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Rheology and microfabric evolution during simple shear in polyphase aggregates: high temperature torsion experiments and electron backscatter diffraction

English title Rheology and microfabric evolution during simple shear in polyphase aggregates: high temperature torsion experiments and electron backscatter diffraction
Applicant Burg Jean-Pierre
Number 103498
Funding scheme Project funding
Research institution Geologisches Institut ETH Zürich
Institution of higher education ETH Zurich - ETHZ
Main discipline Geology
Start/End 01.04.2004 - 31.12.2006
Approved amount 229'381.00
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Keywords (10)

deformation mechanisms; dehydration; fluid-rock interaction; interaction deformation-metamorphism; mantle rheology; microstructures; microsdeformation mechamisms; mantle; rheology; tructures

Lay Summary (English)

Lead
Lay summary
We have started a study on the rheology and microfabric evolution ofpolyphase aggregates with and without reaction. The main aims of thisproject are to determine 1) the effect of mineral transformations ondeformation during the formation of two-phase aggregates, and 2) the roleof microstructures, grain size, lattice orientations, and different volumeproportions of phases on the strength and deformation behavior oftwo-phase aggregates as a function of strain. We have focused on mantlerocks composed of two mineral phases with a high strength contrast andhave started an in-depth study on the dehydration of serpentine duringdeformation.Dehydration of serpentine to olivine, talc and water during deformation iscritical for understanding the possible localization of deformation intoshear zones and the generation of earthquakes along subduction zones. Inorder to investigate the effect of the dehydration reaction on thestrength and ductility of serpentinite, torsion experiments were performedusing a Paterson high PT torsion rig under drained conditions at constantshear strain rates of 10-4 to 10-5 s-1, temperatures of 550 to 750 ºC anda confining pressure of 300 MPa, to local shear strains up to γ = 3.We deformed two types of serpentinite: antigorite from Val Malenco, Italy,a high-temperature phase of serpentine (stable at T <500 ºC), andlizardite from Elba, Italy, a low-temperature phase of serpentine (stableat T <400 ºC).Antigorite deformed in the semi-brittle field at the run conditions.Visible faults formed probably due to reaction-induced fracturing, and thestress started to drop just after the initial peak stress (~350 MPa at 650to 700 ºC and ~280 MPa at 750 ºC). Highly comminuted grains with varioussizes along the faults were identified as partially dehydrated antigorite(H2O ~6 wt%) at 650°C and olivine and talc at >700 °C. Mechanical behaviorafter the peak stress is thought to occur by cataclastic flow, possiblyassisted by diffusion mass transfer processes of these fine-grainedreactant minerals.In contrast to semi-brittle faulting in antigorite, deformation oflizardite at 550 ºC to a bulk shear strain of 0.9 was widely distributed,showing typical ductile microstructures such as boudinage and S-C fabric.A well developed secondary foliation (C-plane) and strong latticepreferred orientations of lizardite grains were observed close to thelocalized shear zones. After the initial peak stress, steady stress valuesof 250 MPa were measured. We intend to focus on how the localized zonesevolve and how the mechanical response changes with increasing shearstrain during the reaction.
Direct link to Lay Summary Last update: 21.02.2013

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Number Title Start Funding scheme
66616 Rheology and microfabric evolution during simple shear in polyphase aggregates: high temperature torsion experiments and electron backscatter diffraction 01.04.2002 Project funding

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