epsilonNd

Epsilon Nd (εNd) is a radiogenic isotope indicator used in geology and geochemistry to express how the 143Nd/144Nd ratio of a sample deviates from a reference chondritic uniform reservoir (CHUR). The value is unitless and is usually reported in parts per ten thousand.

The standard formula is εNd(t) = [(143Nd/144Nd)sample(t) ÷ (143Nd/144Nd)CHUR(t) − 1] × 10^4, where t is the time of interest and CHUR(t) is the expected Nd isotopic composition of the reference reservoir at that time, accounting for the decay of 147Sm to 143Nd. In practice, many studies use present-day CHUR as the reference and report εNd for a given sample at the present, while others explicitly consider the time evolution of CHUR.

Interpretation of εNd is linked to the Sm-Nd radioactive decay system (Sm-147 decays to Nd-143 with a half-life of about 106 billion years). A positive εNd indicates a source derived from depleted mantle with higher initial Sm/Nd and greater radiogenic ingrowth, whereas a negative εNd points to an enriched mantle or crustal material with less radiogenic Nd-143 over time.

Applications include tracing mantle differentiation, crustal formation and recycling, and constraining magma source characteristics in igneous rocks and sediments. Typical natural ranges span from roughly -20 to +15, with continental crust commonly negative and many mantle-derived rocks positive. Accurate interpretation relies on proper reference evolution, careful mass spectrometric analysis, and awareness of Sm/Nd variability in samples.