Mineral reaction and deformation in Plagioclase-Olivine composites : an experimental study

Deformation and metamorphism of rocks are fundamentally interrelated but the ways in which processes of reaction and deformation mutually influence each other are still poorly understood in natural rocks. The aim of this thesis is to investigate relationships between plastic deformation and mineral reactions, by means of rock deformation experiments. Within this broad aim, the thesis focuses on (1) the spatial distribution of deformation and reaction during ductile shear, (2) the effect of mineral reactions on the strength, deformation mechanisms and microstructures of dry rocks and (3) the effect of ductile shear deformation on kinetics and mechanisms of a dry net-transfer reaction. To these ends, water-deficient plagioclase-olivine composites were studied after shear deformation and hydrostatic experiments inside and outside their chemical stability fields, using a Griggs apparatus. Experiments were performed on anorthite-forsterite (An92 -Fo93 ) and labradorite-forsterite (An60 -Fo93 ) composites at 900◦ C, confining pressures between 1000-1600 MPa and with constant shear strain rates of γ ∼5x10−5 s−1 . ˙ The hydrostatic and deformed samples were examined by backscattered scanning electron microscopy (BSEM) and transmission electron microscopy (TEM). At all chosen confining pressures, stable labradorite-olivine composites are found to strain-harden during shear deformation, up to stresses close to the brittle-plastic transition (τ ∼500-780 MPa). Pure olivine and labradorite samples are less strong (τ ∼350 and 100 MPa, respectively) than the labradorite-olivine composites. The pure olivine sample displayed low temperature plasticity, accompanied by some dynamic recrystallisation. The hardening of the labradorite-olivine composites is probably due to the inhibition of grain boundary migration by inter-phase boundaries, which prevent recovery. The prevention of recovery results in labradorite and olivine grains with local high dislocation densities. At all chosen confining pressures, concurrent plastic deformation and reaction of metastable anorthite-olivine composites results in a pronounced decrease in shear stress (τ ∼150 MPa). The onset of weakening coincides with the formation of fine-grained polyphase reaction products (size ∼0.25-1.0 µm). The onset of steady-state flow (γ >5, τ ∼200 MPa) is characterised by the coalescence of these products into interconnecting layers. The fine-grained reaction products deform by grain size sensitive creep. Fabric analysis using the autocorrelation function shows a strong correlation on a sample scale between reaction progress and strain; large shear strain is locally associated with high reaction progress. On a grain scale the applied strain is localised and accommodated in the interconnecting layers of reaction products. Strain accommodation in reaction product layers reduces the strain rate in the reacting anorthite and
olivine grains, which, as a result, are able to undergo recovery by dislocation climb.
The reaction weakening mechanism in anorthite-olivine composites is grain size reduction
by crystallisation of fine-grained polyphase reaction products, which deform
by diffusion-accommodated grain boundary sliding. The reaction causes a change in
deformation mechanism from grain size insensitive creep of the anorthite-olivine composite
to grain size sensitive creep of reaction products. The measured reduction of
shear stress at a constant strain rate confirms this change in the dominant deformation
mechanism of the samples.
The growth rates of enstatite and pyroxene-spinel-garnet reaction rims observed
around olivine and plagioclase indicate that reaction at hydrostatic and water-deficient
conditions is controlled by the limited transport of chemical components. The amount
of pressure overstepping in the experiments affects the reaction progress because the
rate of nucleation increases exponentially with the Gibbs free energy of reaction (the
amount of pressure overstepping for pressure-sensitive reactions). Nevertheless, the
studied reactions display a delayed onset of nucleation of new phases (30 to �80 hrs),
even at pressure overstepping of 700 to 900 MPa.
The plastic deformation of anorthite-olivine composites was found to enhance the
studied mineral reactions at water-deficient conditions. This enhancement is shown by
the increase of reaction progress as well as the increase of the nucleation and growth
rates of reaction rims during deformation. The reaction between anorthite and olivine is
enhanced by an increase in the nucleation rate of new phases. The increased nucleation
rate may be due to high dislocation densities in the reactant grains that deform by
low-temperature plasticity. The mechanical transport of reaction products by grain
boundary sliding may change the local equilibrium conditions, which, in combination
with slow diffusion and fast nucleation, results in the formation of metastable kyanite.
In summary, this experimental study shows that concurrent plastic deformation
and reaction processes in plagioclase-olivine composites positively influence each other:
rheological weakening may result from mineral reactions, and the localisation of reaction
progress in shear zones can be enhanced by plastic deformation. The results of this
thesis imply that concurrent deformation and reaction at water-deficient conditions are
of major importance in explaining how and why strain localisation occurs in polyphase
rocks under a large range of geological conditions.