ausblenden:
Schlagwörter:
subduction zones; rheology; phase transitions; deep-focus earthquakes; grain size; kinetics
DDC:
550 - Earth sciences
Zusammenfassung:
Abstract: We investigate the role of grain-size reduction during the olivine-spinal transformation on rheological properties of subducting slabs on the basis of a scaling model for micro structural development during nucleation and growth. In this model, the size of spinal grains nucleating at olivine grain-boundaries is controlled by the relative rates of nucleation and growth, taking into account the impingement through the collision of grains due to growth. When the volume fraction of spinal reaches a certain threshold value (critical volume fraction ~ 1 to 10 %, depending on the P,T conditions in the slab), the new phase will form a continuous film and will significantly reduce the strength of the two-phase aggregate if spinal grain-size is small. The size of spinal grains delta 0 at this stage is calculated and is shown to be highly sensitive to temperature. At relatively high temperatures (T > 1000 K), delta_0 shows an Arrhenius-type dependence on temperature, that is delta_0 ~ exp(-E* /RT) with E* ~ 400 kJ/mol, whereas a more complicated temperature dependence is found at low temperatures (T < 900 K), where a grain-size reduction of up to 4 orders in magnitude is possible. Strength profiles of slabs due to combined effects of temperatures and of grain-size reduction are calculated. It is shown that: (1) the strength of slabs will have an unusual temperature dependence through the temperature dependence of grain-size; and (2) a subducting slab has a complicated rheological structure containing a weak region below the tip of a metastable olivine-bearing wedge in a cold slab. Possible implications of these anomalous rheological structures of slabs on the dynamics of subduction are discussed including the mechanisms of deep earthquakes.
