P‐ and S‐wave Velocity Measurement of Lunar Orthopyroxene up to 5.5 GPa and 1,273 K: Implication for the Iron Content of the Lunar Upper Mantle

Abstract Seismological observations in the lunar mantle, in conjunction with experimental knowledge on the elastic wave velocities and density of lunar mantle minerals, provide important constraints on the composition and mineralogy of the lunar mantle. Here we report elastic wave velocities and density of a lunar orthopyroxene (Mg 0.84 Fe 0.13 Ca 0.03 SiO 3 ) up to 5.5 GPa and 1,273 K. The result shows that the bulk and shear moduli of orthopyroxene decrease with increasing iron content. Based on the mineral elasticity data, we modeled the P‐ and S‐wave velocities and density of petrologically suggested lunar upper mantle rock composition. The petrological lunar upper mantle rock model shows consistent seismic wave velocities with those observed in the lunar upper mantle whereas markedly lower density. Our modeling suggests an iron‐rich (Fe/(Mg + Fe) = 0.20) lunar upper mantle to explain P‐ and S‐wave velocities and density of the lunar upper mantle at 40–740 km depth.