Density of high-Ti basalt magma at high pressure and origin of heterogeneities in the lunar mantle

摘要

The density of the Apollo 14 black glass melt, which has the highest TiO_2 content of pristine mare glasses, was measured to 4.8GPa and 2100K using an X-ray absorption method. A fit of the pressure-density-temperature data to the high-temperature Birch-Murnaghan equation of state yielded the isothermal bulk modulus K_(T0)=9.0±1.2GPa, its pressure derivative K_0'=16.0±3.4, and the temperature derivative of the bulk modulus (?K_T/?T)_P=-0.0030±0.0008GPa/K at 1700K. The high-Ti basalt magma is less dense than the lunar mantle below about 1.0GPa. Therefore, the high-Ti basalt magma produced in the hybridized source (100-200km) can ascend to the lunar surface. The basalt formed at the higher pressure could not ascend but move downwards, and solidify in the lunar mantle. The solidified high-Ti basalt components can create chemical heterogeneities in the lunar mantle and can cause the low-velocity anomalies observed seismologically.