Sound Velocity of MgSiO3 Majorite Garnet up to 18 GPa and 2000 K.

MgSiO3 majorite is the most significant endmember of the Al‐deficient majorite garnets that form at depths higher than ∼400 km, in the mantle transition zone (MTZ). Here, we report elastic wave velocity measurements on polycrystalline MgSiO3 majorite samples, up to 18.4 GPa and 2000 K by ultrasonic interferometry techniques combined with in situ X‐ray diffraction measurements in a multianvil apparatus. Our data show MgSiO3 majorite has the lowest elastic moduli among silicate garnet endmembers, under the pressure and temperature conditions of the MTZ. These new data combined with those of other garnet endmembers allowed to estimate VP and VS of majorite with compositions relevant to the MTZ. The results suggest that variation of MgSiO3 component in garnet may play a role for interpreting seismic gradients atop the 410‐km discontinuity while below ∼520 km, the velocity contrasts hold too low values to explain the discrepancy between mineral physics data and seismological observations. Plain Language Summary: Majorite is an important constituent of mantle garnet that form at depths higher than ∼400 km, in the Earth's mantle transition zone. Here, we report elastic wave velocity measurements on polycrystalline MgSiO3 majorite samples, up to 18.4 GPa and 2000 K by ultrasonic interferometry techniques combined with in situ X‐ray diffraction measurements in a multianvil apparatus. Our data show MgSiO3 majorite has the lowest elastic moduli among other silicate garnets and therefore may play a role for interpreting the steep seismic gradients across the 410‐km discontinuity while below ∼520 km, it holds too low seismic contrast to explain the discrepancy between mineral physics data and seismological observations. Key Points: Sound velocities of MgSiO3 majorite were measured up to 18.4 GPa and 2000 KElastic softening increase gradually upon increasing MgSiO3 content in mantle garnetsPyrolite and mid‐ocean ridge basalts majorite are unlikely to explain the high VP and VS of the bottom mantle transition region [ABSTRACT FROM AUTHOR]