Sm-Nd are both are both lanthinide or “rare-earth” elements. These lithophile elements behave like hard spheres governed by ionic radius, charge, and how well they can substitute for other elements.
REE show a gradation of chemical properties, with their ionic radii shrinking gradually from La to Lu. As a consequence, their behaviour varies smoothly from highly incompatible (La) to slightly incompatible (Lu). Most REE exist in 3+ state. Eu can exist in both 2+ and 3+ states and displays slightly different behaviour to the other REE in geological systems. The gradational behaviour of the REE is commonly used to investigate magmatic processes, with LREE preferentially concentrated in silicic melts relative to the HREE.
Processes causing large fractionations of Sm from Nd in nature are uncommon. This limits the applications of the Sm-Nd radiogenic decay scheme.
Whole-rock and mineral isochrons: preconditions
- samples must have had the same initial ratio (i.e. co-genetic)
- samples must be the same age
- isotopic (parent-daughter) system must have remained closed since t = 0
If all uncertainties have been correctly estimated and the preconditions do apply, then all scatter about the line can be accounted for as analytical uncertainty.
Common problems with whole-rock Sm-Nd isochron dates:
- similar ionic radii of Sm and Nd (therefore, similar chemical behavior) results in little fractionation of Sm from Nd (range in Sm/Nd) during basalt or granite melt generation and differentiation (thus, imprecise isochron dates)
- to increase the range of Sm/Nd and thus, “improve” precision on Sm-Nd dates, many have given in to the temptation to include samples which are not co-genetic on Sm-Nd isochrons
- mafic melts commonly assimilate significant amounts of wall-rock (i.e. crustal contamination) during eruption, emplacement and differentiation
- migration within the mantle and subcontinental lithosphere of fluids/melts derived by low degrees of partial melting can result in “mantle isochrons”
Correlation of isotopic ratios from Rb-Sr and Sm-Nd systems are found in mantle-derived igneous rocks (mid-ocean ridge and ocean-island basalts), with chondritic Rb/Sr and Sr/Sr, and Sm/Nd and Nd isotopic values at the centre of the Sr-Nd isotopic diagram. This allows comparison of mantle-derived melts with chondritic (bulk earth) values and investigation of the differentiation of the Earth’s mantle.
This has led to new field of isotopic tracer investigations of the Earth system using Sr and Nd. Epsilon notation is used to describe differences from chondritic evolution in 143Nd/144Nd ratio at any specified time, with positive Epsilon meaning radiogenic Nd relative to the contemporaneous chondritic ratio.
Mid-ocean ridge basalts typically have long-term histories of low Rb-Sr and high Sm/Nd and plot within the “depleted” (meaning depleted in “incompatible elements”, generally the lithophile elements, due to past melt extraction) quadrant. This depletion is because melt has been extracted from the Earth’s upper mantle to form the Earth’s continental crust, leaving a melt-depleted mantle residue. So-called “enriched” or long-term high Rb/Sr, LREE/HREE ((low Sm/Nd) ratios found in mantle-derived melts are due to derivation from sources with long histories of past melting and extraction of elements that are incompatible in minerals found in the Earth’s mantle. Most mantle-derived igneous rocks plot along the Mantle Array.