Compressibility of molten high-Ti mare glass: Evidence for crystal-liquid density inversions in the lunar mantle

Static compression density measurements of molten Apollo 14 black glass have been performed at liquidus conditions in the pressure range 1.0–11.5 GPa. A range of values for the Birch-Murnaghan isothermal bulk modulus ( K 2173 K = 13–23 GPa) and the pressure derivative ( dK dP = 7−3 , respectively) fit the data within the uncertainties; K 2173 K = 15.5 GPa and dK dP = 5.2 provides the best fit to the four measured pressure-density pairs corrected to isothermal conditions. The ultramafic high-Ti mare glass is significantly more compressible than other ultrabasic and basic silicate liquids previously studied. The compressibility appears to be related to TiO 2 content. The Apollo 14 black glass sample has the highest TiO 2 content of the pristine mare glasses, which are thought to be primary melts from a differentiated and strongly heterogeneous lunar mantle. Our density measurements confirm the prediction that molten black glass will be denser than equilibrium olivine and orthopyroxene at pressures greater than ∼2.0 and ∼0.5 GPa, respectively. These density inversions correspond to depths greater than approximately 410 and 100 km in the lunar mantle. Since orthopyroxene replaces olivine at ∼1.5 GPa (300 km) on the liquidus, our results constrain the maximum depth of buoyancy for high-Ti magmas at