Experimental Investigation of the Rheology of Eclogite and Its Constituent Minerals at High Pressure

University Of California-Riverside, Riverside CA

Investigators

Abstract

Green, Harry W. EAR-0003631 Eclogite is the densest normal rock exposed at the surface of Earth, typically consisting of approximately equal amounts of garnet and omphacite, +/- small amounts of quartz, kyanite, zoisite, and/or amphibole. Eclogite forms from high-pressure, high-temperature metamorphism of rocks of basaltic composition. This transformation may contribute to the subduction of oceanic crust to great depth, may affect the distribution of intermediate-focus earthquakes, and has been suggested to produce a strong layer that may delaminate from the rest of the lithosphere. The investigators propose a detailed experimental study of the rheology of eclogite and its constituent minerals. They presented preliminary results acquired with their unique, 5 GPa, deformation apparatus showing that it is now technically possible to acquire high-quality data at the pressures required for such studies (2-4 GPa). These preliminary results show that eclogite has a strength comparable to harzburgite, the rock type located immediately below the oceanic crust; this equality is achieved because the great strength of polycrystalline silicate garnet is compensated by the weakness of omphacite. 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. Toward a better understanding of deep subduction processes, the investigators plan to build on their preliminary results with a three-year expanded study of the rheology of eclogite and its constituent minerals at pressures of 2-4 GPa to map out in detail their flow laws, deformation mechanisms and microstructural characteristics. They will: (a) Elaborate the rheology of 50-50 eclogite over a larger domain of temperature and strain rate at 3 GPa and selected experiments at P = 2.0-4.0 GPa; (b) perform analogous experiments on the monomineralic omphacite and garnet end members to determine their flow parameters;(c) conduct comparable experiments and equivalent analysis of specimens with H2O added; and (d) if time permits, they will synthesize majoritic garnet in our multianvil apparatus and conduct qualitative deformation experiments within the majorite stability field using short cylindrical specimens of majoritic and pyropic garnets, one on top of the other, which will determine the relative strengths of these two phases.