Rheology of serpentines and mass transfer in subduction zones
Abstract
Serpentinites have a lower density and lower viscosity than "dry" ultramafic rocks and it was proposed, based on numerical simulations, that they play a major role in mantle-slab decoupling, and in downward (sink) or upward (exhumation) motion of eclogites and ultra-high pressure (UHP) rocks in subduction zones (Schwartz et al., 2001). Numerical models either assume arbitrary viscosity or use rheological laws derived from low-pressure data (Rayleigh and Paterson, 1965). Extrapolation of less than 0.5 GPa data to subduction zone pressures may be the source of significant error in the physical models. We present here new rheological data on antigorite, the stable variety of serpentine in subduction zones, obtained over a P-T range of 1-4 GPa and 200-500 /deg C that cover most of its stability field. The experiments were carried out in a D-DIA apparatus installed at GSECARS on the 13-BM-D line of APS. Strain rates and stresses were obtained respectively from in-situ monitoring the sample length with X-ray radiographs, and azimuthal dependence of d-spacings on diffraction patterns. The deformed samples show textures resembling those of natural samples, suggesting similar deformation mechanisms. The determined stress-strain curves were fitted to a power-law equation including both temperature and pressure dependence. When compared with extrapolation of power-laws derived from Raleigh and Paterson (1965) data, the stress dependency and pre-exponential factor are identical within mutual uncertainties. However, our apparent activation energy at a given pressure remains less than half of those obtained in previous estimations. Thus the temperature dependence is very low, indicating that antigorite and antigorite-dominated rocks have low viscosities down to the lowest temperatures encountered in subducted slabs. The pressure-induced change in viscosity is significant but the viscosity increase between 1 and 5 GPa remains below 2 orders of magnitude. The present results confirm that serpentinites will indeed act as a weak layer that allows significant mass transfer along the "serpentinized channel" and dynamic processes such as mantle slab decoupling, and mantle wedge convection. They can be applied with little P-T extrapolation to the modeling of subduction zone processes. Raleigh CB, Paterson MS (1965) Experimental Deformation of Serpentinite and its Tectonic Implications J Geophys Res 70: 3965-3985 Schwartz S, Allemand P, Guillot S (2001) Numerical model of the effect of serpentinites on the exhumation of eclogitic rocks: insights from the Monviso ophiolitic massif (Western Alps) Tectonophysics 342: 193-206
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.T21G..08H
- Keywords:
-
- 3902 Creep and deformation;
- 5120 Plasticity;
- diffusion;
- and creep;
- 8160 Rheology: general (1236;
- 8032);
- 8170 Subduction zone processes (1031;
- 3060;
- 3613;
- 8413)