Fractionation, Mass Independent and Dependent

Definition

Isotope fractionation is referred to as mass dependent when observed isotopic abundances deviate smoothly and monotonically with the masses of the isotopes from those in the reference material. Both kinetic and equilibrium processes are known to account for this most common form of isotope fractionation. Alternate patterns are referred to as mass independent and are known for O and S in some specific environments and are often related to photochemistry in the solar nebula and in planetary atmospheres.

Overview

Isotope fractionation varies approximately with the difference of the isotopic masses. Using oxygen isotopes as an example, the mass difference ¹⁸O − ¹⁶O is +2 and δ¹⁸O will therefore be twice the δ¹⁷O $\left(\mathrm{where},\;\mathrm{e}.\mathrm{g}.,\;{\delta}^{18}\mathrm{O}\; is\left[{\left({}^{18}\mathrm{O}/{}^{16}\mathrm{O}\right)}_{\mathrm{sample}}/{\left({}^{18}\mathrm{O}/{}^{16}\mathrm{O}\right)}_{\mathrm{std}}-1\right]\right).$

In a δ¹⁷O versus δ¹⁸O diagram,...