page_title: Samarium–neodymium dating
section_title: The depleted-mantle model
hierarchical_section_title: Samarium–neodymium dating / The depleted-mantle model
caption_attribution_description: English: Graph to show the depleted mantle model of DePaolo (1981)
context_page_description: Samarium–neodymium dating is a radiometric dating method useful for determining the ages of rocks and meteorites, based on the radioactive decay of a long-lived samarium isotope to a radiogenic neodymium isotope. Neodymium isotope ratios together with samarium-neodymium ratios are used to provide information on age information and the source of igneous melts. 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.
  The usefulness of Sm–Nd dating stems from the fact that these two elements are rare earth elements and are thus, theoretically, not particularly susceptible to partitioning during sedimentation and diagenesis. Fractional crystallisation of felsic minerals changes the Sm/Nd ratio of the resultant materials.
context_section_description: Despite the good fit of Archean plutons to the CHUR Nd isotope evolution line, DePaolo 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). This led to the realization that Archean continental igneous rocks that plotted within the error of the CHUR line could instead lie on a depleted-mantle evolution line characterized by increasing Sm/Nd and ¹⁴³Nd/¹⁴⁴Nd ratios over time. To further analyze this gap between the Archean CHUR data and the young volcanic samples, a study was conducted on the Proterozoic metamorphic basement of the Colorado Front Ranges (the Idaho Springs Formation). The initial ¹⁴³Nd/¹⁴⁴Nd ratios of the samples analyzed are plotted on a ɛNd versus time diagram shown in the figure. DePaolo (1981) fitted a quadratic curve to the Idaho Springs and average ɛNd for the modern oceanic island arc data, thus representing the neodymium isotope evolution of a depleted reservoir. The composition of the depleted reservoir relative to the CHUR evolution line, at time T, is given by the equation
  ɛNd(T) = 0.25 T² – 3 T + 8.5.
  Sm-Nd model ages calculated using this curve are denoted as TDM ages. DePaolo (1981) argued that these TDM model ages would yield a more accurate age for crustal formation ages than TCHUR model ages – for example, an anomalously low TCHUR model age of 0.8 Gy from McCulloch and Wasserburg's Grenville composite was revised to a TDM age of 1.3 Gy, typical for juvenile crust formation during the Grenville orogeny.
caption: Graph to show the depleted-mantle model of DePaolo (1981)
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