GRAIN-SIZE EVOLUTION IN SUBDUCTED OCEANIC LITHOSPHERE ASSOCIATED WITHTHE OLIVINE-SPINEL TRANSFORMATION AND ITS EFFECTS ON RHEOLOGY

We investigate the role of grain-size reduction during the olivine-spi nel transformation on rheological properties of subducting slabs on th e basis of a scaling model for microstructural development during nucl eation and growth. In this model, the size of spinel grains nucleating at olivine grain boundaries is controlled by the relative rates of nu cleation and growth, taking into account the impingement through the c ollision of grains due to growth. When the volume fraction of spinel r eaches a certain threshold value (critical volume fraction similar to 1-10%, depending on the P-T conditions in the slab), the new phase wil l form a continuous firm and will significantly reduce the strength of the two-phase aggregate, if spinel grain size is small. The size of s pinel 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) similar to exp(-E/RT) with E* similar to 400 kJ/mol , whereas a more complicated temperature dependence is found at low te mperatures (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 tempera ture dependence through the temperature dependence of grain size; and (2) a subducting slab has a complicated rheological structure containi ng a weak region below the tip of a metastable olivine-bearing wedge i n a cold slab. Possible implications of these anomalous rheological st ructures of slabs on the dynamics of subduction are discussed, includi ng the mechanisms of deep earthquakes.