Long-lived replenishment of magmas in the oceanic upper mantle revealed by chromite nodules in ophiolites

Magmas produced at depth pass through the upper mantle before they form oceanic crust at spreading centers, but the timescales and thermal evolution of magma replenishment remain poorly constrained. Chromite nodules of ophiolites formed in magma conduits in the upper mantle and record these processes. Here, we show that some of these nodules exhibit uniform sizes, monocrystalline textures, and low dihedral angles (~20°), characteristic of textural evolution within basaltic magma with complete, isotropic grain-boundary wettability. We therefore use a Monte Carlo Potts model to numerically simulate this forming process, demonstrating that chromite nodules form via Ostwald ripening within magma conduits. Calibrated with experiments, the growth kinetics suggest that these nodules form up to 10⁴–10⁶ years. Our study reveals a prolonged melt flux in the upper mantle with cooling rates of ~10⁻² to 10⁻⁴ °C/yr, lower than magmas in the lower crust (~10⁻¹–10⁻² °C/yr), indicative of a strong contrast in geothermal gradients across the Moho boundary. Chromite nodules grow through Ostwald ripening over timescales of 10⁴ to 10⁶ years, recording prolonged magma replenishment and slow cooling in the oceanic upper mantle, according to combined microstructural observations and Monte Carlo Potts modeling of chromite nodule formation in ophiolites.