[P-PS08] Lunar Science and Exploration

[PPS08-20] Laboratory measurements of P- and S-wave velocities of lunar orthopyroxene at high pressure and high temperature

*Yoshihiro Inoue¹, Yoshio Kono², Steeve Greaux¹, Sho Kakizawa³, Itaru Ohira⁴, Jiejun Jing¹, Yuji Higo³ (1.Geodynamics Research Center, Ehime University, 2.Department of Physics and Astronomy, Kwansei Gakuin University, 3.Japan Synchrotron Radiation Research Institute, 4.Department of Chemistry, Faculty of science, Gakushuin University)

Keywords:Lunar upper mantle, Orthopyroxene, Seismic wave velocity, High pressure

Understanding the composition and mineralogy of the lunar mantle is important to discuss nature and evolution of the Moon and other telluric planets. Petrological studies aiming at understanding the crystallization of lunar magma ocean have suggested that the lunar upper mantle should be composed mainly of Fe-bearing olivine and orthopyroxene (e.g., Lin et al., 2017). On the other hand, seismological studies have reported seismic wave velocity structure models of the lunar interior based on seismic observations from the Apollo mission data (e.g., Weber et al., 2011). Despite these knowledge, no studies have attempted to investigate if the olivine-pyroxene aggregate proposed by experiments can explain the seismic observations of the lunar upper mantle. Laboratory measurement of P- and S-wave velocities and density of minerals under high-pressure provides ways to compare elastic wave velocities of lunar mantle minerals with seismic models of the Moon's interior, which should provide important constraints on the composition and mineralogy of the lunar mantle.
In this study, we investigated P- and S-wave velocities and density of an orthopyroxene (Mg_0.84Fe_0.13Ca_0.03SiO₃) polycrystalline sample, which was synthesized after the results of lunar magma ocean crystallization experiments (Lin et al., 2017). We conducted ultrasonic measurement combined with synchrotron X-ray measurements in a multi-anvil press at high pressure and high temperature conditions up to 5.5 GPa and 1273 K at the BL04B1 beamline in SPring-8. P- and S-wave velocities, and unit cell volume of the orthopyroxene were analyzed by means of a finite strain equation of state to the 3^rd order, which yielded the elastic parameters: V_0,300 = 839.48(28) ų, K_S0,300 = 110.7(8) GPa, K_S' = 7.5(3), (∂K_S0,T/∂T)_P = -0.0207(42) GPa.K^-1, G_0,300 = 73.7(5) GPa, G' = 2.0(1), (∂G_0,T/∂T)_P = -0.0111(14) GPa.K^-1 and a₀ = 3.12(65) 10^-5.K^-1. Our new lunar pyroxene elasticity dataset was combined with elasticity data of olivine reported by previous study to model P- and S-wave velocities and density of lunar upper mantle rock aggregates with varying Fe-content. The models showed that lunar mantle aggregates containing 13 mol.% Fe, as reported by petrological studies, are relatively consistent with observed seismic wave velocities of the lunar upper mantle (e.g., Weber et al., 2011), but they cannot explain its density structure. Our model suggests that a Fe-enriched composition, with 20 mol.% iron in orthopyroxene and olivine, is required to explain both the observed seismic wave velocities and densities of the lunar upper mantle at the depths of 40-740 km.