Low-Frequency Seismic Properties of Olivine-Orthopyroxene Mixtures

Although the seismic properties of polycrystalline olivine have been the subject of systematic and comprehensive study at seismic frequencies, the role of orthopyroxene as the major secondary phase in the shallow parts of the Earth's upper mantle has so far received little attention. Accordingly, we have newly prepared three synthetic melt-free polycrystalline specimens containing different proportions of olivine (Ol, Fo₉₀) and orthopyroxene (Px, En₉₀) by hot pressing precursor powders produced with the solution-gelation method at 1,200°C and 300 MPa. The resulting specimens (of Ol₉₅Px₅, Ol₇₀Px₃₀, and Ol₅Px₉₅ phase proportions) were mechanically tested by torsional forced-oscillation from 1,300 or 1,200–400°C during staged cooling under a confining pressure of 200 MPa. Within the observational window (1–1,000 s), the shear modulus and dissipation vary monotonically with period and temperature for each of the tested specimens. There is no evidence of the superimposed high-temperature dissipation peak reported by Sundberg and Cooper (2010, https://doi.org/10.1080/14786431003746656) for an Ol₆₀Px₄₀ specimen derived from natural precursor material and containing ∼1.5% melt. The forced-oscillation data for each specimen are well-described by a model based on an extended Burgers-type creep function. The findings suggest that an olivine-based model for high-temperature viscoelasticity in upper mantle olivine requires only modest adjustment of the unrelaxed shear modulus to accommodate the role of orthopyroxene.