"Laboratory measurements of seismic wave speeds and attenuation in upper-mantle materials"
Ian Jackson
Professor of Research School of Earth Sciences, Australian National University
Foreign visiting Professor, GRC.
30 July 2003 17:00-18:00
Meeting room, 6th floor, Advanced Research Building, Ehime University
Fine-grained polycrystalline aggregates of Fo90 olivine, with and without
added basaltic melt, have been prepared by hot-isostatic pressing and characterised
by light and electron microscopy, including TEM and EBSD, and infrared
spectroscopy. Mechanical tests have been performed in an internally heated
gas-medium high-pressure apparatus with torsional forced oscillation/microcreep
techniques that provide access to low shear-strain amplitudes (< 10-5)
and seismic periods (1-1000 s) at high temperature T (to 1300C) and 200
MPa confining pressure. Significant strain-energy dissipation Q-1 and associated
frequency dependence (dispersion) of the shear modulus, both indicative
of viscoelastic rheology, seen above 900C. The melt-free and melt-bearing
materials display qualitatively different behaviour. In the absence of
melt (melt fraction << 0.1%), olivine grains meet at triple junctions
< 2 nm in diameter and the strain-energy dissipation 1/Q varies monotonically
with frequency and temperature in the manner described as high-temperature
background. The presence of a small basaltic melt fraction (0.004 - 0.037
at maximum temperatures of 1240-1300C) results in the rounding of olivine
grain edges at triple junction tubules now > 200 nm in diameter. During
staged cooling below 1200?C the melt forms a fine intergrowth of plagioclase
crystallites and residual glass. The presence of a broad dissipation peak
superimposed on the monotonically frequency and temperature dependent background
clearly distinguishes materials containing as little as ~ 0.5 % melt from
their melt-free equivalents. In both types of material the observed dissipation
is attributed to grain-boundary sliding involving a mixture of elastic
and diffusional accommodation. The difference in mechanical behaviour is
ascribed to the rounding of olivine grain edges at melt-bearing grain-edge
tubules - allowing sufficient localisation of dissipation in frequency-temperature
space to produce a resolvable peak attributable to elastic accommodation.
Empirical models have been developed that adequately describe the dependence
of Q-1 upon period, temperature, average grain size and, for the melt-bearing
materials, maximum melt fraction. These models have been extrapolated to
the larger grain sizes and P-T conditions of the upper mantle. The inferred
levels of attenuation for melt-free material are broadly consistent with
those observed seismologically suggesting that grain-boundary diffusional
processes might account for much of the wave speed variability and attenuation
in the mantle. The superposition of the melt-related dissipation peak upon
the monotonic background is such that, for appropriate combinations of
grain size, melt fraction and temperature, nearly frequency-independent
seismic-wave attenuation is expected.
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