Olivine-spinel in single crystals: can water’s effect on interface kinetics be distinguished from that on creep?
|対象||教職員向け 大学院生向け 学部生向け|
|イベント概要||Prof. Stephen Morris
(University of California, Berkley)
With increasing pressure, olivine, the chief constituent of earth’s upper mantle, undergoes a series of densification phase changes, first to a modified spinel structure, then to the spinel structure. Deviatoric stress thus generated has been proposed as a cause for deep earthquakes within subducting oceanic plates. The mechanism by which compaction at the grain-scale produces deviatoric stress at the seismic scale is not understood. However, the grain-scale is accessible experimentally, and, if the physics at that scale were understood, the behaviour of polycrystals could be treated by computation. One major physical issue is that transformation is accelerated by the presence of hydrogen at concentrations likely to occur within the bulk of the upper mantle; the effect of hydrogen at the lower concentrations likely within unmetasomatized oceanic plates is unknown. There is an opportunity for theoretical and experimental studies of the grain scale. In particular, it is not known whether hydrogen affects the transformation via the bulk phases, or via the atomically thin interphase region. To argue that experiments on single crystals can be used to discriminate between those possibilities, we analyse growth of a spinel rim on an olivine sphere. Initially, the sample is at uniform pressure. Though its outer surface is kept at that constant pressure, rim growth causes the pressure within the core of unconverted olivine to differ from the applied pressure. As a result, rim growth involves coupling between interface kinetics and deformation of the phases. We compare predictions obtained from two models.