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

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We present in situ measurements of the unit-cell volume of a natural terrestrial ilmenite (Jagersfontein mine, South Africa) and a synthetic reduced ilmenite (FeTiO 3 ) at simultaneous high pressure and high temperature up to 16 GPa and 1273 K. Unit-cell volumes were determined using energy-dispersive synchrotron X-ray diffraction in a multi-anvil press. Mössbauer analyses show that the synthetic sample contained insignificant amounts of Fe 3+ both before and after the experiment. Results were fit to BirchMurnaghan thermal equations of state, which reproduce the experimental data to within 0.5 and 0.7 GPa for the synthetic and natural samples, respectively. At ambient conditions, the unit-cell volume of the natural sample [V 0 = 314.75 ± 0.23 (1σ) Å 3 ] is significantly smaller than that of the synthetic sample [V 0 = 319.12 ± 0.26 Å 3 ]. The difference can be attributed to the presence of impurities and Fe 3+ in the natural sample. The 1 bar isothermal bulk moduli K T0 for the reduced ilmenite is slightly larger than for the natural ilmenite (181 ± 7 and 165 ± 6 GPa, respectively), with pressure derivatives K 0 ′ = 3 ± 1. Our results, combined with literature data, suggest that the unit-cell volume of reduced ilmenite is significantly larger than that of oxidized ilmenite, whereas their thermoelastic parameters are similar. Our data provide more appropriate input parameters for thermo-chemical models of lunar interior evolution, in which reduced ilmenite plays a critical role.

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Although orthopyroxene (Opx) is present during a wide range of magmatic differentiation processes in the terrestrial and lunar mantle, its effect on melt trace element contents is not well quantified. We present results of a combined experimental and computational study of trace element partitioning between Opx and anhydrous silicate melts. Experiments were performed in air at atmospheric pressure and temperatures ranging from 1,326 to 1,420°C in the system CaO-MgO-Al 2 O 3 -SiO 2 and subsystem CaOMgO-SiO 2 . We provide experimental partition coefficients for a wide range of trace elements (large ion lithophile: Li, Be, B, K, Rb, Sr, Cs, Ba, Th, U; rare earth elements, REE: La, $ 0:058, and are all virtually independent of temperature. Cr and Co are the only compatible trace elements at the studied conditions. To elucidate charge-balancing mechanisms for incorporation of REE into Opx and to assess the possible influence of Fe on Opx-melt partitioning, we compare our experimental results with computer simulations. In these simulations, we examine major and minor trace element incorporation into the end-members enstatite (Mg 2 Si 2 O 6 ) and ferrosilite (Fe 2 Si 2 O 6 ). Calculated solution energies show that R 2? cations are more soluble in Opx than R 3? cations of similar size, consistent with experimental partitioning data. In addition, simulations show charge balancing of R 3? cations by coupled substitution with Li ? on the M1 site that is energetically favoured over coupled substitution involving Al-Si exchange on the tetrahedrally coordinated site. We derived best-fit values for ideal ionic radii r 0 , maximum partition coefficients D 0 , and apparent Young's moduli E for substitutions onto the Opx M1 and M2 sites. Experimental r 0 values for R 3? substitutions are 0.66-0.67 Å for M1 and 0.82-0.87 Å for M2. Simulations for enstatite result in r 0 = 0.71-0.73 Å for M1 and *0.79-0.87 Å for M2. Ferrosilite r 0 values are systematically larger by *0.05 Å for both M1 and M2. The latter is opposite to experimental literature data, which appear to show a slight decrease in r M2 0 in the presence of Fe. Additional systematic studies in Febearing systems are required to resolve this inconsistency and to develop predictive Opx-melt partitioning models for use in terrestrial and lunar magmatic differentiation models.