Single-crystal elasticity of iron-bearing majorite to 26 GPa: Implications for seismic velocity structure of the mantle transition zone

Single-crystal elastic moduli of synthetic iron-bearing majoritic garnet were measured using Brillouin spectroscopy up to a pressure of 26 GPa. We found that the iron-bearing majoritic garnet (Mg_0.79Fe_0.08Al_0.30Si_0.84O₃) remains elastically isotropic throughout the pressure range of this study. The shear and adiabatic bulk moduli and their pressure derivatives at zero pressure were determined to be μ₀ = 88.7(7) GPa, K_S0 = 166(2) GPa, (∂μ / ∂P)₀ = 1.28(8), (∂K / ∂P)₀ = 4.2(2). These values are almost indistinguishable, within the experimental uncertainties, from those obtained in previous studies of pure MgSiO₃ majorite and other solid solutions of the majorite-pyrope join. Our direct acoustic velocity measurements of iron-bearing majorite indicate that the incorporation of iron into majoritic garnet does not drastically affect its elastic moduli and their pressure derivatives. The measured pressure derivatives of iron-bearing majorite are about 3.6 times lower than required to account for the high seismic velocity gradients observed in the mantle transition zone.