Extreme Mantle Heterogeneity Revealed by Geochemical Investigation of In Situ Lavas at the Central Mohns Ridge, Arctic Mid-Ocean Ridges

Mid-ocean ridge basalts reflect the mantle’s composition and reveal processes from melting to eruption. The Mohns and Knipovich Ridges have ultraslow spreading rates, low magma budgets and erupted lavas indicating various mantle domains. Here, we use geochemistry and isotope systematics of in situ samples from two axial volcanic ridges (AVRs) to study mantle heterogeneity and melt production. By linking chemical variations to high-resolution bathymetry and age data, we document systematic changes over time in the mantle source of the volcanic sequence. At Mohns Ridge AVR-M10 (72.3°N), we observed significant variations in chemistry (e.g., (La/Sm)_N from 0.7 to 2.9) and isotope systematics in basaltic samples from a small area (∼1 km²), suggesting the emplacement of multiple small-volume lava flows. Pb isotope variations, for example, ²⁰⁶Pb/²⁰⁴Pb (17.91–18.76), are comparable with the observed range along the entire Mohns and Knipovich Ridges. Temporal constraints document that erupted basalts have changed from highly radiogenic Pb compositions to a more depleted signature within 30 ka. To explain the extreme variations in the erupted lavas at the Mohns Ridge, the mantle would need to be highly heterogeneous in composition with effective melt extraction and limited mixing prior to eruption. We use the highly heterogenous mantle underneath the Mohns Ridge to understand the melt extraction processes and mixing of melts and propose a two-stage melting model: continuous generation of enriched melts from a deep and fertile source in the first stage, while depleted melts from a shallower and more refractory mantle occur sporadically and simultaneously with the intermittent ascent of diapirs.