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 (Mg0.79Fe0.08Al0.30Si0.84O3) 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, KS0 = 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.