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, Fo(90)) and orthopyroxene (Px, En(90)) by hot pressing precursor powders produced with the solution-gelation method at 1,200 degrees C and 300 MPa. The resulting specimens (of Ol(95)Px(5), Ol(70)Px(30), and Ol(5)Px(95) phase proportions) were mechanically tested by torsional forced-oscillation from 1,300 or 1,200-400 degrees 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, ) for an Ol(60)Px(40) specimen derived from natural precursor material and containing similar to 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.

Automated summary

This study experimentally investigated the seismic properties of olivine and orthopyroxene in the mantle, highlighting the significant role of orthopyroxene in the shallow parts of the mantle. The findings suggest that adjustments to the shear modulus and dissipation of high-temperature mantle olivine are needed to accommodate the contribution of orthopyroxene.