Early-crystalized bridgmanite as a source of primitive noble gases

Ocean island basalts (OIB) provide a unique window into the deep Earth, with one of the most valuable signals coming from the He and Ne noble gases. The isotopes of these elements point to a “primitive” volatile reservoir in the deep interior that has remained far less degassed than the upper mantle sampled by MORBs. However, fundamental questions remain unresolved. In particular, where do these volatiles reside in the mantle and how have they persisted through billions of years of mantle convection and planetary differentiation? Here, we use ab initio calculations to determine the partitioning behaviour of He and Ne between Al-bearing bridgmanite and silicate melt under lower-mantle conditions. Our results show that substantial quantities of both noble gases will be incorporated into bridgmanite crystallized from the magma ocean, particularly near the core-mantle boundary. The concentration of He and Ne is so high that if even tiny fractions of just 0.3% or less of the early-crystallized bridgmanite avoided degassing after initial crystallisation, it could preserve enough primordial He and Ne to supply the OIB noble gas flux throughout Earth’s history. Mantle convection models show that the preservation of such small amounts is indeed likely and so this is a simple and natural explanation for the noble gas signatures at OIBs.