Experimental Investigation on the Deformation of Eclogite at High Temperature and High Pressure

Abstract: Eclogite serves as a reliable marker of high-pressure environments in collision zone terranes. The eclogitization of subducted crust and its reincorporation into the mantle are a critical part of the cycle of mantle convection. The transformation of basalt/granulite/gabbro to eclogite may contribute to the subduction and delamination of continental crust at depths, may produce a strong layer that separates from the rest of the lithosphere, and has been suggested to affect the distribution of intermediate-focus earthquakes. However, largely for technical reasons, there has been no experimental investigation on the deformation of eclogite at high temperature and high pressure. As a consequence, the deformation behavior of eclogite and its role in the deeper levels of subduction zones and mantle remain poorly understood. To address this important problem, we have performed preliminary studies on eclogite under high temperature and high pressure, including an experimental investigation on the rheology of UHP eclogite, an experimental investigation on the hydroxyl dehydration embrittlement of UHP eclogite, a FTIR study on the hydroxyl in UHP eclogite and associated rocks and a preliminary EBSD study on the deformation microstructure of UHP eclogite.We have conducted experiments at strain-rates of 10-4 -10-5/s, pressures of 2.5-3.5 GPa, and temperatures of 1300-1700K to determine the power flow law parameters for deformation of eclogite: A = 102.0 0.9, n = 3.4±0.5, Q = 420±70 kJ/mol and V = 19.1 ±5.5 cm3/mol. Our observations suggest that the strength contrast among garnet, omphacite and quartz is garnet > omphacite > quartz. The strain of eclogite is mostly accommodated by the deformation of weak quartz and omphacite, while garnet serves as a rigid body. We also conducted preliminary experiments on the strengths of the two minerals that constitute eclogite. These preliminary data show that (i) The two principal minerals of eclogite have greatly different strengths; the creep strength of eclogite appears to be comparable with that of harzburgite, the rock type located immediately below the oceanic crust, within experimental error, but 2-3 times that of omphacitite and no more than half that of garnetite; (ii) Progressive increase of garnet results in a smooth increase in strength, closely consistent with theoretical expectations; (iii) Eclogite has strength comparable to harzburgite; this equality is achieved because the great strength of polycrystalline silicate garnet is compensated by the weakness of omphacite. The latter is surprising from the experimental side, given previous data on diopside and enstatite showing strength much greater than olivine, but is consistent with predictions made from micro structures of naturally deformed eclogites. These results explain why eclogites from subduction-zone terranes exhibit evidence of extensive flow rather thanremaining undeformed eternally, (iv) These data further suggest that, during subduction, delamination of the oceanic crust from the underlying mantle due to contrasting rheologies is unlikely, except perhaps at depths of the mantle transition zone, (v) The extraordinary strength of polycrystalline silicate garnet supports previous suggestions that the lower portion of the mantle transition zone (-500-700 km) may be a layer of enhanced viscosity for it has the highest garnet contents in the upper mantle. We have performed preliminary deformation experiments at 3 GPa on a reconstituted natural eclogite that contains a significant hydroxyl concentration in both pyroxene and garnet. These preliminary data revealed faulting phenomena under stress within hydroxyl-enriched eclogite at temperatures between the H2O-saturated and dry solidi. The flow behavior of these eclogites is controlled by temperature and strain rate but their failure strength is not temperature or strain rate sensitive. Microstructural observations suggested exsolution of dissolved H2O to grain boundaries and within abundant Mode I microcracks followed by very small amounts of melting as ultra-thin film…
Key words: Eclogite; High P-T; Rheology; Dehydration-induced faulting; Hydroxyl; Crystallographic preferred orientation; FTIR; EBSD

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