COMPRESSIBILITY OF MOLTEN HIGH-TI MARE GLASS - EVIDENCE FOR CRYSTAL-LIQUID DENSITY INVERSIONS IN THE LUNAR MANTLE

摘要

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-2173K = 13-23 GPa) and the pressure derivative (dK/dP = 7-3, respectively) fit the data within the uncertainties; K-2173K = 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 TiO2 content. The Apollo 14 black glass sample has the highest TiO2 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 similar to 2.0 and similar to 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 similar to 1.5 GPa (300 km) on the liquidus, our results constrain the maximum depth of buoyancy for high-Ti magmas at <400 km in the lunar mantle. [References: 47]