Magmatic filtering of mantle compositions at mid-ocean-ridge volcanoes

Earth’s dominant form of magmatism occurs at mid-ocean ridges (MORs), producing the igneous crust for two-thirds of the planet’s surface and conveying significant heat and material fluxes from the mantle to the world’s oceans. Mid-ocean-ridge basalt (MORB) magmas form from upwelling compositionally heterogeneous mantle1 by aggregation of near-fractional melts beneath spreading centres2. Multiple processes modify magmatic composition during ascent and storage before eruption3–5. MORBs can be used to probe upper-mantle composition1,5, provided the chemical signals imparted by processes such as melting, magma transport, melt-rock reactions, mixing and differentiation can be disentangled from those of the mantle source rocks. Melting conditions vary beneath MORs in multiple ways. Models predict that mantle composition and flow can affect the shape of the MOR melting region6, and that mantle of heterogeneous composition will melt progressively7. Uranium-series (U-series) radioactive disequilibria in MORBs constrain the melting, melt extraction and melt accumulation rates from decamillenial8,9 to decadal timescales10. Globally, MORB chemistry depicts variations in how MOR volcanism samples the mantle in space and time — geography, spreading rate and crustal structure emerge as important controlling variables1,3,5. Lava composition patterns at 10 to 100-km length scales mimic physical ridge segmentation, and related variations in melt supply and uppermost mantle and crustal temperatures. The oceanic crust imparts multiple chemical fingerprints on erupted compositions. A relatively small fraction of MOR magma erupts to form MORB (typically <20%); the rest crystallizes as plutons at midto lower-crustal levels, forming a substrate that melts can react with11. Magma mixing and differentiation in composite reservoirs also significantly affects MORB compositon12. The global MOR is a large and complex volcanic system in which site-specific geological conditions affect MORB chemistry. These effects are being studied at high resolution, in ever more locales to investigate the scales and magnitudes of mantle compositional heterogeneity. Integrated geochemical, geophysical and geological data indicate that mantle compositions are significantly modified in range, magnitude and length scale at MORs by pre-eruptive Magmatic filtering of mantle compositions at mid-ocean-ridge volcanoes