HR: 1330h
AN: S22A-0427    [PDF]
TI: Effect of Dehydration Reaction on Serpentinite Deformation in Torsion
AU: * Hirose, T
EM: hirose@erdw.ethz.ch
AF: Geologisches Institut, ETH Zentrum, Zurich, 8092 Switzerland
AU: Bystricky, M
EM: misha@erdw.ethz.ch
AF: Geologisches Institut, ETH Zentrum, Zurich, 8092 Switzerland
AU: St\"{u}nitz, H
EM: holger.stuenitz@unibas.ch
AF: Department of Earth Science, Basel University, Basel, 4056 Switzerland
AU: Kunze, K
EM: karsten.kunze@erdw.ethz.ch
AF: Geologisches Institut, ETH Zentrum, Zurich, 8092 Switzerland
AB: Dehydration of serpentine to olivine, talc and water during deformation is critical for understanding the possible localization of deformation into shear zones and the generation of earthquakes along subduction zones. In order to investigate the effect of the dehydration reaction on the strength and ductility of serpentinite, torsion experiments were performed using a Paterson high PT torsion rig at constant shear strain rates of 10$^{-4}$ to 10$^{-5}$ s$^{-1}$, temperatures of 550 to 750 $\deg$C and a confining pressure of 300 MPa, to local shear strains up to $\gamma$ = 3. We deformed two types of serpentinite: antigorite from Val Malenco, Italy, a high-temperature phase of serpentine (stable at T $<$500 $\deg$C), and lizardite from Elba, Italy, a low-temperature phase of serpentine (stable at T $<$400 $\deg$C). Most of the samples were shaped in dog-bone geometry with a central hole along their axial direction which acted as a fluid conduit, enabling an easy escape for any released fluid during the dehydration reaction. We also deformed solid bone-shaped specimens to compare the mechanical behavior of solid and hollow specimens. In both cases, porous alumina spacers were placed on both end sides of specimen and led to the atmosphere through the pore pressure line. Thus our experiments were performed under drained conditions. Antigorite deformed in the semi-brittle field at the run conditions. Visible faults formed probably due to reaction-induced fracturing, and the stress started to drop just after the initial peak stress ($\sim$350 MPa at 650 to 700 $\deg$C and $\sim$280 MPa at 750 $\deg$C). Highly comminuted grains with various sizes along the faults were identified as partially dehydrated antigorite (H$_{2}$O $\sim$6 wt$%$) at 650 $\deg$C and olivine and talc at $>$700 $\deg$C. Mechanical behavior after the peak stress is thought to occur by cataclastic flow, possibly assisted by diffusion mass transfer processes of these fine-grained reactant minerals. We have also investigated the effect of pre-heating on the strength of antigorite. The peak strength of a sample pre-heated at 750 $\deg$C for 3 hr and then deformed at 700 $\deg$C is 70 MPa lower than a sample deformed directly at 700 $\deg$C. This stress reduction occurred due to the presence of a mechanically weaker reaction product (talc) rather than elevated pore pressure because the sample was deformed under completely drained conditions. By contrast, a solid sample was weaker than hollow ones by $\sim$100 MPa, mainly due to low permeability of the serpentinite. Excess fluid pressure in solid specimens leads to a drop in the effective pressure and appears to have enhanced the dehydration reaction along micro-fractures. Our data shows that dehydration weakening of serpentinite is caused not as much because of excess pore pressure but more because of the weaker mineral assemblages from the reaction. In contrast to semi-brittle faulting in antigorite, deformation of lizardite at 550 $\deg$C 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 lattice preferred orientations of lizardite grains were observed close to the localized shear zones. After the initial peak stress, steady stress values of 250 MPa were measured. We intend to focus on how the localized zones evolve and how the mechanical response changes with increasing shear strain during the reaction.
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
DE: 5104 Fracture and flow
DE: 5120 Plasticity, diffusion, and creep
SC: Seismology [S]
MN: 2003 Fall Meeting