RheoMan: a five-year, ERC-funded (Advanced Grant), project to model the rheology of the Earth's mantle

Nov 10, 2015 Rheology of olivine under lithospheric conditions Results

Combining experiments, microstructural characterization and numerical modelling, we revisit the rheology of olivine under lithospheric conditions in a paper recently published in Earth and Planetary Science Letter:

F. Boioli , A. Tommasi, P. Cordier , S. Demouchy, A. Mussi,  Low steady-state stresses in the cold lithospheric mantle inferred fromdislocation dynamics models of dislocation creep in olivine,  Earth and Planetary Science Letters 432 (2015) 232–242.

 

Low steady-state stresses in the cold lithospheric mantle

 

At high temperatures dislocation climb, i.e. dislocation motion driven by absorption or emission of point defects, is an important recovery mechanism that can strongly influence the plastic behaviour. At low/moderate temperature, climb is expected to play a minor role in plastic deformation. This is true at laboratory strain rates. Still, TEM investigations of olivine sample deformed in laboratory conditions demonstrate that recovery processes induced by climb, as dipole break-down, do occur. They are too slow however to play a significant role.

 

Figure 1

 

In this work we show that at geological strain-rates, these recovery processes allow steady-state deformation. By using a 2D dislocation dynamics (DD) model we investigated dislocation creep in olivine at low to moderate temperatures, relevant for the lithospheric mantle. Similarly to high temperature creep, the interplay between glide and climb leads to steady state deformation conditions.

 

Figure 2

 

Based on those calculations, we propose a new flow law for the lithospheric mantle (Figure 2) which reproduces well experimental results at laboratory strain rates. Applied at mantle strain rates of 10-14 s-1, which are relevant for the lithospheric mantle, we find stress well below the values obtained from extrapolations of high-temperature flow laws.

 

In the Figure below we report the steady state creep strain rates, as a function of the reciprocal temperature. The DD simulation results (colour lines) are compared with the strain rates inferred for lithospheric plates in different geodynamic situations (grey squares in the Figure).

 

Figure 3

 

For more details, see the paper just published F. Boioli , A. Tommasi, P. Cordier , S. Demouchy,

A. Mussi,  Low steady-state stresses in the cold lithospheric mantle inferred from

dislocation dynamics models of dislocation creep in olivine,  Earth and Planetary Science Letters 432 (2015) 232–242.