Mixing at mid-ocean ridges controlled by small-scale convection and plate motion

Abstract

The geochemical variability of lavas erupted at mid-ocean ridges is lowest where plate spreading rates are high, implying that large-scale plate motions mix the mantle—yet lavas erupted at slow-spreading ridges are also quite homogeneous. Numerical simulations suggest that small-scale convection in the mantle mixes and homogenizes the lavas erupted at slow-spreading ridges. Oceanic lavas are thought to be derived from different sources within the Earth’s mantle, each with a distinct composition1,2,3,4. Large-scale plate motions provide the primary mechanism for mixing these sources, yet the geochemical signature of lavas erupted at different mid-ocean ridges can still vary significantly5,6. Geochemical variability is low where plate spreading rates are high, consistent with plate-scale mixing5,6. However, slow-spreading centres, such as the Southwest Indian Ridge in the Indian Ocean, are also geochemically homogeneous, which is inconsistent with plate-scale mixing6,7. Here we use numerical simulations of mantle flow to study mantle mixing at mid-ocean ridges, under conditions with variable plate length and spreading rate. Our simulations reveal that small-scale convection in the mantle contributes significantly to mantle mixing at slow spreading rates; faster plate velocities and smaller plates inhibit small-scale convection. We conclude that whereas fast-spreading ridge lavas are well mixed by plate-scale flow, slow-spreading ridge lavas are mixed by small-scale convection.