Plate tectonic cycling modulates Earth's 3 He/ 22 Ne ratio

•The MORB mantle has an elevated 3He/22Ne relative to planetary precursors.•A new experiment verifies faster He than Ne lattice diffusion in olivine.•Kinetic fractionation beneath mid-ocean ridges elevates mantle 3He/22Ne.•MORB source mixing timescales for the 3He/22Ne system are ≥400 Myr.•3He/22Ne suggests the mantle is >96% degassed in primordial volatiles.