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International audienceThe absence of moonquakes originating deeper than about 1,100 km (ref. 1) implies that the lower mantle of the Moon couldbepartiallymolten.Upto30%meltbyvolumehas been estimated to exist between about 1,200 and 1,350km depth2. 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

We have studied the formation conditions of Al-rich chondrules by doing isothermal and dynamic crystallization experiments at one atmosphere on four different chondrule analogue compositions within the pure CaO-MgO-Al 2 O 3 -SiO 2 system. For the dynamic crystallization experiments, we cooled from both liquidus and subliquidus peak temperatures (T max ), at cooling rates from 5-1000°C/h. The starting compositions include two with anorthite and two with forsterite as the dominant liquidus phases, all at or near spinel-saturation. One of each pair evolves towards diopside crystallization, and the others cordierite or enstatite crystallization, giving a total of four completely different crystallization sequences analogous to the four basic varieties of Al-rich chondrule recently proposed. Bulk composition is the main controlling factor, both in terms of mineralogy and texture. The textures of the anorthite-rich compositions are more sensitive to T max than they are to cooling rate, whereas the textures of the forsterite-rich compositions are more sensitive to cooling rate. Comparisons of natural Al-rich chondrules having similar compositions to our synthetic analogues indicate that the natural objects reflect a range of peak heating temperatures, $1400-1500°C, and cooling rates of 10-500°C/h for porphyritic chondrules and possibly higher (1000°C/h) for barred chondrules. These conditions are consistent with the conditions inferred for ferromagnesian chondrules but differ from those inferred for some calcium-aluminum-rich inclusions.