Interplay between mineral reaction and deformation via structural defects

Non-isostatic stresses created by tectonic forces or volume changes during mineral reactions at high temperature induce inelastic creep of rocks that may interact with the reaction progress. Simultaneously, metamorphic reactions may considerably alter the bulk rheological behaviour. We propose to investigate the interplay between mineral reaction and plastic deformation via structural defects at controlled laboratory conditions. The suggested system CaCO3 – MgCO3 allows to study a) the growth of dolomite from calcite and magnesite single crystals and b) the dissolution of Mg in calcite from calcite in contact with dolomite. Tests will be performed at temperatures up to about 900°C and confining pressures of 400 MPa by applying axial and shear stresses under triaxial and torsion conditions using a Paterson-type deformation apparatus. Presumably, the dolomite rim growth rates as well as the formation rates of magnesian calcite depend on differential stress through induced crystalline defects. These comprise point defects, e.g. vacancies or interstitials, line defects (dislocations), and planar defects as for example high angle grain boundaries or twin boundaries. The predominant defect budget and associated texture and grain boundary character distribution depend on the applied stress and strain, which can be analyzed using modern electron microscopy. In addition, ongoing reactions may change the aggregate strength via solute drag, precipitation hardening, increased point defect concentration, or by a switch in the prevailing deformation mechanism. The rheological behaviour can be deduced from mechanical data combined with microstructure analysis. The project is partly a continuation and extension of the former project 6 (‘feedback between transport controlled mineral reactions und differential stress’) examined during the first application phase. Based on the previous findings we focus on a quantitative examination of the effect of stress-strain induced structural defects on the kinetics of mineral reactions and on the strength evolution.

Schematic drawing of the pressure vessel (a) and sample assembly (b) used for high pressure, high temperature axial compression and torsion experiments with the Paterson deformation apparatus.