Neutral buoyancy of titanium-rich melts in the deep lunar interior

The absence of moonquakes originating deeper than about 1,100 km (ref. 1) implies that the lower mantle of the Moon could be partially molten. Up to 30% melt by volume has been estimated to exist between about 1,200 and 1,350 km depth(2). However, the absence of recent volcanic activity at the Moon's surface implies that such deep partial melts must be at least as dense as their surroundings. Here we use a combination of in situ synchrotron X-ray absorption techniques and molecular dynamics simulations to determine the density range of primitive lunar melts at pressures equivalent to those in the lunar interior. We find that only melts that contain about 16 wt% titanium dioxide are neutrally buoyant at depths corresponding to the top of the proposed partial melt zone. These titanium-rich melts are formed by deep partial melting of titanium-rich rocks. As such rocks are thought to have formed at shallow levels during crystallization of the lunar magma ocean, we infer that a significant vertical transport of mass occurred before melt formation. Our measurements therefore provide evidence for a large-scale overturn of the lunar mantle shortly after crystallization of the magma ocean and point to the continuing influence of a dense, titanium-rich reservoir on lunar interior evolution.