into mafic minerals, so a mafic rock which crystallises mafic minerals will concentrate neodymium in the melt phase relative to samarium.Through the analysis of isotopic compositions of neodymium, De Paolo and Wasserburg (1976) discovered that terrestrial igneous rocks at the time of their formation from melts closely followed the "chondritic uniform reservoir" or "chondritic unifractionated reservoir" (CHUR) line – the way the Nd ratio increased with time in chondrites.Chondritic meteorites are thought to represent the earliest (unsorted) material that formed in the Solar system before planets formed.This therefore allows crustal formation ages to be calculated, despite any metamorphism the sample has undergone.Despite the good fit of Archean plutons to the CHUR Nd isotope evolution line, De Paolo and Wasserburg (1976) observed that the majority of young oceanic volcanics (Mid Ocean Ridge basalts and Island Arc basalts) lay 7 to 12 ɛ units above the CHUR line (see figure).