Intracrystalline deformation of calcite

Abstract

It is well established from observations on natural calcite tectonites that intracrystalline plastic mechanisms are important during the deformation of calcite rocks in nature. In this thesis, new data are presented on fundamental aspects of deformation behaviour of calcite under conditions where 'dislocation creep' mechanisms dominate. The data provide a better understanding of the rheological behaviour of calcite rocks, and provide a basis for meaningful texture (crystallographic preferred orientation) modelling for calcite polycrystals. In chapter 1, previous work on intracrystalline plastic mechanisms in calcite is summarized. Aspects of deformation behaviour hitherto insufficiently understood are highlighted, thus defining the scope for the present study. Chapter 2 describes uniaxial compression experiments performed on optical quality calcite single crystals at temperatures and constant strain rates in the range 400 to 800°C and 3x10-4 to 3x10-8 sec-1 respectively (mostly under controlled CO2 pressure). The tests were carried out with the compression direction parallel to [4041], Le. parallel to the intersection of two cleavage rhombs. At temperatures below -600°C, the crystals deformed largely bye-twinning. At higher temperatures, deformation occurred by slip on a single r<2021> system plus a single f system in the so-called positive sense, as identified by slip line analysis. The effective slip direction within the active f-plane was of <1 oT1> type rather than the <0221> type reported previously. In the slip dominated regime, the samples exhibited steady state flow behaviour. The flow stresses were found to be relatively insensitive to strain rate and can be empirically described by a power law creep equation with a stress exponent ranging from -13 at 550-600 °C to -9.5 at 700-800 °C. Irregular dislocation networks, observed in TEM, are prominent features of the dislocation substructure. These networks may have developed from dislocation interactions involving double cross slip. A related network recovery mechanism is implied by the steady state flow behaviour of the single crystals. In chapter 3, results are presented for uniaxial compression experiments on calcite single crystals in a second orientation, namely with the compression direction at 30° to the c-axis and 23° to the pole on a cleavage rhomb (Le. subparallel to [2243]). The tests were performed at temperatures in the range 300-800 °C, mostly at a constant strain rate of 3x10-5 sec-1. The stress-strain curves exhibited multistage hardening behaviour, steady state only being approached at higher temperatures and/or high strains (>10%). The active glide systems were found to be two r<2021> systems and one single f<1 oT1> system, all in the so-called negative sense. In addition, at T;:::600 °C, definitive evidence was found for slip on the basal plane. The observed work hardening behaviour is attributed to the absence of a network-related recovery mechanism. Chapter 4 compares the strength characteristics of the r, f and c glide systems in calcite. Based on yield data obtained from multi-stage stress-strain curves, it is shown that no significant difference exists in strength between positive and negative glide on the r<2021> and 1<1011> systems. Considering present results as well as previous data, it is also concluded that two regimes of slip system activity exist: 1) a low temperature regime involving e-twinning, r<2021> glide and 1<0221> glide, and 2) a higher temperature regime characterized by r<2021 >, 1<1011> and c<a> slip. A major texture transition is to be expected passing between these regimes In chapter 5, the stress (0") vs. dislocation density (p) relation for calcite single crystals is experimentally determined. The relationship obtained is found to be in good agreement with the well-known theoretical relation, 0" DC pO.5, based on theory of dislocation interaction. Data on calcite polycrystals, however, deviate from this. Using a concept of non-homogeneous deformation related to grain size, a simple model is put forward to account for this. In chapter 6, the flow data obtained for single crystals compressed in the [4041] orientation (chapter 2) are fitted to various microphysical models of dislocation creep. By considering the fitting results and microstructural observations, and comparing these with existing data on other materials, it is proposed that the steady state deformation of single crystals is best explained by a dislocation cross slip controlled creep mechanism. Mechanical behaviour and microstructures characteristic of calcite polycrystals, deformed at roughly identical conditions, show various similarities with the single crystals, and their creep behaviour may well be rate-controlled by the same mechanism. Finally, chapter 7 reports observations on deformed limestones from a small scale shear zone in SW Wales, UK, where maximum PT conditions were 130 MPa, and 200°C respectively. Flattened, strained calcite grains, high dislocation densities and a weak but distinct crystallographic preferred orientation indicate that deformation occurred predominantly by intracrystalline mechanisms. Using conventional paleopiezometers (dislocation density, recrystallized grain size, twinning frequency) and failure criteria, a paleostress within the range 70-410 MPa was inferred for the shear zone. Values computed by extrapolating various experimentally determined flow equations for calcite materials show a far wider range of stresses. Notably, power laws are unsuccesful in reliably predicting paleostresses in deformed calcite rocks at low metamorphic grade. In contrast, the cross slip controlled creep equation established for single crystals in the [4041] orientation may offer a method for estimating minimum flow stresses in limestones deformed at low temperature.