Measurement of Dislocation Creep

Based on:

Low-Stress High-Temperature Creep in Olivine Single Crystals

D.L. Kohlstedt and C. Goetze, 1974

Picture from Couvy et. al, 2004

I. The experiment

II. A closer look at dislocation creep

Designing an experiment to model mantle flow processes

• Goal: produce a steady strain rate at a constant stress

Olivine single crystals

• High temperature (1450-1650°C) is needed for strain to occur fast enough to measure readily in the laboratory.

• Natural peridotite contains other phases, lowering the solidus below experimental temperatures

• Use of single crystal avoids grain boundary issues

San Carlos Peridot

Experimental setup

• Furnace

• Method of applying precise load

• Method of measuring strain

The Apparatus

• Molybdenum vs. graphite

• Gas inlet for H2, CO2, controls O2 fugacity

• Crystals dry rapidly at >1000°C and Atmospheric pressure

Results

101 102 103 104

σ1 – σ3 (bars)

Microstructures

Dislocation Creep: A Mechanism for Plastic Flow

Edge dislocations and glide: the rug analogy

Screw dislocation

Slide on Burgers vectors?

Slide on Power law creep equation?

Dislocation tangles & strain hardening

Edge dislocation pile-ups in olivineThese sorts of

dislocation tangles were commonly

observed in crystals deformed at differential stresses above 1 kbar.

Climb and

vacancy diffusion

Evidence for climb in olivine

In samples deformed under lower stress, dislocation structures appear to have reached an equilibrium concentration, implying the existence of some annealing process such as climb.

Conlusions

• Basic laboratory experiments can be used to hypothesize flow laws for the mantle

• Dislocation creep is a viable mechanism for plastic flow at high temperature and low differential stress