Thus, the concentration of samarium and neodymium in rocks, and the ratios between different isotopes of neodymium, can reveal how long ago a rock sample crystallized from magma, and thus how old it is.
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.
This, in turn, influences the Nd ratio of the isotope system provides important information on crustal formation and the isotopic evolution of the solar system.
The concentration of Sm and Nd in silicate minerals increase with the order in which they crystallise from a magma according to Bowen's reaction series.
Prior to 1905 the best and most accepted age of the Earth was that proposed by Lord Kelvin based on the amount of time necessary for the Earth to cool to its present temperature from a completely liquid state.
Although we now recognize lots of problems with that calculation, the age of 25 my was accepted by most physicists, but considered too short by most geologists. Recognition that radioactive decay of atoms occurs in the Earth was important in two respects: Principles of Radiometric Dating Radioactive decay is described in terms of the probability that a constituent particle of the nucleus of an atom will escape through the potential (Energy) barrier which bonds them to the nucleus.
The production of radiogenic Sr can be expressed with respect to a reference isotope is the y-intercept ( which corresponds to Basalitc Achondrite Best Initial (or BABI) value for achondrite meteorites which gives an estimate of the initial Sr isotopic composition of the earth when it formed out of the solar nebula) and the slope of this line is proportional to the amount of time the rock has remained in a closed system with respect to Rb and Sr.