Mid-Ocean Ridge Mantle Heterogeneity: Insights from the 8°20' N Near-Axis Seamount Chain

Abstract:

Models for mid-ocean ridge (MOR) mantle composition and melting systematics predominantly arise from decades of research on axial MOR basalts. The MOR axis is where melts from the mantle focus and are known to homogenize to various extents in shallow magma chambers before eruption. The resulting re-equilibration and mixing of magmas prior to erupting biases interpretations of upper mantle source compositions and melting systematics beneath MORs. To better understand oceanic crustal formation and mantle melting, we examine the composition of lavas erupted along the 8°20’ N near-axis seamount chain, a ~200 km long volcanic lineament orientation perpendicular to the East Pacific Rise axis. These seamount lavas provide a unique opportunity to study near-ridge mantle processes, largely absent the effects of melt focusing and homogenization that occur beneath the axis. High-resolution bathymetric maps are combined with geochemical analyses of ~350 basalts collected during two research cruises in 2016 and 2018 on the R/V Atlantis using human-operated vehicle Alvin to sample lavas at locations mapped at high resolution by the autonomous underwater vehicle Sentry. Major and trace element concentrations and radiogenic isotope ratios are extremely variable along the seamount chain and often within a single seamount. The suite of samples represents a geochemical continuum of depleted, normal, and a high abundance of enriched basalts, spanning the full range of MOR and seamount compositions in the northeast Pacific. Petrologic modeling of the 8°20’ N sample suite suggests multiple mantle sources melted to variable extents are required to produce the heterogeneity observed on and within the seamounts. These results confirm that the sub-ridge mantle is much more heterogeneous than is observed by studying on-axis basalts alone, necessitating a substantive revision for models of upper mantle source variability and melting systematics at fast-spreading MORs.