“Minerals exercise a considerable degree of selectivity in admitting REEs into their crystal structures (Faure and Mensing 2005). Feldspar, biotite, and apatite tend to concentrate the light REEs (the Ce group), whereas pyroxenes, amphiboles, and garnet concentrate the heavy REEs (the Gd group). The selectivity of the rock-forming minerals for the light or heavy REEs obviously affects the REE concentrations of the rocks in which those minerals occur. Sm and Nd both belong to the light REEs, so they tend to concentrate in feldspar, biotite, and apatite. Thus the Sm and Nd concentrations in calc-alkaline plutonic and volcanic igneous rocks range from <1 ppm in ultramafic rocks to about 8 ppm Sm and 45 ppm Nd in granite. Alkali-rich igneous rocks have consistently higher Sm and Nd concentrations than the calc-alkaline suite, ranging up to about 15 ppm Sm and 85 ppm Nd. “Minerals exercise a considerable degree of selectivity in admitting REEs into their crystal structures (Faure and Mensing 2005). Feldspar, biotite, and apatite tend to concentrate the light REEs (the Ce group), whereas pyroxenes, amphiboles, and garnet concentrate the heavy REEs (the Gd group). The selectivity of the rock-forming minerals for the light or heavy REEs obviously affects the REE concentrations of the rocks in which those minerals occur. Sm and Nd both belong to the light REEs, so they tend to concentrate in feldspar, biotite, and apatite. Thus the Sm and Nd concentrations in calc-alkaline plutonic and volcanic igneous rocks range from <1 ppm in ultramafic rocks to about 8 ppm Sm and 45 ppm Nd in granite. Alkali-rich igneous rocks have consistently higher Sm and Nd concentrations than the calc-alkaline suite, ranging up to about 15 ppm Sm and 85 ppm Nd.
“Sm and Nd exhibit an unusual geochemical behavior, which arises from what is known as the “lanthanide contraction” (Faure and Mensing 2005). This contraction results from the way electrons fill their f shell orbitals. As a consequence, the ionic radius of Sm (Z = 62) is smaller than that of Nd (Z = 60). Even though the difference in the radii is small (Nd3+ = 1.08 Å; Sm3+ = 1.04 Å), Nd is preferentially concentrated in the liquid phase during partial melting of silicate minerals, whereas Sm remains in the residual solids. For this reason, basalt magmas have lower Sm/Nd ratios than the source rocks from which they formed. Thus this preferential partitioning of Nd into the melt phase has caused the rocks of the continental crust to be enriched in Nd relative to Sm compared to the residual rocks in the lithospheric mantle. “Sm and Nd exhibit an unusual geochemical behavior, which arises from what is known as the “lanthanide contraction” (Faure and Mensing 2005). This contraction results from the way electrons fill their f shell orbitals. As a consequence, the ionic radius of Sm (Z = 62) is smaller than that of Nd (Z = 60). Even though the difference in the radii is small (Nd3+ = 1.08 Å; Sm3+ = 1.04 Å), Nd is preferentially concentrated in the liquid phase during partial melting of silicate minerals, whereas Sm remains in the residual solids. For this reason, basalt magmas have lower Sm/Nd ratios than the source rocks from which they formed. Thus this preferential partitioning of Nd into the melt phase has caused the rocks of the continental crust to be enriched in Nd relative to Sm compared to the residual rocks in the lithospheric mantle.
“Even though the concentrations of Sm and Nd reach high values in accessory phosphate minerals such as apatite and monazite and in carbonatites, these minerals and carbonatites are still more enriched in Nd than in Sm and hence their Sm/Nd ratios are less than 0.32. Among the rock-forming silicate minerals, garnet is the only one with a high Sm/Nd ratio (0.54) even though its concentrations of Sm and Nd are both low (1–2 ppm). Several other rock-forming silicate minerals, such as K-feldspar, biotite, amphibole, and clinopyroxene, have higher Sm and Nd concentrations than garnet, but their Sm/Nd ratios are less than 0.32 in most cases. “Even though the concentrations of Sm and Nd reach high values in accessory phosphate minerals such as apatite and monazite and in carbonatites, these minerals and carbonatites are still more enriched in Nd than in Sm and hence their Sm/Nd ratios are less than 0.32. Among the rock-forming silicate minerals, garnet is the only one with a high Sm/Nd ratio (0.54) even though its concentrations of Sm and Nd are both low (1–2 ppm). Several other rock-forming silicate minerals, such as K-feldspar, biotite, amphibole, and clinopyroxene, have higher Sm and Nd concentrations than garnet, but their Sm/Nd ratios are less than 0.32 in most cases.
“Sm and Nd each have seven naturally-occurring isotopes. Of these 147Sm, 148Sm, and 149Sm are all radioactive, but the latter two have such long half-lives (about 1016 years) that they are not capable of producing measurable variations in the daughter isotopes 144Nd and 145Nd, even over supposed conventional cosmological intervals (1010 years) (Dickin 2005). Yet 147Sm only has an abundance of 15.0% in naturally occurring Sm (Lide and Frederikse 1995). Although the half-life of 147Sm is also very long (currently determined as 106 billion years), it decays by α-particle emission to 143Nd, a stable isotope of Nd. The relevant decay scheme is often depicted as: “Sm and Nd each have seven naturally-occurring isotopes. Of these 147Sm, 148Sm, and 149Sm are all radioactive, but the latter two have such long half-lives (about 1016 years) that they are not capable of producing measurable variations in the daughter isotopes 144Nd and 145Nd, even over supposed conventional cosmological intervals (1010 years) (Dickin 2005). Yet 147Sm only has an abundance of 15.0% in naturally occurring Sm (Lide and Frederikse 1995). Although the half-life of 147Sm is also very long (currently determined as 106 billion years), it decays by α-particle emission to 143Nd, a stable isotope of Nd. The relevant decay scheme is often depicted as:
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