Deep evolution of carbonated magmas controls ocean island basalt chemistry

The composition of ocean island basalts (OIBs) is key to understanding mantle differentiation and quantifying intra-plate carbon outflux. Existing petrogenesis models fail to simultaneously reproduce the low SiO₂ and low SiO₂/FeO^T characteristics of alkalic OIBs and ignore melt-orthopyroxene reactions in the lithosphere that may further elevate the SiO₂ content of primary magmas. Here we show experimentally that high-degree (>50%) high-pressure crystallization of carbonated primary magmas at the base of lithosphere drastically reduces both the SiO₂ content and SiO₂/FeO^T ratio, due to the combined effects of clinopyroxene and garnet precipitation and carbonates dissolution. The major-element chemistry of alkalic OIBs can be quantitatively reproduced by considering varying degrees of crystallization, melt-orthopyroxene reactions, and source CO₂ content. Our results imply high intra-plate carbon outfluxes and support the observed association of low OIB SiO₂ contents with low mantle potential temperatures, as slower magma transport at lower temperatures leads to more extensive crystallization and reaction.