ΔNb is a geochemical construct of Fitton et al. (1997) based on two trace-element ratios, Nb/Y and Zr/Y, plotted against one another on a log-log diagram. ΔNb is defined in relation to a diagonal line on the diagram separating Nb-enriched Icelandic tholeiite from Nb-depleted mid-ocean ridge basalt (MORB). Neither crystallization differentiation nor partial melting of a peridotite mantle source can drive compositions below the line, with negative ΔNb, to locations above the line, with positive ΔNb, or vice versa. The parameter ΔNb thus is taken to indicate distinct mantle sources.

Crystallization differentiation along the East Pacific Rise has almost no...