Experimental Evidence Supporting an Overturned Iron‐Titanium‐Rich Melt Layer in the Deep Lunar Interior

Dense Fe‐Ti‐rich cumulates, formed as the last dregs of the lunar magma ocean, are thought to have driven a large‐scale overturn of the lunar mantle over 4 Ga ago. Analysis of lunar seismic data has implied that some of the overturned bodies may have reached the lunar core‐mantle boundary and remained there until the present day as a partially molten layer. However, whether such a molten layer could be stable during >4 Ga of post‐magma‐ocean lunar history and explain lunar seismic observations remains poorly constrained. Here, we report the first sound velocity measurements on a Fe‐Ti‐rich lunar melt up to conditions of the lowermost lunar mantle. Our results suggest that a partial melt layer with at least 20% overturned Fe‐Ti‐rich melt can be trapped atop the lunar core‐mantle boundary until the present day, strongly influencing the thermochemical evolution of the lunar interior. Plain Language Summary The Moon may have formed from a lunar magma ocean (LMO). During the late stage of LMO, a layer of dense Fe‐Ti‐rich phases would crystallize, causing the lunar mantle overturn due to gravitational instability. Such a process could introduce Fe‐Ti‐rich bodies into the lunar interior, where they may become part of the source for Fe‐Ti‐enriched lunar basalts and pyroclastic volcanic glasses. Apollo seismic data have implied the potential presence of a partially molten region in the deep lunar interior, likely corresponding to the melting of those overturned Fe‐Ti‐rich bodies. However, whether such a molten layer could be stable and match with lunar seismic observations is still not well‐known. In this study, we have experimentally determined the sound velocity of a lunar Fe‐Ti‐rich melt for the first time, up to conditions of the deep lunar mantle. Our new data help tightly constrain the velocity and density profiles of Fe‐Ti‐rich melt in the lunar interior. By comparing with lunar seismic observations, we find that a partial melt layer with at least 20% overturned Fe‐Ti‐rich melt could be stable above the lunar core‐mantle boundary. Key Points First sound velocity measurements on a Fe‐Ti‐rich lunar melt up to the deep lunar mantle conditions A partial melt layer with at least 20% overturned Fe‐Ti‐rich melt could be stable atop the lunar core‐mantle boundary Fe‐Ti‐rich melts are able to rise from their estimated source regions to form the Fe‐Ti‐enriched lunar basalts and glasses