How can we calculate the Sm/Nd elemental ratio for the depleted upper mantle and continental crust, given their respective ¹⁴³Nd/¹⁴⁴Nd ratios (0.5131 for the mantle and 0.5118 for the continental crust)?
a) By dividing the ¹⁴⁴Nd/¹⁴⁴Nd ratio of the mantle by its Sm concentration
b) By subtracting the ¹⁴⁴Nd/¹⁴⁴Nd ratio of the continental crust from its Sm concentration
c) By multiplying the ¹⁴⁴Nd/¹⁴⁴Nd ratio of the mantle by its Nd concentration
d) By dividing the ¹⁴⁴Nd/¹⁴⁴Nd ratio of the mantle by its Nd concentration