Samarium-neodymium dating

Samarium-neodymium dating is useful for determining the age relationships of rocks and meteorites. Sm/Nd ratios are used to provide information on the source of igneous melts as well as to provide age data. The various reservoirs within the solid earth will have different values of initial ¹⁴³Nd/¹⁴⁴Nd ratios, especially with reference to the mantle.

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The usefulness of Sm-Nd dating is the fact that these two elements are rare earths. They are thus, theoretically, not particularly susceptible to partitioning during melting of silicate rocks. The fractionation effects of crystallisation of felsic minerals (see above) changes the Sm/Nd ratio of the resultant materials. This, in turn, influences the ¹⁴³Nd/¹⁴⁴Nd ratios with ingrowth of radiogenic ¹⁴³Nd.

The mantle is assumed to have undergone chondritic evolution, and thus deviations in initial ¹⁴³Nd/¹⁴⁴Nd ratios can provide information as to when a particular rock or reservoir was separated from the mantle within the Earth's past.

In many cases, Sm-Nd and Rb-Sr isotope data is used together.

Sm-Nd radiometric dating

Samarium has five naturally occurring isotopes and neodymium has seven.
The two elements are joined in a parent-daughter relationship by the alpha-decay of ¹⁴⁷Sm to ¹⁴³Nd with a half life of 1.06 x 10¹¹ years.

¹⁴⁶Sm is an extinct nuclide which decays via alpha emission to produce ¹⁴²Nd, with a half-life of 1.08 x 10⁸ years.

¹⁴⁶Sm is itself produced by the decay of ¹⁵⁰Gd via alpha-decay with a half-life of 1.06 x 10¹¹ years.

An isochron is calculated normally. As with Rb-Sr and Pb-Pb isotope geochemistry, the initial ¹⁴³Nd/¹⁴⁴Nd ratio of the isotope system provides important information on crustal formation and the isotopic evolution of the solar system.

Sm and Nd geochemistry

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. Samarium is accommodated more easily into mafic minerals, so a mafic rock which crystallises mafic minerals will concentrate neodymium in the melt phase faster relative to samarium. Thus, as a rock undergoes fractional crystallization from a mafic to a more felsic composition, the abundance of Sm and Nd changes, as does the ratio between Sm and Nd.

Thus, ultramafic rocks have low Sm and Nd and high Sm/Nd ratios. Felsic rocks have high concentrations of Sm and Nd but low Sm/Nd ratios (komatiite has 1.14ppm Sm and 3.59ppm Nd versus 4.65ppm Sm and 21.6ppm Nd in rhyolite).

The importance of this process is apparent in modeling the age of continental crust formation.

Crustal segregation modeling

The process of modelling crustal segregation ages work like this; We must assume a chondritic evolution for the Earth. In other words, we assume the Earth is formed from chondritic material, with a Nd/Nd ratio comparable to chondrite meteorites, and with similar bulk Sm/Nd ratio and abundances.

Because, as ¹⁴⁷Sm decays to ¹⁴³Nd at a known rate and we know the Sm/Nd ratio and current Nd/Nd ratio of the chondrites, the chondritic uniform reservoir (CHUR) initial Nd/Nd ratio can be mathematically modelled for any point in time. For certain assumptions CHUR is assumed to represent the mantle.

We take a rock, for example a granulite from the lower continental crust, and measure the Sm and Nd elemental abundances, and the abundances of isotopes of these elements within the rock. We can then calculate the present-day ¹⁴⁷Sm:¹⁴³Nd ratio and the current ¹⁴³Nd/¹⁴⁴Nd ratio. This can derive a radiometric age of the rock.

We can then mathematically calculate, given the current ¹⁴³Nd/¹⁴⁴Nd ratio and Sm:Nd elemental ratio of the granulite, an age at which the granulite ¹⁴³Nd/¹⁴⁴Nd ratio was equivalent to the CHUR ¹⁴³Nd/¹⁴⁴Nd ratio.

This calculation derives a time at which, hypothetically, the granulite could have been formed from the assumed reservoir by a melting event. This is known as the segregation age.