Hydrogen isotopes reveal water leakage from the core.

Chemical heterogeneities in Earth's deep mantle may record either preserved primordial reservoirs or later core-mantle exchange, but distinguishing between these origins remains challenging. Hydrogen isotopes offer a sensitive tracer, yet whether magma ocean crystallization could generate a deuterium-depleted deep reservoir has not been quantitatively evaluated. Here, we use machine learning-accelerated path-integral simulations to determine equilibrium hydrogen isotope fractionation between silicate melt and bridgmanite, ringwoodite, and wadsleyite under magma ocean conditions. Incorporating these fractionation factors into a magma ocean crystallization model shows that mineral-melt fractionation is intrinsically weak, producing an essentially homogeneous D/H distribution in the primitive mantle. Magma ocean crystallization therefore cannot generate the extremely low δD values observed in some ocean island basalts. Combined with exceptionally high 3He/4He ratios, these signatures more plausibly reflect selective transfer of primordial volatiles from Earth's core, implying sustained volatile exchange across the core-mantle boundary.