Abstract
Oceanic crusts at slow-spreading ridges are created either by symmetric spreading dominated by magmatic accretion or asymmetric spreading controlled by tectonic extension. Consecutive change in the spreading mode at the same ridge has been commonly attributed to variation in magma supply, but the mechanism controlling magma supply remains unclear. Here, we present geochemical analyses of peridotites and basalts from the Mid-Atlantic Ridge at 23°N, a region that has shifted from asymmetric spreading to symmetric spreading over the past 3.3 million years. Our results indicate that the asymmetric phase was characterized by a low magma flux, resulting from inherited ancient melt depletion in the asthenosphere. The subsequent increase in magma supply and shift to symmetric spreading corresponded with the arrival of more fertile mantle material. This work provides evidence of shifts between spreading modes driven by changing mantle compositions, highlighting the crucial role of asthenospheric heterogeneity in controlling the spreading modes at slow-spreading ridges.