Nd Isotopic Equilibration During Channelized Melt Transport Through the Lithosphere: A Feasibility Study Using Idealized Numerical Models

Abstract

This study is motivated by the observed variability in trace element isotopic and chemical compositions of primitive (Si52 wt %) basalts in southwest North America (SWNA) during the Cenozoic transition from subduction to extension. Specifically, we focus on processes that may explain the enigmatic observation that in some localities, basalts with low Ta/Th, consistent with parental melts in a subduction setting, have ${\epsilon }_{Nd}$ signatures consistent with continental lithospheric mantle (CLM). In locations with the oldest CLM (Proterozoic and Archean), Cenozoic basalts with low Ta/Th have ${\epsilon }_{Nd}$ well below zero. We model channelized magma transport through the CLM using simple 1D transport models to explore the extent to which diffusive and reactive mass exchange can modify Nd isotopic compositions via open system melt-wallrock interactions. For geologically reasonable channel spacings and volume fractions, we quantify the reactive assimilation rates required for incoming melt with a different ${\epsilon }_{Nd}$ than the wall-rock to undergo a substantial isotopic shift during $\approx$10 km channelized melt transport. In the presence of grain boundaries, enhanced diffusion between melt-rich channels and melt-poor surrounding rock contributes to isotopic equilibration, however this effect is not enough to explain observations; our models suggest a significant contribution from reactive assimilation of wall-rock. Additionally our models support the idea that the observed covariability in Ta/Th and ${\epsilon }_{Nd}$ in Cenozoic basalts cannot be attributed to transport alone and must also reflect the transition from subduction-related to extension-related parental melts in SWNA.