Mantle heterogeneities induce time- and space-varying magmatism at the migrating ultraslow Southwest Indian Ridge

The link between mantle heterogeneity as depicted by geochemical and seismic studies and the magmatism emplaced on the divergent plates remains elusive. Here, we derive variations of magmatism from marine magnetic data and trace mantle heterogeneities beneath the eastern Southwest Indian Ridge over 27 Ma. Three states of magmatism, robust, intermediate, and starved, are identified from the amplitude of magnetic anomalies. Comparing the distribution of magmatism on orthogonal and oblique ridge sections supports an asthenospheric source, not moving with the plates and ridge, as the predominant source of magmatic variations. These variations are therefore linked to different amounts of fertile and refractory mantle in the asthenosphere sampled by the ultraslow spreading center. In the study area, fertile mantle heterogeneities are 10 to 60 km-wide with the ridge migrating at 5 km/Myr in a N30°E direction. This study offers a new approach to constrain deep mantle heterogeneities from shallow crustal observations.