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

This study is motivated by the observed variability in trace element isotopic and chemical compositions of primitive (SiO2< ${\mathrm{O}}_{2}< $52 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 εNd ${\varepsilon }_{Nd}$ signatures consistent with continental lithospheric mantle (CLM). In locations with the oldest CLM (Proterozoic and Archean), Cenozoic basalts with low Ta/Th have εNd ${\varepsilon }_{Nd}$ 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 εNd ${\varepsilon }_{Nd}$ 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 εNd ${\varepsilon }_{Nd}$ 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. Plain Language Summary: Over the past half century, the abundances and isotopic ratios of trace elements such as Nd in basalts have been widely used to infer aspects of the source region where mantle melting occurs. This assumes that when the mantle melts, trace element characteristics of source rocks are inherited by the generated melts and these are not further modified as melts ascend through the tectonic plate (lithosphere) to be erupted as basalt. We reassess this assumption inspired by enigmatic observations from southwest North America, where basalts that have undergone minimal processing (primitive basalts), show trace element characteristics of both the mantle lithosphere and of the asthenosphere. We use 1D transport models to show that melts moving in channels through the lithosphere may undergo significant trace element changes by interacting with the walls as they ascend, thereby explaining the observations with geologically reasonable scenarios. Key Points: Trace elements in primitive basalts from southwest North America suggest both asthenosphere‐ and lithosphere‐related signaturesOpen system interactions between ascending channelized melts and wallrock may affect the trace element composition of erupted basalts1D transport models quantify the role of diffusion and wallrock assimilation to produce significant Nd isotopic shifts over 10 km of transport [ABSTRACT FROM AUTHOR]