　　The 6th Global COE (24th GRC) International Frontier Seminar

"Laboratory-based Interpretation of Upper-mantle Seismic Tomograms: Progress and Prospects"

Prof. Ian Jackson
Research School of Earth Sciences, Australian National University, Australia

3 March 2009, 17:00-18:00
Room 101, First floor, Kogi-to Building, Faculty of Science, Ehime University

　　　　　　　　　
　During the past decade, torsional forced-oscillation techniques have been used intensively to probe the high-temperature viscoelastic behaviour of fine-grained synthetic polycrystalline olivine. The result has been substantial progress in understanding the grain-boundary processes responsible for the attenuation and associated shear modulus dispersion in such materials. We review the experimental dataset from the ANU and University of Wisconsin (UW) laboratories for fine-grained olivine polycrystals, derived from either natural or synthetic precursors, and for a dunite mylonite. The ANU data have been re-processed to correct for the newly documented influence of interfacial compliance as part of an updated assessment - with emphasis on the grain-size sensitivity of the viscoelastic relaxation and the influence of a small melt fraction. There is a striking qualitative similarity between the data obtained in the ANU laboratory and by Cooper and his colleagues at UW. Quantitatively, the reconstituted dunite specimens tested by Cooper et al. are consistently more lossy, and of correspondingly lower shear modulus, than the ANU melt-free specimens made from pure laboratory reagents by a solution-gelation process. This difference is plausibly attributed to the presence of a small melt fraction in the UW reconstituted dunites. Alternative strategies for description of the contrasting behaviour of genuinely melt-free and melt-bearing olivine are reviewed, and their differing seismological implications are addressed. Finally, we emphasise that the experimental work of the past decade has been focussed on grain-size sensitive viscoelastic relaxation attributed to grain-boundary sliding in essentially dry fine-grained polycrystals. Much more experimental work is required to provide a comprehensive picture of seismic wave dispersion and attenuation in the upper mantle. We highlight emerging opportunities for experimental studies of dislocation-mediated viscoelastic relaxation and the influence of water.

　Contact: T. Irifune 　phone: +81-89-927-9645 　e-mail: irifune@dpc.ehime-u.ac.jp
　http://www.ehime-u.ac.jp/~grc