The global distribution of CO₂-rich magmas is determined by lithospheric thickness

Metasomatized lithospheric mantle plays a critical role in the petrogenesis of CO₂-rich magmas, which are important hosts of rare-earth element deposits. However, the relationship between the structure of the lithosphere and the global distribution of CO₂-rich magmas remains poorly quantified. Here we analyse the locations of young (<200 million years ago) continental intraplate CO₂-rich silicate magmas and magmatic carbonatites in conjunction with upper-mantle shear-wave velocity anomalies and lithospheric thickness estimates. Our results document systematic increases in lithospheric thickness with estimated magma CO₂ content from basanites (<5 wt% CO₂), which erupt through seismically slow and thin non-cratonic lithosphere, to nephelinites, melilitites and ultramafic lamprophyres, which occur within progressively faster, thicker lithosphere and, finally, to lamproites and kimberlites (<20 wt% CO₂), which are emplaced on thick cratonic lithosphere. Carbonatites are associated with lithospheric thicknesses similar to those of nephelinites, melilitites and ultramafic lamprophyres, implying the derivation of carbonatites from these mafic CO₂-rich silicate magmas via liquid immiscibility and/or fractional crystallization. We illustrate our lithospheric thickness-magma type relationship using Cretaceous-Pleistocene alkaline magmatism across western North America, ultimately demonstrating how lithospheric thickness controls the global occurrence of CO₂-rich magmas and, consequently, their associated rare-earth element deposits.