Tracking magma: Using Nd isotopic ratios as geochemical beacons

A fundamental assumption in interpreting trace-element isotopic ratio measurements is that ratios like ¹⁴³Nd/¹⁴⁴Nd of erupted basalts directly reflect the initial isotopic ratios within the melt source at the time of melting. This is a reasonable assumption, given that partial melting begins at the pore-scale, where melts and mineral grains are in equilibrium. However, as those melts are transported to the surface (e.g., within melt-rich channels), it is generally assumed that chemical exchange in magma-rock interaction leaves these isotopic ratios unchanged. In this work we challenge and investigate this assumption by quantitatively modeling disequilibrium magma-rock interaction during transport using 1-D two-phase advection-diffusion-exchange equations. We determine the extent to which this interaction can ``overprint’’ the starting magma isotopic compositions with the isotopic compositions of the surrounding rock. Because Nd isotopes are used as a fundamental probe of mantle melting over time, this work has important implications for our understanding of plate tectonics and Earth evolution.