Shear wave attenuation and dispersion in melt‐bearing olivine polycrystals: 2. Microstructural interpretation and seismological implications

Abstract

[1] The torsional forced oscillation tests of melt-bearing olivine aggregates reported by Jackson et al. [2004] consistently show a peak in attenuation that is absent from melt-free aggregates tested under similar conditions and grain sizes. Characterization by SEM shows that the melt resides in triple junction tubules and larger pockets as previously described. TEM imaging and EDS analysis reveals that olivine-olivine grain boundaries are characterized by a region 1 nm wide which is structurally and chemically distinct from olivine grain interiors. From the possible mechanisms that can produce an anelastic attenuation peak, melt squirt can be eliminated for our samples and experimental conditions. We attribute the observed attenuation peak to elastically accommodated grain boundary sliding, requiring that the grain boundaries are weak relative to olivine grain interiors but have a significantly higher viscosity than bulk melt. While the nanometer scale grain boundary structure in the melt-bearing aggregates is essentially the same as for melt-free aggregates studied previously, elastically accommodated sliding in the latter is apparently inhibited by tight three-grain edge intersections. The exponentially increasing high temperature background attenuation in both types of aggregate is attributed to diffusionally accommodated grain boundary sliding. Extrapolation to mantle grain sizes shows that the broad peak may be responsible for nearly frequency independent attenuation in partially molten regions of the upper mantle.

Local Earthquake Magnitude Scale andb‐Value for the Danakil Region of Northern Afar

“…Seismic attenuation in the upper mantle is thought to occur through two potential mechanisms: grain boundary sliding (Faul et al, 2004) and melt squirt (Mavko and Nur, 1975;O'Connell and Budiansky, 1977). Grain boundary sliding has been shown to cause a broad absorption peak in the seismic frequency band and so cannot explain the low attenuation that we observe (Faul et al, 2004). Hammond and Humphreys (2000) modeled the effect that melt squirt has on attenuation in the upper mantle with melt percentages ranging from 0% to 3%.…”

A rootless rockies—Support and lithospheric structure of the Colorado Rocky Mountains inferred from CREST and TA seismic data

“…For example, an $2.5% shear wave velocity decrement per 1% melt volume is commonly reported [Hier-Majumder and Courtier, 2011; Kreutzmann et al, 2004;Takei, 2000] while Hammond and Humphreys [2000] found velocity reductions in excess of 7.9% per 1% melt. The effect of melt on seismic attenuation is also unclear [Faul et al, 2004;Gribb and Cooper, 2000]. Surface and body-wave tomography results suggest that most of the upper mantle velocity variation in the western United States, including the CRM, can be largely attributed to temperature differences [Goes and van der Lee, 2002;Schmandt and Humphreys, 2010].…”

(An)elastic softening from static grain boundaries and possible effects on seismic wave propagation

“…In this paper we exclude features such as melts on interfaces and refer to Hiraga et al (2002) for intergranular melt phases with a characteristic thickness in the range of 0.5-10 nm and de Kloe et al (2000), Tan et al (2001) and Faul and Jackson (2004) for careful analysis of meltbearing systems involving olivine. While it is possible to extract information about the melt, we focus here on meltfree interfaces without grain boundary sliding or any other movement which trivially leads to dissipation of strain energy.…”