epsilon142Nd

epsilon142Nd is a geochemical parameter that expresses the deviation of a sample’s 142Nd/144Nd ratio from a reference chondritic uniform reservoir (CHUR). The quantity is reported in epsilon units, defined as ε142Nd = [(142Nd/144Nd)sample / (142Nd/144Nd)CHUR − 1] × 10^4, meaning deviations are given in parts per ten thousand. Measurements rely on high-precision mass spectrometry and careful corrections for mass fractionation, often using Nd standards.

142Nd is a radiogenic daughter product of the now-extinct radionuclide 146Sm, which decays with a half-life of about 68 million years. Because 146Sm decays relatively quickly in geological time, the 142Nd abundance in different reservoirs preserves information about early Sm/Nd fractionation in the first tens of millions of years after solar system formation. Consequently, epsilon142Nd values provide constraints on the timing and nature of early differentiation processes, such as mantle-crust separation on Earth and differentiation in planetary bodies like the Moon and Mars.

In practice, epsilon142Nd is used alongside other Nd isotopic measurements (for example 143Nd/144Nd) to interpret early differentiation histories and to compare terrestrial rocks, meteorites, and lunar samples. CHUR remains a common reference, though some studies employ alternative chondritic or planetary bulk reservoirs to test robustness of interpretations. Limitations arise from the analytical challenge of measuring 142Nd precisely and from potential post-formation alteration of Nd isotope systems. Overall, epsilon142Nd is a key tool in understanding early solar system evolution and planetary differentiation.