Experimental investigation of the rheology and faulting of eclogite at high temperature and pressure.

摘要

Eclogite plays an important role for mantle convection and geodynamics in subduction zones. I report in this dissertation the rheology and faulting of eclogite at high temperature and pressure. I conducted a systematic experimental investigation on the deformation behaviors of eclogite and its constituent minerals at high temperature and pressure. My results demonstrate that eclogite is not a rock with expected extreme strength because the great strength of garnet is compensated by the weakness of omphacite. The presence of dissolved water can cause a significant decrease in creep strength and a pronounced change of deformation microstructure. Eclogite has strength comparable to harzburgite, suggesting that delamination of the oceanic crust from the underlying mantle due to contrasting rheologies is unlikely.; Electron backscattered diffraction (EBSD) analyses revealed near random crystallographic preferred orientations (CPO) in garnet and pronounced CPOs in omphacite under both dry and wet conditions. The former is consistent with rigid round garnets in dry eclogite and recrystallized garnets in wet eclogite. I argue that grain boundary effects dominate the deformation of garnet and lead to the strong shape preferred orientation under wet conditions. The latter can be further elaborated as an S-type fabric in foliated samples and an L-type fabric in lineated samples. They are consistent with the abundant dislocations revealed by TEM studies. I conclude that omphacite deformation is accomplished by the combined slip systems of {lcub}110{rcub}1/2[110], {lcub}110{rcub}[001] and (100)[001]. The transition between S-type and L-type omphacite fabric is associated with strain kinematics (shearing or flattening).; I present also in this dissertation that a hydroxyl-enriched eclogite develops a faulting instability associated with precipitation of water at grain boundaries and the production of very small amounts of melt ( 1 vol%) at high pressure. This new faulting mechanism satisfactorily explains high-temperature earthquakes in subducting oceanic crust and could potentially be involved in much deeper earthquakes in connection with similar precipitation of water in the mantle transition zone (400--700 km).