NEARLY 60 years ago, Bernal1 proposed that a polymorphic phase transformation in olivine might be responsible for the seismic velocity discontinuity near 410 km depth in the mantle. Phase equilibria experiments2,3 have since shown that the olivine (α) to wadsleyite (β) transition in (Mg,Fe)2SiO4 occurs at the appropriate pressure (13.8 GPa) under mantle conditions. Comparison of laboratory measurements of the acoustic velocity contrast in the α–β system to the magnitude of the seismically observed discontinuity at 410 km provides a way to constrain the olivine content of the mantle at this depth. Here we report measurements of the full set of elastic moduli of single-crystal forsterite (α-Mg2SiO4) at pressures between 3 and 16 GPa, using Brillouin scattering in a diamond anvil cell. At 13.8 GPa, the aggregate compressional-and shear-wave velocities of α-Mg2SiO4 are 2.7 ± 0.7% lower than predicted from earlier low-pressure data4,5. From our data, and assuming a homogeneous mantle composition, the seismic velocity contrast at 410 km depth can be satisfied only by a mantle containing less than ~40% olivine. This is well below the olivine abundance assumed in peridotite-based upper-mantle models.
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