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An experimental investigation of the mechanical behaviour of synthetic calcite-dolomite composites Kushnir, Alexandra Roma Larisa 2012

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AN EXPERIMENTAL INVESTIGATION OF THE MECHANICAL BEHAVIOUR OF SYNTHETIC CALCITE-DOLOMITE COMPOSITES by  Alexandra Roma Larisa Kushnir  B.Sc. The University of British Columbia, 2008  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  MASTER OF SCIENCE  in The Faculty of Graduate Studies (Geological Sciences)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)  September 2012  © Alexandra Roma Larisa Kushnir, 2012  Abstract The role of dolomite on the strength and evolution of calcite-dolomite fold and thrust belts is largely unknown. Field investigations indicate that, under upper- to mid-crustal conditions, strain in natural systems is localized in calcite, resulting in a ductile response, while dolomite deforms in a brittle manner. The effect of dolomite on limestone rheology, and the potential for strain localization in composites have not yet been fully quantified. I conducted 11 constant displacement rate (3x10-4 and 10-4 s-1), high confining pressure (300 MPa), and high temperature (750°C and 800°C) torsion experiments to address the role of dolomite on the strength of calcite-dolomite composites. Starting materials were formed by hot isostatic pressing mixtures of dolomite and calcite powders (given as Dm%: Dm25, Dm35, Dm51, and Dm75) and were deformed up to a maximum shear strain of ~5. Mechanical data show a considerable increase in yield strength with increasing dolomite content. Microstructural analysis shows that dolomite grains <~50 µm are characterized by diffuse and poorly defined grain boundaries; in Dm25 and Dm35, high aspect ratio dolomite grains are aligned into a foliation. Dolomite grains >~50 µm are characterized by well-defined grain boundaries and cleavage-controlled fracture. Electron backscatter diffraction (EBSD) shows no crystallographic preferred orientation (CPO) development in dolomite, but optical microscopy confirms brittle deformation of dolomite grains by Mode I cracks, shear fractures, and subsequent grain size reduction. Calcite grains are internally strain-free, equiaxed to tabular in shape, and characterized by triple-junction grain boundaries. EBSD confirms a distinct CPO of calcite c-axes perpendicular to the direction of maximum stretching. The microstructure of calcite aggregates suggests grain boundary sliding, accommodated by diffusion and dislocation glide, which accommodates high shear strains without significant change in grains shape and size. Dolomite is essentially undeformed in run products with less than 35% dolomite; calcite accommodates most of the displacement in these experiments. In contrast, for dolomite contents greater than 51%, dolomite accommodates displacement by brittle processes.  ii  My experiments provide insights into the processes controlling rheology within bimodal calcite-dolomite systems, suggesting that a minimum dolomite-content exists (between 35% and 51%) above which dolomite significantly influences composite strength.  iii  Preface The work presented in this thesis is, in part, a collaboration between the Centre for the Experimental Study of the Lithosphere (CESL) at the University of British Columbia (UBC) and the Experimental Rock Deformation Laboratory (ERDL) at the Swiss Federal Institute of Technology in Zürich (ETH- Zürich). The starting powders were mixed and hot isostatically pressed at ETH- Zürich by Dr. L.A. Kennedy and Dr. P. Benson. I characterized the starting material by thin section, scanning electron microscope (SEM), electron backscatter diffraction (EBSD), and X-ray diffraction (XRD) methods. I performed the torsion experiments at ETH- Zürich under the supervision of Dr. P. Benson and Dr. S. Misra. Additionally, I was responsible for characterization of the deformed materials by thin section, SEM, EBSD, XRD, and microprobe methods.  iv  Table of Contents 	
    Abstract	
  ...................................................................................................................................................	
  ii	
   Preface	
  ....................................................................................................................................................	
  iv	
   Table	
  of	
  Contents	
  ..................................................................................................................................	
  v	
   List	
  of	
  Tables	
  ........................................................................................................................................	
  vii	
   List	
  of	
  Figures	
  ....................................................................................................................................	
  viii	
   Acknowledgements	
  .............................................................................................................................	
  x	
   Dedication	
  ..............................................................................................................................................	
  xi	
   Chapter	
  1:	
  Introduction	
  .....................................................................................................................	
  1	
   Chapter	
  2:	
  The	
  State	
  of	
  the	
  Art	
  .........................................................................................................	
  3	
   2.1	
  Calcite	
  and	
  Dolomite	
  ...............................................................................................................................	
  3	
   2.2	
  Deformation	
  Mechanisms	
  in	
  the	
  Earth’s	
  Crust	
  ..............................................................................	
  5	
   2.2.1	
  Brittle	
  Deformation	
  .............................................................................................................................................	
  5	
   2.2.2	
  The	
  Brittle-­‐Ductile	
  Transition	
  and	
  Ductile	
  Deformation	
  ....................................................................	
  7	
   2.3	
  Experimental	
  Rock	
  Deformation	
  ........................................................................................................	
  7	
   2.3.1	
  Calcite	
  Rheology	
  ...................................................................................................................................................	
  7	
   2.3.2	
  Dolomite	
  Rheology	
  ...........................................................................................................................................	
  13	
   2.4	
  Deformation	
  in	
  Bimineralic	
  Crystalline	
  Rocks	
  ...........................................................................	
  15	
   2.5	
  Decarbonation	
  .......................................................................................................................................	
  16	
   Chapter	
  3:	
  Experimental	
  Methods	
  ...............................................................................................	
  18	
   3.1	
  Deformation	
  Apparatus	
  and	
  Techniques	
  ......................................................................................	
  18	
   3.2	
  Sample	
  Assembly	
  ..................................................................................................................................	
  23	
   3.3	
  Microstructural	
  and	
  Textural	
  Analyses	
  .........................................................................................	
  26	
   3.4	
  X-­‐ray	
  Diffraction	
  ....................................................................................................................................	
  26	
   3.5	
  Microprobe	
  ..............................................................................................................................................	
  27	
   Chapter	
  4:	
  Starting	
  Material	
  Preparation	
  and	
  Characterization	
  .......................................	
  28	
   4.1	
  Sample	
  Preparation	
  .............................................................................................................................	
  28	
   4.1.1	
  Starting	
  Powders	
  ...............................................................................................................................................	
  28	
   4.1.2	
  Hot	
  Isostatic	
  Pressing	
  (HIP)	
  .........................................................................................................................	
  29	
   4.1.3	
  Chemistry	
  .............................................................................................................................................................	
  29	
   4.1.4	
  Porosity	
  .................................................................................................................................................................	
  30	
   4.1.5	
  Sample	
  Assembly	
  Preparation	
  ....................................................................................................................	
  32	
   4.2	
  Microstructure	
  .......................................................................................................................................	
  32	
   4.3	
  Textures	
  and	
  Fabrics	
  ...........................................................................................................................	
  39	
   Chapter	
  5:	
  Results	
  .............................................................................................................................	
  43	
   5.1	
  Sample	
  Decarbonation	
  ........................................................................................................................	
  43	
   5.2	
  Mechanical	
  Results	
  ...............................................................................................................................	
  43	
   5.2.1	
  Strain	
  Rate	
  Stepping	
  Experiment	
  ...............................................................................................................	
  45	
   5.2.2	
  Yield	
  Experiments	
  .............................................................................................................................................	
  48	
   5.2.3	
  High	
  Strain	
  Experiments	
  ................................................................................................................................	
  48	
   5.2.4	
  High	
  Temperature	
  and	
  Low	
  Strain	
  Rate	
  Experiments	
  ......................................................................	
  49	
   5.3	
  Microstructure	
  .......................................................................................................................................	
  51	
   5.3.1	
  Yield	
  Experiments	
  .............................................................................................................................................	
  51	
   v  5.3.2	
  High	
  Strain	
  Experiments	
  ................................................................................................................................	
  54	
   5.3.3	
  High	
  Temperature	
  Experiments	
  .................................................................................................................	
  59	
   5.4	
  Texture	
  Analyses	
  ...................................................................................................................................	
  59	
   5.4.1	
  ! = !	
  .......................................................................................................................................................................	
  59	
   5.4.2	
  Yield	
  Experiments	
  .............................................................................................................................................	
  59	
   5.4.3	
  High	
  Strain	
  Experiments	
  ................................................................................................................................	
  60	
   5.4.4	
  High	
  Temperature	
  Experiment	
  ...................................................................................................................	
  60	
   5.4.5	
  Calcite	
  Clumps	
  ....................................................................................................................................................	
  61	
   5.5	
  Chemical	
  Changes	
  Attending	
  Deformation	
  ..................................................................................	
  73	
   5.5.1	
  Energy-­‐dispersive	
  X-­‐ray	
  Spectroscopy	
  ....................................................................................................	
  73	
   5.5.2	
  Microprobe	
  ..........................................................................................................................................................	
  73	
    Chapter	
  6:	
  Discussion	
  ......................................................................................................................	
  76	
   6.1	
  Overview	
  of	
  Results	
  ..............................................................................................................................	
  76	
   6.1.1	
  Mechanical	
  Data	
  .................................................................................................................................................	
  76	
   6.1.2	
  Microstructure	
  and	
  Texture:	
  Calcite	
  .........................................................................................................	
  78	
   6.1.3	
  Microstructure	
  and	
  Texture:	
  Dolomite	
  ....................................................................................................	
  78	
   6.2	
  Deformation	
  Mechanisms	
  ..................................................................................................................	
  79	
   6.2.1	
  Dislocation	
  Creep	
  with	
  Dynamic	
  Recrystallization	
  ............................................................................	
  79	
   6.2.2	
  Superplastic	
  Flow	
  and	
  Calcite	
  ......................................................................................................................	
  79	
   6.2.3	
  Calcite	
  Clumps:	
  Analogues	
  for	
  Veins	
  in	
  Nature?	
  ..................................................................................	
  81	
   6.2.4	
  The	
  Role	
  of	
  Dolomite:	
  Brittle	
  and	
  Ductile	
  Behaviour	
  ........................................................................	
  81	
   6.3	
  Application	
  to	
  Natural	
  Systems	
  ........................................................................................................	
  85	
   6.4	
  Summary	
  ..................................................................................................................................................	
  85	
   Chapter	
  7:	
  Conclusions	
  ....................................................................................................................	
  87	
   Bibliography	
  .......................................................................................................................................	
  89	
   Appendix	
  A:	
  Matlab	
  Code	
  ................................................................................................................	
  94	
   A.1	
  Grain	
  Size	
  Analysis	
  ...............................................................................................................................	
  94	
   A.2	
  Paterson	
  Deformation	
  Analysis	
  .......................................................................................................	
  94	
   A.2.1	
  Torsion_data_processing_n_factor.m	
  .......................................................................................................	
  94	
   A.2.2	
  Torsion_load_data.m	
  .......................................................................................................................................	
  95	
   A.2.3	
  Torsion.m	
  .............................................................................................................................................................	
  96	
   A.3	
  Microprobe	
  Analysis	
  ............................................................................................................................	
  97	
   Appendix	
  B:	
  X-­‐ray	
  Diffractograms	
  ...............................................................................................	
  99	
   B.1	
  Starting	
  Powders	
  ................................................................................................................................	
  100	
   B.2	
  Hot	
  Isostatic	
  Pressing	
  Product	
  .......................................................................................................	
  101	
   B.3	
  Deformed	
  Run	
  Product	
  .....................................................................................................................	
  103	
   Appendix	
  C:	
  Microprobe	
  Data	
  .....................................................................................................	
  104	
    vi  List of Tables Table 2.1 Identified glide planes in calcite and conditions for activation of slip and twinning along them…………………………………………………………………9 Table 2.2 Identified glide planes in dolomite and conditions for activation of slip and twinning along them………………………………………………………………15 Table 4.1 HIP conditions and properties of HIP product………………………………………30 Table 5.1 List of deformation experiments performed…………………………………………45 Table 5.2 List of sample slices used for microstructural analysis………...……………………51  vii  List of Figures Figure 2.1 Hexagonal structure shared by calcite and dolomite…………………………………..4 Figure 2.2 Deformation of a circle by simple shear………………………………………………6 Figure 2.3 Stress-strain relationships for Wombeyan marble…………………………………….6 Figure 2.4 Brittle and ductile fabrics for sinistral shear systems……………………………..….11 Figure 2.5 Stereographic projection showing important slip and twinning planes for calcite and dolomite………………………………………………………………………….…11 Figure 2.6 Deformation map for Solnhofen limestone………………………………………….12 Figure 2.7 Deformation map for dolomite………………………………………………………14 Figure 2.8 Thermal dissociation of confined dolomite………………………………………….17 Figure 3.1 Schematic of the Paterson deformation rig with torsion actuator……………...........21 Figure 3.2 Schematic of sample geometry………………………………………………...…….22 Figure 3.3 Relationship between stress exponent (n) and shear stress………………………….22 Figure 3.4 Torsion rig sample assemblies……………………………………………………….24 Figure 3.5 Schematic diagram of the three principal thin section cuts of a rock deformed by torsion……………………………………………………………………………………25 Figure 4.1 Volume percent grain size distributions of starting material powders………….……31 Figure 4.2 Backscatter electron (BSE) images of Dm25 starting material…………..…………..34 Figure 4.3 Backscatter electron (BSE) images of Dm35 starting material………..……………..35 Figure 4.4 Backscatter electron (BSE) images of Dm51 starting material……………..………..36 Figure 4.5 Backscatter electron (BSE) images of Dm75 starting material………………………37 Figure 4.6 Backscatter electron images of a pure calcite clump in Dm25 starting material….....38 Figure 4.7 Electron backscatter diffraction (EBSD) analysis of Dm25 starting material……….40 Figure 4.8 Electron backscatter diffraction (EBSD) analysis of Dm75 starting material……….41 Figure 4.9 Area fraction grain size distributions of hot isostatically pressed starting materials...42 Figure 5.1 Thermal dissociation equilibrium of confined dolomite………………………..……44 Figure 5.2 Strain rate-steeping experiment, P1529 (Dm35)…………………………….……….47 Figure 5.3 Mechanical data for all experiments…………………………………………………50 Figure 5.4 Scanning electron microscopy (SEM) images of yield experiments……………..….52 Figure 5.5 Optical and scanning electron microscopy (SEM) images of deformed materials…..53 Figure 5.6 High strain deformed material: Dm25 (P1527)………………………………...…….55 Figure 5.7 High strain deformed material: Dm75 (P1538)…………………………..…………..56 viii  Figure 5.8 Dm51 deformed at high temperature (P1534)………………………………………..57 Figure 5.9 Electron backscatter diffraction (EBSD) analysis of 0-strain: Dm25…………...…...62 Figure 5.10 Electron backscatter diffraction (EBSD) analysis of 0-strain: Dm75…………...….63 Figure 5.11 Electron backscatter diffraction (EBSD) analysis of yield experiment: Dm51…….64 Figure 5.12 Electron backscatter diffraction (EBSD) analysis of yield experiment: Dm75….…65 Figure 5.13 Electron backscatter diffraction (EBSD) analysis of yield experiment: Dm75…….66 Figure 5.14 Electron backscatter diffraction (EBSD) analysis of high strain experiment: Dm25………………………………………………………………………………….…67 Figure 5.15 Electron backscatter diffraction (EBSD) analysis of high strain experiment: Dm35…………………………………………………………………………………….68 Figure 5.16 Electron backscatter diffraction (EBSD) analysis of high strain experiment: Dm75…………………………………………………………………………………….69 Figure 5.17 Area fraction grain size distributions of high strain experiments………………..…70 Figure 5.18 Electron backscatter diffraction (EBSD) analysis of high temperature experiment: Dm51………………………………………………………………………………...…..71 Figure 5.19 Crystallographic preferred orientation (CPO) development near calcite clumps…..72 Figure 5.20 Energy-dispersive X-ray spectroscopy (EDS) maps of magnesium concentration for experiment P1527 (Dm25)…………………………………………....74 Figure 5.21 Microprobe analysis maps for high strain experiments P1527 (Dm25) and P1538 (Dm75)…………………………………………………………………………………...75 Figure 6.1 Evolution of crystallographic preferred orientation (CPO) with strain………………77 Figure 6.2 Comparison of study data with the reported deformation behaviour of Solnhofen limestone…………………………………………………………………………………83 Figure 6.3 Comparison of study data with the reported deformation behaviour of Madoc dolomite………………………………………………………………………………….84 Figure 6.4 Field examples of coexistant micrite and dolomite, Toscana Strata of the Apennines in the Gulf of La Spezia………………………………………………………….…...…86  ix  Acknowledgements I sincerely thank Dr. L.A. Kennedy for constantly challenging me to ask penetrating questions and guiding me in the pursuit of answers. It has truly been a pleasure discussing science. I offer my enduring gratitude to Prof. J.K. Russell for always having an open door and for saying it like it is. The guidance and opportunities he has given me have truly been one of the most rewarding aspects of my degree. I thank Dr. C. Sinclair for his eternal enthusiasm and lively discussions. To J. Kabel and G. LeFebvre, I owe many thanks for their support and unwavering patience in guiding me through my exhaustive hours of EBSD analysis. To Dr. R. Bruijn, Dr. P. Benson, and Dr. S. Misra, I am grateful for their advice and assistance during the experimental program at ETH-Zürich. The learning curve was steep and the challenge was invigorating. To my lab family: M. Campbell, D. Woodell, S. Kolzenburg, L. Hilchie, B. Friedlander, C. Ryane, L. Porritt, A. Ryan, and J. Welles. You have made the last two years an utter pleasure and the end of this period of time is most bittersweet. Your endless support and humour have kept me going. To M. Heap for keeping me from losing my enthusiasm and always being just a phone call away. My funding was provided by the Alexander Graham Bell Canada Graduate Scholarship (CGS) from the National Sciences and Engineering Research Council of Canada  x  Dedication  For my parents, without whom very little of what I have achieved would have been possible.  xi  Chapter 1: Introduction Approximately 20% of the sedimentary rock in the Earth’s crust is limestone. Calcite (CaCO3) is the major constituent of limestone (i.e. rock containing > 95% wt% calcite), however, calcite is commonly partially or completely replaced by dolomite (MgCa(CO3)2) through the enrichment of magnesium ions, a process called dolomitization. The deformation mechanisms and rheology of calcite aggregates have been explored extensively (Barnhoorn et al., 2003, 2005a; Bruijn et al., 2010; Casey et al., 1997; De Bresser et al., 2005; Kennedy and Logan, 1997; Kern and Wenk, 1983; Leiss and Molli, 2002; Molli et al., 2011; Oesterling et al., 2006; Paterson and Olgaard, 2000; Pieri et al., 2000a; Pieri et al., 2000b; Rutter et al., 1993; Rutter, 1972, 1995; Schmid et al., 1977; Schmid et al., 1987; Schmid et al., 1980; Walker et al., 1990). The rheology of dolomite is not well defined under crustal conditions (Austin and Kennedy, 2005; Austin et al., 2005; Barber et al., 1994; Davis et al., 2008; Delle Piane et al., 2007; Delle Piane et al., 2008; Leiss and Barber, 1998; Newman and Mitra, 1994). Calcite and dolomite commonly occur together and behave quite differently when involved in crustal-scale deformation (Erikson, 1994; Woodward et al., 1988). Dolomitic rocks, in particular, play an important structural role in controlling the strength of thrust faults and nappes. For example, the varying degree of dolomitization within rocks of the Naukluft Nappe Complex and Naukluft Thrust, central Namibia, likely played a significant role in their structural evolution (Miller et al., 2008; Viola et al., 2006). Ductile deformation occurs within calcitemylonites, whereas the basal Naukluft Fault is localised along a pre-existing massive, dolomite layer through both brittle and ductile reworking (Miller et al., 2008). Dolomite within continental low-angle normal faults, such as the Zuccale Fault in Italy (Smith et al., 2007), is responsible for their profound and long-term weakness (Collettini et al., 2009). Additionally, dissolution of calcite is, in part, responsible for devastating earthquakes in active fault systems (e.g. Fucino Fault System of the Central Apennines; Carcaillet et al. (2008)). Despite the structural and chemical similarities of calcite and dolomite, dolomitic rocks (i.e., rocks containing > 95% wt.% of dolomite) are apparently stronger than their calcite-rich counterparts (i.e. limestone) at low temperatures (< 700°C). At high temperatures (> 700°C), fine-grained dolomite rocks weaken significantly compared to similar experiments on calcite rocks (Delle Piane et al., 2009a). To date, the influence of dolomite content on the strength of limestone under both ambient and high temperature conditions is not well constrained in the laboratory. 1  In this study, synthetic hot isostatically pressed (HIP) calcite-dolomite (Cc-Dm) composites are deformed in a torsion apparatus under elevated temperature and confining pressure to determine the rheological behaviour of the composites and to evaluate the effect of dolomite content on rock strength. The composites are composed of 25 %, 35%, 51%, and 75% coarse-grained dolomite with the remainder being fine-grained calcite. The experimental campaign comprises a total of 12 rock deformation experiments; the conditions of experimentation are temperatures, T, of 750°C and 800°C, confining pressure, Pc, of 300 MPa, and total shear strains between 0.16 and 5.5. These experiments directly address variations in the mechanical behaviour of composite materials as a function of composition and grain size. I show that with increasing dolomite content, these synthetic composites demonstrate a two-fold increase in mechanical strength. Previous studies (see Delle Piane et al., 2009) of similar ilk demonstrate that dolomite can decrease in yield strength when relatively small proportions of dolomite are of the same grain size as the surrounding calcite. In this study I explore the implications of composition and grain size on strain localization in calcite-dolomite composites. The results of this study can be applied to the rheological behaviour of dolomite-rich limestones in foreland fold and thrust belts (e.g. Canadian Rockies, Apennines, Italy).  2  Chapter 2: The State of the Art 2.1 Calcite and Dolomite Subtle differences in crystal structure set calcite and dolomite apart both in physical properties and in available mechanisms of deformation. Calcite (CaCO3) is hexagonal (!3! symmetry) with interlayers of Ca and triangular CO3 perpendicular to the crystal c-axis (see Figure 2.1A). The density of calcite is 2.71 g/cm3. Dolomite (CaMg(CO3)2) is hexagonal (!3 symmetry) with Ca, Mg, and triangular CO3 layers interlayered perpendicular to the c-axis (see Figure 2.1B). The density of dolomite is 2.88 g/cm3. The fundamental structural difference between calcite and dolomite is the introduction of Mg layers in the dolomite, resulting in lower structural symmetry. The two cations occupy specific structural positions and, therefore, there is no implied solid solution between CaCO3 and MgCO3 at low temperatures. For temperatures in excess of 1000°C, a complete solid solution exists between calcite and dolomite. The solid-solid mineral reaction between calcite and dolomite is a complex process. Chemical interactions between calcite and dolomite phases have been well documented, especially for the establishment of geothermometry techniques (Goldsmith and Newton, 1969). Significantly, deformation-enhanced chemical exchange has been noted in calcite-dolomite (CcDm) composites deformed to high strains, complicating the geothermometry of naturally deformed systems (Delle Piane et al., 2009a).  3  c  A c  c  a1  a2 c  -a3  a1 a2  -a3  B  a1  a2 -a3  Figure 2.1 Hexagonal structure shared by calcite and dolomite. Green rhombohedron is the primitive, acute rhombohedron of both minerals. The red parallelogram is subject to three-fold rotation to produce the hexagonal structure. A. Calcite. B. Dolomite. Red dots are Ca, green dots are Mg, and the blue triangles sit in the CO3 position.  4  2.2 Deformation Mechanisms in the Earth’s Crust Stress (σ; shear stress: τ) is the force on an area divided by its area and generally reported in the earth sciences in units of MPa. Strain is a unitless measure of the change in shape of a material under stress. For example, absolute axial strain of a given material is given by ! =  !! !  ,  where L is the starting length of material and ΔL is the change in length during deformation (Middleton and Wilcock, 1994). Shear strain, !, is calculated in non-coaxial deformation regimes by ! = tan ! for small !, where ! is the angular shear (Figure 2.2) (Middleton and Wilcock, 1994). In the crust, the mode of deformation depends on a wide range of strain rates, temperatures, and confining pressures. Deformation can be broadly described as either brittle or ductile. These terms describe the macroscopic deformation behaviour of a material and are not mechanism specific. I discuss these failure modes broadly in the following sections.  2.2.1 Brittle Deformation Brittle deformation refers to macroscopic deformation of a material that loses its ability to support load, see Figure 2.3A (Rutter, 1986). This type of deformation typically occurs at low pressure-temperature conditions. Catastrophic failure is accommodated by the coalescence of microfractures (formed by dilatant microcracking) along a macroscopic shear plane (Paterson and Wong, 2005).  5  γ = tanψ  a  s = γz  a  z  ψ x b  b  Differential Stress (σ1-σ3)/MPa  Figure 2.2 Deformation of a circle by simple shear. x and z are the directions of maximum stretching and shortening, respectively of the original circle. Shear strain is calculated γ = tanψ, where ψ is the angular shear.  200 B σ3 = 35 MPa  100 0  A σ3 = 3.5 MPa 0  0.01  0.02 0.03 Absolute Strain  0.04  Figure 2.3 Stress-strain relationships for Wombeyan marble deforming by A. brittle failure and B. ductile processes. This schematic shows the effect of confining pressure on the brittle-ductile transition in Wombeyan marble (Paterson and Wong, 1977).  6  2.2.2 The Brittle-Ductile Transition and Ductile Deformation Ductile deformation refers to macroscopic deformation of a material that does not lose its ability to support load (see Figure 2.3B). The transition between brittle and ductile behaviour can be facilitated by increases in temperature or pressure, decreases in strain rate, or changes in the material properties (e.g. composition) of the material deformed. The geometry of fabrics in brittle and ductile shear systems is similar, however, the mechanisms of deformation giving rise to the fabrics are different (see Figure 2.4). A number of operative deformation mechanisms (including dilatant microcraking and cataclasis) contribute to ductility, making ductility a complicated material behaviour. Possible ductile mechanisms include: pressure solution, mechanical twinning, dislocation glide, dislocation creep, diffusion creep, and grain boundary sliding. An extensive description of these mechanisms can be found in Passchier and Trouw (2005).  2.3 Experimental Rock Deformation A number of experimental apparatuses have been designed to study the deformation of the crust. Uniaxial and triaxial compression apparatuses can only access strains up to a maximum of approximately 5-10%. However, the crust can behave in a ductile manner to very large strains, which are only replicated experimentally through non-coaxial experimental configurations such as the torsion rig (Paterson and Olgaard, 2000). Torsion experimental studies typically target large, high strain systems such as orogenic development.  2.3.1 Calcite Rheology Given the relative ease of deforming carbonates and their global profusion in continental tectonics, the mechanical behaviour of carbonate rocks has been extensively studied. The body of knowledge on the ductile deformation of calcite is especially exhaustive in the field (Kennedy and Logan, 1997; Leiss and Molli, 2002; Molli et al., 2011; Oesterling et al., 2006), in single crystals (De Bresser and Spiers, 1990, 1993; De Bresser and Spiers, 1997; Handin et al., 1967), in compression (De Bresser et al., 2005; Kern and Wenk, 1983; Rutter et al., 1993; Rutter, 1972, 1995; Schmid et al., 1977; Schmid et al., 1987; Schmid et al., 1980; Walker et al., 1990), and in torsion (Barnhoorn et al., 2003, 2005a; Bruijn et al., 2010; Casey et al., 1997; Oesterling et al., 2006; Paterson and Olgaard, 2000; Pieri et al., 2000a; Pieri et al., 2000b) Intracrystalline deformation (dislocation glide and creep) is dominated by the movement of dislocations along specific glide planes. Dominant glide planes in calcite have been identified 7  by means of single crystal deformation experiments and are summarized in Table 2.1. These glide planes are stereographically presented in Figure 2.5A. Of particular interest to this study is the behaviour of fine-grained calcite at high temperature and low strain rates. Depending on stress, strain rate, and grain size, calcite deforms at high temperatures and low strain rates by either linear viscous creep (involving the movement of point defects and grain boundary sliding) or dislocation creep (involving the movement of dislocations) (De Bresser and Spiers, 1990, 1993; Herwegh et al., 2003; Rutter et al., 1994; Schmid, 1976; Schmid et al., 1980; Walker et al., 1990). Grain-size sensitive flow, or diffusion creep, dominates the deformation of calcite rocks for grain sizes less than 10-40 µm (Brodie and Rutter, 2000). Intracrystalline deformation (i.e. dislocation creep dominated; the rate controlling step is diffusion assisted) is dominant in fine-grained calcites at high to moderate strain rates and high to moderate differential stresses (regimes 1 and 2, see Figure 2.6) (Schmid et al., 1977). The behaviour of calcite under these conditions is best described by an exponential stress dependence (regime 1) or power-law creep (regime 2)(Schmid et al., 1977). In the context of rock deformation, superplastic flow resides in regime 3 of Figure 2.6 and is strongly grain size dependent; strain increases with a decrease in grain size for any given stress. Grain boundary sliding is the dominant deformation mechanism assisting deformation in this field.  8  Table 2.1 Identified glide planes in calcite and conditions for activation of slip and twinning along them. Modified from De Bresser and Spiers (1997) and Wenk et al. (1983). System  !  T  Pc  τ  (s )  (°C)  (MPa)  (MPa)  2.5x10-4  800  500  ?  2.5x10-4  300  500  ?  3x10-5  600-800  unconf.  ?  ?  300, 500  500  ?  3.3x10-7  300  500  ?  -1  Reference  Slip  ! 0001 1210  ! 1210 2021  Griggs et al. (1960) Turner and Orozco (1976) De Bresser and Spiers (1993) Paterson and Turner (1970) Turner and Heard (1965) Turner et al. (1954); Spiers and Wenk  !  !  1012 1011  3x10-4 – 3x10-8  (1980); 550-800  unconf.?  18  De Bresser and Spiers (1990); De Bresser and Spiers (1997)  ! ! 1012 2201 0221 ! ! 1012 2201 0221 ! ! 1012 1011 ! ! 1012 0221  ! ! 1014 2021  2.5x10-4  20  500  ?  Turner et al. (1954)  2.5x10-4  600-800  500  ?  Griggs et al. (1960)  2.5x10-5  575-650  unconf.  ?  3x10-5  600-800  unconf.  ?  10  -4  25-800  210-16.5  ?  300  500  ?  1x10-4  460-550  unconf.  ?  2.5x10-5  350-650  unconf.  ?  550-800  unconf.  ?  600  ?  >16  -4  3x10 – 3x10  -8  10-5  Spiers and Wenk (1980) De Bresser and Spiers (1993) Turner et al. (1954) Weiss and Turner (1972) Braillon and Serughetti (1976) Spiers and Wenk (1980) De Bresser and Spiers (1993) Spiers and Wenk (1980)  9  System  ! ! 1014 2021  ! ! 1014 2021  !  T  Pc  τ  (s-1)  (°C)  (MPa)  (MPa)  10  -1  300-500  64-18  10  -7  300-500  30-13  Reference  Turner et al. (1954)  2.5x10-4  20-400  300-1000  ?  Turner et al. (1954)  2.5x10-4  300-00  500  ?  Griggs et al. (1960)  25-500  500  ?  3x10-5  300-800  unconf.  ?  2.5x10-4  20-300  500-100  2.5x10-4  300-800  500  ?  ?  20  300  > 12  ?  300  500  ?  ?  300?  500  ?  4x10-1 3x10  -7  Turner and Heard (1965) De Bresser and Spiers (1993)  Twinning  !  !  1018 4041  11.5-6.5 8.0-8.5  ! ! 1014 2021  ! ! 1012 1011  Turner et al. (1954) Griggs et al. (1960) Borg and Handin (1967) Weiss and Turner (1972) Paterson and Turner (1970)  10  A  B P  R’ Y  R  C  C’  S  Figure 2.4 Brittle fabric (Riedel shear, A) and ductile fabric (S-C fabric, B) for sinistral shear systems. The geometry of fabrics is similar for both shear systems, however, the mechanisms of deformation giving rise to the fabrics are different. A represents shallow level fault zones while B represents deeper level fault zones.  +a3  +a3  e1  f1  f1 r2  r3 c f2 e2  A  +a1  c  +a2 f3  r1  r2  r3  f2  e3  +a2 f3  r1 B  +a1  Figure 2.5 Stereographic projection (upper hemisphere equal angle) showing important slip and twinning planes for A. calcite (modified from De Bresser and Spiers, 1997), and B. dolomite (modified from Barber and Wenk, 2001).  11  Regime 1 Regime 2  2  Regime 3 60  1.5  0˚C 70  1  0˚C 0˚C  80  0.5 0  0˚C  90  log10 Differential stress (MPa)  2.5  −0.5 −1 0  2  4  6 −1  −log10 Strain rate (s )  8  Figure 2.6 Log-log plot of the differential stress vs. strain rate for compression deformation experiments on Solnhofen limestone (Schmid et al., 1977). Regime 1: Exponential relationship between strain rate and stress; Regime 2: Power-law creep; Regime 3: Superplasticity. Regimes 1 and 2 are characterized in the microstructure by dislocaiton glide and/or creep. Regime 3 is characterized in the microstructure by grain boundary sliding.  12  2.3.2 Dolomite Rheology Dolomite rheology is not well understood. Studies have included deformation investigation in the field (Leiss and Barber, 1998; Newman and Mitra, 1994), in single crystals (Barber et al., 1981; Barber and Wenk, 2001; Higgs and Handin, 1959; Turner et al., 1954), in compression (Austin and Kennedy, 2005; Austin et al., 2005; Barber et al., 1994; Davis et al., 2008), and in torsion (Delle Piane et al., 2007; Delle Piane et al., 2008). Field observations suggest that dolomite is stronger than calcite; dolomite is frequently observed fractured and adjacent to calcite, which has undergone extensive internal strain (Erikson, 1994; Woodward et al., 1988). Under similar test conditions, dolomite rock is stronger and less ductile than calcite limestone (Griggs et al., 1951, 1953; Handin and Fairburn, 1955; Higgs and Handin, 1959). Dominant dolomite slip systems are summarized in Table 2.2 and plotted in Figure 2.5B. The role of grain size sensitive deformation mechanisms in fine-grained dolomite has been recently investigated (Davis et al., 2008; Delle Piane et al., 2007; Delle Piane et al., 2008). Mechanical and microstructural evidence suggests that fine-grained dolomites (<40 µm) deform in a linear viscous manner accommodated by diffusion creep associated with grain boundary sliding (Delle Piane et al., 2008). Low to moderate temperature deformation is assisted by basal slip, while at moderate to high temperatures deformation twinning is active (Delle Piane et al., 2008). Fine-grained dolomite deforms by diffusion creep at strengths more comparable to finegrained calcite. Coarse-grained dolomites show significant strength compared to analogous calcite grain sizes, emphasizing the critical nature of grain size in determining the rheology contrast between calcite and dolomite. Coarse-grained dolomite is stronger than calcite in the crystal plastic and dislocation creep fields (Davis et al., 2008). Figure 2.7 is a deformation map for coarse-grained dolomite (grain size = 100 µm) showing the dislocation creep, diffusion creep, and twinning fields. The shaded grey region shows the field for fine-grained dolomite experiments performed by Davis et al. (2008).  13  -1 1000  Crystal Plasticity and Twinning  10-14 s-1 σ1-σ3 (MPa)  100 10 1  10-5 s-1 10-6 s-1  Dislocation Creep  10-8 s-1 Diffusion Creep  10-14 s-1  0.1 100  300  500  700  10-10 s-1 900  T (˚C) Figure 2.7 Deformation map for dolomite of grain size d = 100 μm. Temperature is normalized by a melting temperature for dolomite of 1100˚C. The stress-temperature field for synthetic dolomite experiments is shown by the shaded box, (Davis et al., 2008).  14  Table 2.2 Identified glide planes in dolomite and conditions for activation of slip and twinning along them. Modified from Wenk et al. (1983). !   (s-1)  T (°C)  τ (MPa)  ! 0001 2110  10-5  25-700  50-130  ! ! 1012 2201  10-5  25-700  170-100  ! ! 1014 1210  10-5  > 500  10-5  250-600  System  Reference  Slip Barber et al. (1981); Barber and Wenk (2001) Barber et al. (1981) Barber et al. (1981)  Twinning ! 1012 1011  90-1000  Barber et al. (1981)  2.4 Deformation in Bimineralic Crystalline Rocks Deformation of bimineralic rocks has been addressed in a variety of studies including studies of calcite-anhydrite (in compression: Bruhn and Casey (1997); Bruhn et al. (1999); in torsion: Barnhoorn et al. (2005b)), calcite-muscovite (in torsion: Delle Piane et al. (2009b)), calcite-halite (in torsion: Jordan (1988)), calcite-quartz (in compression: Siddiqi (1997); in torsion: Rybacki et al. (2003)), and calcite-dolomite composites (in torsion: Delle Piane et al. (2009a)). Local heterogeneities of phase distribution can lead to strain localization in originally homogeneous material. Chemical reactions and ion exchange are common between unlike phases and encourage stain localization in poly-mineralic rocks (Olgaard, 1990). However, dragging and pinning at grain boundary contacts by secondary phases (e.g. pores, different mineral phases) inhibits grain boundary mobility (Olgaard, 1990). Strain localization has been associated with a change in deformation mechanism of the ‘weak’ phase (e.g. anhydrite in the calcite-anhydrite system, Barnhoorn et al. (2005b)) from dislocation creep to diffusion creep and/or grain boundary sliding. In fine-grained calcite-dolomite composites, strain localization and foliation development is attributed to grain boundary sliding and diffusion processes accommodated by dislocation activity in dolomite (Delle Piane et al., 2009a). Strong phases do not significantly assist deformation and, therefore, material strength increases with the addition of a strong phase (Rybacki et al., 2003). 15  2.5 Decarbonation High temperatures can facilitate intense mineralogical, chemical, and textural modifications of carbonate rock, leaving it acutely altered, fractured, and thus weakened (Chen et al., 2009; Homand-Etienne and Troalen, 1984; Mao et al., 2009). Calcite dissociates at high temperature according to the reaction (Samtani et al., 2002): CaCO3(s) → CaO(s) + CO2(g).  (1)  The thermal decomposition of calcite produces lime (CaO) and CO2 gas. The thermal decomposition of dolomite differs slightly: it is a two-stage process involving the reaction (Maitra et al., 2005; McIntosh et al., 1990): CaMg(CO3)2(s) → CaCO3(s) + MgO(s) + CO2(g).  (2)  The solid calcite products of equation (2) then break down as per reaction equation (1) and the net dolomite decomposition reaction can be rewritten: CaMg(CO3)2(s) → MgO(s) + CaO(s) + 2 CO2(g).  (3)  Previous studies show that the onset of decarbonation in dolomite occurs at lower temperatures than in calcite (between 600°C and 850°C) (Gunasekaren and Anbalagan, 2007). In unvented experimental setups, an infinitesmial decarbonation reaction occurs when the system is taken out of the stability field for Cc and Dm (Figure 2.8). This fills any pore space with CO2, imparting a pore fluid pressure on the system and reducing the effective confinement of samples.  16  P(CO2) (MPa)  200  MgCa(CO3)2  MgO+CaCO3+CO2 CaCO3 + MgCa(CO3)2 Stability field  150  100  50  0 300  400  500  600  700  800  900  T (˚C) Figure 2.8 From Davis et al., 2008. Thermal dissociation equilibrium of confined dolomite (Goldsmit and Newton, 1959). Dissociation begins at 560˚C. Unvented, jacketed samples containing dolomite taken to ambient temperatures above this dissociation threshold dissociate and generate a CO2 pore fluid pressure as indicated by the curve.  17  Chapter 3: Experimental Methods To model the behaviour of rock deformation experimentally, rocks are typically collected from nature and cored to dimensions that can be deformed within the constraints of current laboratory technology. The experimentalist attempts to deform material under feasible crustal pressure, temperature, pore fluid, and strain rate conditions, but realistic strain rates are generally sacrificed in the interest of producing mechanical results in a timely manner (i.e. not on typical geological timescales). Traditionally, rock mechanics testing has employed coaxial deformation of rock cores in uniaxial or triaxial compression experiments. The geometric limitations of such apparatuses limit total absolute strain to << 1 - typically maxima of 0.05 - 0.1. With the advent of the torsion actuator (Paterson and Olgaard, 2000), high-temperature rock deformation experiments can be performed to very high values of total strain - strains in excess of 50. This apparatus, therefore, allows the experimentalist to more closely simulate conditions in the Earth for at least two reasons. Firstly, natural systems are active for hundreds of millions of years, resulting in absolute strains in excess of 1. Secondly, the geometry of torsion experiments is more physically comparable to non-coaxial movement along fault surfaces. An additional benefit is that the geometry of torsion experiments (explained below) results in a deformed sample that captures the entire range of strain, from 0 to the final strain. This allows for detailed microstructural investigations of the strain evolution in a single sample. The Paterson rig with torsion actuator is used in this experimental program in order to assess high strain in a non-coaxial system and thereby provide better quantification of the textural development of calcite-dolomite composites. Ultimately, this elucidates the role of increasing dolomite content on the mechanical behaviour and strength of calcite limestones.  3.1 Deformation Apparatus and Techniques I performed all experiments in this study in an internally heated, argon-confining medium, triaxial deformation apparatus with torsion actuator (the Paterson rig), described by Paterson and Olgaard (2000) (Figure 3.1). The Paterson rig is capable of deforming samples in torsion under the following temperature (T), confining pressure (Pc), shear strain rate (!), and internal torque (M) conditions: 0℃ < ! < 1350℃, 0  !"# < !! < 500  !"#, 5×10!!   ! !! < ! < 5×10!!   ! !! , and 0  !" < ! < 200  !". The experiments were performed at constant 18  shear strain rates set at either 1×10!! ! !! or 3×10!! ! !! . Both confining pressure and temperature were held constant at 300 MPa and either 750°C or 800°C, respectively. Once the sample was seated in the confining vessel, a minimal axial load was used to couple all segments of the sample assembly and confining pressure was increased to ~250 MPa, as a minimum of 50 MPa is required to operate the furnace. This confining pressure accommodated expansion of the argon gas as it was heated by the furnace. Sample temperature was monitored using a K-type thermocouple placed 3mm above the sample. The thermal profile along the sample was calibrated to be consistent within 1°C. The sample was heated and cooled by 10°C/min. Following sample heating to the target temperature (750°C or 800°C), confining pressure was increased to 300 MPa. The torsion actuator applies a torque to the sample assembly, twisting the assembly (clockwise, looking down) from the top. Slots at the base of the confining vessel hold the sample assembly fixed. The strength of the spacers and the friction between spacers in the sample assembly must exceed the strength of the rock sample so that all strain is localized within the rock sample. If the sample strength is higher than that of the sample assembly components, the jacketing will rupture and the system will degas, terminating the experiment. As deformation is by torsion, the centre of the sample experiences zero torque and the strain recorded there is 0. Strain increases linearly within the sample with increasing radius: !! =  !"  (1)  !  where !! is shear strain at radius, r; l is the sample length; and θ is the angular displacement (Figure 3.2). !! is maximum at the sample surface. This geometry allows observation of the microstructural evolution of a given sample along its radius. Bulk shear strain is calculated from the set shear strain rate, !, multiplied by the length of the experiment. The applied torque was measured using an internal load cell with calibrated linear variable differential transformers (LVDTs). Measured torque was corrected for the strength of the iron jacket (Barnhoorn, 2003) and converted to shear stress at the sample surface: !=  ! ! !! !  ! !!  !  (2)  where τ is shear stress, M is internal torque, d is the diameter of the sample, and n is the stress exponent (Paterson and Olgaard, 2000). n is an unknown, empirical parameter that is used to describe the deformation mechanisms active in the power law creep domain. In this study, the power law creep relationship used is:  19  !!  ! = !! ! ! !"  (3)  where A, n, and Q (the activation energy) are constants, T is the temperature, and R is the gas constant (Paterson and Olgaard, 2000). Equation 2 assumes homogeneous rheological properties throughout the sample and shear stress increases with decreasing n (Figure 3.3). The stress exponent, n, is experimentally determined for a given sample by conducting a strain rate stepping experiment and plotting the total torque response, M, to changing strain rate, !.   As M is linearly related to τ, the slope of the log-log plot ! vs. M yields the stress exponent, n, according to: ! !" !  ! = ! !" !  (4)  Differential stress (!! − !! ) is calculated for comparison between axial and torsion experiments: !! − !! = 3!  (5)  where !! is maximum compressive stress, !! is the minimum compressive stress, and ! is shear stress.  20  Thermocouple  Torsion  Actuator  Servo-Motor  Furnace  External Load Cell  Sample Sample Assembly  Argon In  Internal Load Cell  Figure 3.1 Schematic of the Paterson deformation rig with torsion actuator. Modified from Paterson and Olgaard, 2000 and Barnhoorn, 2005.  21  d  θ  r  l  Figure 3.2 Schematic of sample geometry (Paterson and Olgaard, 2000). d and l and the diameter and length of the sample core, respectively. r denotes the radius of the core for which strain and strain rate are integrated. θ is the angular displacement between originally concomitant points  Shear Stress (MPa)  150  100  50 0  5  10  Stress Exponent (n)  15  20  Figure 3.3 Relationship between stress exponent (n) and shear stress (τ) for a constant internal torque, M = 50 Nm, and sample diameter, d = 15 mm (modified from Barnhoorn, 2003).  22  3.2 Sample Assembly Samples were mounted between alumina and zirconia spacers to ensure a constant temperature profile across the sample (Figure 3.4). The sample assembly was encased in iron jacketing and placed within the confining vessel, which was fitted with a high temperature furnace. A jacket thickness of 0.25 mm was used for 15 mm diameter samples. Jackets for 10 mm diameter samples were swaged to the correct inner diameter resulting in thickening of the jacket wall to 0.4 mm (Figure 3.4). Prepared jackets were checked for punctures and swaging did not damage the jacket material. The mechanical data were corrected for jacket rheology according to Barnhoorn (2003). The deformed sample was placed in epoxy resin and vacuum cured to protect its integrity. Figure 3.5 (modified from Paterson and Olgaard (2000)) illustrates the kinematic planes of the deformed samples, tangential to concentric cylinders within the deformed samples. Each sample was sawn in half. Each half was then sawn to provide an exposed longitudinal tangential kinematic plane (see Figure 3.5), the intermediate kinematic plane (representing one half of the total achieved strain), and a 0-strain plane at the sample centre. This process resulted in two sets of cuts of the maximum, intermediate (50%), and zero-strain kinematic planes. One set of these cuts was sent for thin sectioning. The remaining sample cuts were mounted in epoxy for Scanning Electron Microscopy (SEM) and Electron Backscatter Diffraction (EBSD) analysis. These sections were polished using a rotary polishing wheel and silica colloidal solution to within 200 nm. Polishing resulted in the loss of surficial material on each sample, thus the strain represented by this surface is less than anticipated. For instance, polishing of the maximum shear strain kinematic plane results in a plane of strain less than the maximum strain achieved. Considering the polishing technique used, this loss of material cannot be quantified, but is considered to be trivial in most cases.  23  Alumina Spacers  Sample  Zirconia Spacer  Alumina Spacers  Zirconia Spacer  Iron Jacket  15 mm sample assembly  10 mm swaged sample assembly  10 cm  Figure 3.4 Torsion rig sample assemblies. Top: Sample assembly without jakcet, showing alumina and zirconia spacers. Middle: 15 mm sample assembly with iron jacket. Bottom: 10 mm sample assembly with swaged iron jacket.  24  l z y γmax  γ=0  γ = 0.5γmax  d Longitudinal Axial  l z x Longitudinal Tangential  d y  x  Transverse Figure 3.5 Schematic diagram of the three principal thin section cuts of a rock sample deformed by torsion (Paterson and Olgaard, 2000). The longitudinal axial cut captures intermmediate strains along the centre axis of the segment. The longitudinal tangential segment captures the maximum strain kinematic plane of the sample. Finally, the transverse cut is taken perpendicular to the sample long axis. The x, y, and z directions refer to the kinematic coordinate system, where x is the direction of maximum stretching, z is the direction of maximum shortening, and y records no kinematic movement. d and l are the diameter and length of the sample core, respectively.  25  3.3 Microstructural and Textural Analyses Backscatter electron (BSE) and secondary electron (SE) SEM images were collected using a thermal field emission type Zeiss Sigma SEM with 1.3 nm resolution at 20 kV acceleration voltage. Probe current was 1.37 nA. Electron Backscatter Diffraction (EBSD) analysis was completed to map the evolution of the crystallographic preferred orientation (CPO) of the starting and deformed materials. EBSD measurements were completed using an EDAX DigiView EBSD camera. Samples were inclined to the electron beam at 70° to produce clear diffraction patterns (Kikuchi lines) for automated identification using Orientation Imaging Microscopy (OIM™) Data Collection and Data Analysis software. Dolomite and calcite are exceedingly difficult to differentiate by EBSD due to their similar crystallographic structures. Therefore, energy-dispersive X-ray spectroscopy (EDS) data were collected in conjunction with EBSD diffraction patterns to chemically identify the presence of Mg and Ca in concentrations sufficiently large to identify dolomite. All EBSD data were subsequently processed to take into account approximate 1:1 Ca:Mg-ratios as a defining parameter of dolomite presence. EDS was performed on all analyzed samples using an Apollo XL Silicon Drift Detector (SDD) at a typical working distance of 14 mm. The ideal EDS working distance for this particular system is 8 mm, however, sample geometry dictated that a longer working distance be used to avoid damaging the secondary electron detector within the SEM chamber. Despite this, EDS data were of sufficient quality to establish relative Mgconcentrations throughout the sample and lead to dolomite-calcite discrimination.  3.4 X-ray Diffraction X-ray diffraction (XRD) was used to identify compositional phases and determine proportions of these phases in the starting powders, HIP products, and experimentally deformed sample cores. For Rietveld analysis samples were powdered to <10 µm using a McCrone Micronising Mill. Phase identification was performed by smear mounting hand-ground powders on quartz plates. Data were collected using a Bruker D8 FOCUS powder diffractometer (Bragg-Bentano geometry), equipped with an Fe monochromator foil, a 0.6 mm (0.3°) divergence slit, incidentand diffracted-beam Soller slits, and a LynxEye detector. The long time-focus Co X-ray tube was operated at 35 kV and 40 mA, using a take-off angle of 6°. Diffractograms were collected 26  between 3° and 80° 2θ with a step of 0.4° 2θ. Because so little of the deformed material was available, XRD analysis was qualitative and powders were smear mounted. These samples were spun at 60 rpm using an effective counting time of 0.9 seconds per step during data collection. Powder mounts for Rietveld analysis of starting powders and HIP material were not spun during data collection. Mineral identification was done on all samples using Bruker Diffrac plus EVA™ analysis software (using the International Centre for Diffraction Database PDF-4). The background was removed using the EVA built-in tools and the diffractogram was aligned to a quartz standard peak (analysis 00-046-1045) (to account for changes in the sample geometry). Alignment in all samples was smaller than 0.05° 2θ. Exact proportions of calcite and dolomite in the starting powders and HIP products were determined by Rietveld refinement (Raudsepp et al., 1999) using TOPAZ™.  3.5 Microprobe Electron-probe micro-analyses of deformed samples were done to confirm exact grain composition. Data were collected on a fully automated CAMECA SX-50 instrument, operating in the wavelength-dispersion mode with the following operating conditions: excitation voltage: 15 kV; beam current: 10 nA; peak count time: 20 s; background count-time: 10 s; spot diameter: 10 µm. Data reduction was done using the 'PAP' φ(ρZ) method (Pouchou and Pichoir, 1985). Oxygen was determined by stoichiometry and carbon by difference. For the elements considered, the following standards, X-ray lines, and crystals were used: dolomite, MgKα, TAP; calcite, CaKα, PET; rhodochrosite, MnKα, LIF; siderite, FeKα, LIF. The data were subsequently converted from wt% to Ca and Mg fraction.  27  Chapter 4: Starting Material Preparation and Characterization This study comprises experiments performed on synthetic carbonate rocks, rather than natural carbonates (e.g., Carrara marble, Solnhofen limestone; e.g. Rutter et al. (1994), Schmid et al. (1977), and Barnhoorn et al. (2003)). Natural rocks have unavoidable, though perhaps limited, variation in grain size, porosity, texture, and composition. This reality motivates deformation of synthetic rocks of controlled composition, grain size, and porosity (Olgaard and Fitz Gerald, 1993). The goal of this study is to understand the role of dolomite in controlling the mechanical properties and behaviour of natural limestones. To isolate the effects of dolomite content on the experimental response, I used hot isostatic pressing (HIP) techniques to make synthetic calcitedolomite (Cc-Dm) composite samples from pure end member powders of the following compositions: Cc75:Dm25, Cc65:Dm35, Cc49:Dm51, and Cc25:Dm75. Herein these compositions are denoted Dm25 , Dm35, Dm51, and Dm75.  4.1 Sample Preparation 4.1.1 Starting Powders Pure reagent calcite powder and crushed natural dolomite marble were combined to produce composite powders for HIP. The reagent-grade calcite powder (Minema 1™) was supplied by Alberto Luisoni Ag, Mineral- & Kunststuffe and has a modal grain size of 9 µm. The full grain size distribution for the calcite powder, as measured with a Mastersizer 2000 laser diffraction particle size analyser (Malvern Instruments Ltd.), is shown in Figure 4.1A. The reagent calcite powder contains equiaxed calcite grains exhibiting rare growth twins. X-Ray Diffraction (XRD) analysis of the calcite powder confirms its composition to be 99% CaCO3 with the remaining constituents below detection. See Appendix B for XRD data. The dolomite end-member powder derives from a 4 kg block of Badshot marble, a natural dolomite marble sourced from the Selkirk Mountains of British Columbia (Austin, 2003). Badshot dolomite is characterized by coarse dolomite grains featuring lobate grain boundaries and fine, polygonal grains. Cleavage and twinning are prevalent in most grains (Austin, 2003). Crushing of the Badshot marble produces a powder having a modal grain size of ~120 µm. The full grain size distribution for the crushed marble is shown in Figure 4.1A. XRD analysis of the Badshot powder indicates a mineralogy that is ~98% dolomite (see Appendix B). Based on thin 28  section analysis there are trace (<< 1%) quantities of pyrite, apatite, calcite, tremolite, and white mica; these accessory phases are sufficiently low in abundance to be undetected by XRD analysis. The grain size distributions of the mixed starting powders (i.e. Dm25, Dm35, Dm51, and Dm75) are shown in Figure 4.1B. The starting powders have a bimodal grain size distribution, reflecting the proportion of dolomite powder.  4.1.2 Hot Isostatic Pressing (HIP) All powder mixtures were dried for at least 24 hours in an oven at 110°C. Powders were cold pressed into steel canisters (outer diameter: 35 mm; length: 165 mm) using an Enerpac-HFrame 50-t press to a maximum stress of approximately 200 MPa. The ends of the canisters were filled with alumina powder to provide a reservoir for CO2 produced during decarbonation of dolomite during the HIP process (Delle Piane, 2009). The canisters were then incrementally filled with the powder mixtures by adding 20 g at a time. Incremental cold pressing ensures that the powders are homogeneously packed. Subsequently, the canisters were welded shut and sandblasted to remove any surface rust. HIP was performed in a large volume, internally heated, argon gas apparatus at ETH-Zürich under a confining pressure of 170 MPa (Delle Piane et al., 2009a) and a temperature of 700°C for 4 hours. The resulting product is a suite of coherent, sintered material of known compositions and consistent grain size, texture, and porosity.  4.1.3 Chemistry Material from each canister was crushed and powder-mounted for XRD and Rietveld refinement to confirm the compositional proportions of Cc and Dm in the HIP products, herein referred to as the starting material. Rietveld refinement confirms that there was little decarbonation of dolomite or calcite during HIP as neither periclase (MgO) nor lime (CaO) was detected. Rietveld refinement confirms that the compositions of the starting materials are 25%Dm, 35%-Dm, 51%-Dm, and 75%-Dm, with calcite comprising the remaining proportion. See Appendix B for diffractograms.  29  4.1.4 Porosity The skeletal and isolated pore space volumes, Vs+i, of each sample were determined by use of a Micrometritics Multivolume Pycnometer 1305 helium pycnometer. Reproducibility was determined by running samples at least 3 times. Connected porosity,   !, is calculated from the geometric bulk volume, !! , and skeletal and isolated pore volume: ! = 1−  !!!! !!  ×100%  (1)  The final composition, porosity, and density of the starting materials are given in Table 4.1. Table 4.1 Hot isostatic pressing (HIP) conditions and properties of HIP product. Dm is dolomite content (%), φ is connected sample porosity, and ρ is sample density. Dm  φ  ρ  (%)  (%)  (kg m-3)  25  3.3±0.2  2.76  35  3.3±0.2  2.77  51  2.7±0.3  2.80  75  5.2±0.3  2.85  30  7  Cc  Volume Percent (%)  7  8  A  6  6 5  5 4  4  3  3  2  Dm  1 0  Dm−25 Dm−35 Dm−51 Dm−75  B  Volume Percent (%)  8  2 1  0  10  1  10  Grain Size (�m) μ  2  10  0  0  10  1  10  μ Grain Size (�m)  2  10  Figure 4.1 Volume percent grain size distributions of starting material powders. A. Grain size distributions of pure calcite and pure dolomite powders. Modal grain size of the calcite and dolomite powders are 9 μm and 120 μm, respectively. B. Grain size distributions of powder mixtures used for hot isostatic pressing of synthetic samples.  31  4.1.5 Sample Assembly Preparation In total, 3 canisters of each composition were lithified by HIP and, from each canister, three cores were recovered for deformation. Dm25 and Dm35 were drilled to produce 15 mm diameter experimental cores; this size was chosen to fit flush with the sample assembly spacers and pistons. In contrast, synthetic samples Dm51 and Dm75 were drilled to produce smaller diameter (10 mm) experimental cores. The smaller core diameter is needed to offset the strengths of the higher dolomite content samples. Even at high temperature, Dm51 and Dm75 are too strong to deform in torsion using a 15 mm diameter sample. All samples were hand ground using 800 grit sand paper to parallelism. Samples were stored in a 110°C oven until they were mounted in the sample assembly for deformation.  4.2 Microstructure Backscattered electron (BSE) images obtained from scanning electron microscopy (SEM) confirm that, for all starting material compositions, the calcite is equant in shape and generally has straight grain boundaries. Calcite grains are closely packed with triple junction grain boundaries (Figures 4.2B, 4.3B, 4.4A, and 4.5B). Porosity is isolated and therefore not accessed by the helium gas pycnometer, thus, the porosity data obtained from pycnometry is considered a lower limit. The Dm25 and Dm35 starting materials contain randomly distributed circular concentrations of calcite observed throughout the sample in thin section and SEM (Figure 4.6). These concentrations are spherical throughout the samples and accreted during mechanical shaking of the starting powders. The margins of the circular cross-sections of these calcite “clumps” are accentuated by concentrations of dolomite grains, such that the long axes of dolomite grains are tangential to the spherical calcite clumps. Dolomite grains are, generally, homogeneously distributed throughout all synthetic starting materials (Figures 4.2A and 4.4C), with the exception of the calcite clumps. Locally, however, coarse-grained dolomite grains can form clusters (Figure 4.5C). Coarse dolomite grains can be encased in a halo of predominantly fine-grained calcite (Figure 4.3C), inferred to be developed by accretion of fine grained material during mechanical shaking of the starting powder during HIP preparation.  32  Dolomite grains are angular to subangular and are fractured; straight fractures appear to follow cleavage planes but curved fractures also exist (see Figures 4.2C, 4.3A, 4.4B, and 4.5A). These fractures are a result of the crushing process used to produce the starting dolomite powder and do not continue into the calcite matrix. Intergranular porosity is greatest at Dm-Cc interfaces.  33  A  B  20 μm  100 μm  C  10 μm Figure 4.2 Backscatter electron images of Dm25 starting material. The cold pressing direction is into the page for all images. A. Subangular to subrounded dolomite grains in calcite matrix. There is significant isolated porosity at grain boundaries. Dolomite grains with high aspect ratios are randomly oriented. B. Equiaxed calcite grains, closely packed with straight grain boundaries forming triple junctions. There is significant intergranular porosity. C. Subangular, fractured dolomite grains; fractures are generally along cleavage planes, but some fractures are curved.  34  A  B  100 μm  10 μm  C  100 μm Figure 4.3 Backscatter electron images of Dm35 starting material. The cold pressing direction is into the page for all images. A. Large, angular to subrounded, fractured dolomite grains in micritic calcite matrix. Porosity is greatest at grain boundaries between dolomite and calcite. B. Equiaxed calcite grains, closely packed with straight grain boundaries forming triple junctions. Porosity is intergranular and isolated. C. Subangular to subrounded dolomite grains heterogeneously distributed. Note the dolomite-deficient zone encircling the large dolomite grain in the centre. Long axes of high aspect ratio dolomite grains surrounding this grain are tangential to the dolomite grain’s edge, suggesting that this zone formed by accretion of fine grained material, primarily calcite, during the starting powder preparation. 35  A  B  20 μm  1 μm  C  100 μm Figure 4.4 Backscatter electron images of Dm51 starting material. The cold pressing direction is into the page for all images. A. Equiaxed calcite grains, closely packed with straight grain boundaries forming triple junctions. Porosity is concentrated at dolomite boundaires and between calcite grains. B. Large, subangular to subrounded, fractured dolomite grains in micritic calcite matrix. Porosity is greatest in calcite matrix in narrow areas between dolomite grains. C. Dolomite grain distribution, homogeneous on this scale.  36  A  B  ue  q Opa  10 μm  10 μm  C  100 μm Figure 4.5 Backscatter electron images of Dm75 Starting material. The cold pressing direction is into the page for all images. A. Subangular dolomite grains. Bright white grain is an opaque mineral identified in thin section as pyrite. Note significant porosity (black) in proximity to dolomite grains. B. Equiaxed calcite grains, closely packed with straight grain boundaries forming triple junctions. Porosity is concentrated at dolomite boundaires. C. Heterogeneous distribution of coarse-grained dolomite throughout Dm-75 starting material. Note region at centre of BSE image that is populated exclusively by dolomite grains smaller than 50 μm.  37  100 μm  100 μm  Figure 4.6 Backscatter electron images of a pure calcite clump in Dm35 starting material. The diameter of the imaged clump ~275 μm. High aspect ratio dolomite grains are oriented such that their long axes are tangential to the circumference of the clump. Dashed white line identifies the boundary between pure calcite and calcite+dolomite.  38  4.3 Textures and Fabrics Individual calcite grains lack internal distortion of the crystal lattice; that is, undulose extinction and/or subgrain development is rare or absent (Figures 4.7 and 4.8). For all starting material compositions, stereographic data obtained from electron backscatter diffraction (EBSD) analysis indicates that there is a weak crystallographic preferred orientation (CPO) of the c-axis of the calcite grains. This is oriented parallel to the long axis, b, of the HIP canisters (Figures 4.7 and 4.8). Calcite grains are generally equiaxed, with limited growth twinning. Regardless of calcite volume %, the CPO of calcite aggregates does not significantly vary. Upper hemisphere stereographic projections of the dominant calcite slip systems show a diffuse concentration of the c-axis, oriented perpendicular to load direction during cold pressing of the material in the canister before HIP, as observed by Rutter et al. (1994) (i.e. c-axis CPOs may occur broadly parallel to the b direction of the canisters). Dolomite in the starting material does not show a CPO. EBSD indicates no preferred orientation along any of the known dolomite glide planes; though data are limited by the small number of dolomite grains measured as a result of the coarse grain size (Figures 4.7 and 4.8). Grain size distributions based on two-dimensional images for Dm25 and Dm75 were computed using Orientation Imaging Microscopy (OIM™) data analysis software by fitting a model ellipse to each crystallographically identified grain (Figure 4.9). Both starting compositions show similar calcite grain size distributions; Dm25 and Dm75 have modal calcite grain sizes of 5.5 µm and 4.5 µm, respectively. Dolomite grain size distributions are not as precise, reflecting the small number of dolomite grains in the scan areas.  39  A a  b  100 μm 20 μm  B  Calcite  C  Dolomite  100 μm  100 μm  b  b  b  b  a  a  a  a  a  a  a  a  Figure 4.7 Electron backscatter diffraction (EBSD) analysis of Dm25 starting material. Hot isostatic pressing conditions: Confining pressure 170 MPa; Temperature 700˚C; 4 hours. The a and b axes refer to the axes on the stereographic projections. The cold pressing direction is into the page for all images. A. Low magnification BSE image, with inset to scale. B. Calcite: Inverse pole figure (top); upper hemisphere contoured stereoplots (bottom) for the c, r, a, and f slip systems, clockwise from top left. C. Dolomite: Inverse pole figure (top); upper hemisphere contoured stereoplots (bottom) for the c, r, a, and f slip systems, clockcwise from top left. Blackened portions of the IPFs are components of different phases, not indexed.  40  A a  b  20 μm  B  Calcite  100 μm  C  Dolomite  100 μm  b  b  b  b  a  a  a  a  a  a  a  a  Figure 4.8 Electron backscatter diffraction (EBSD) analysis of Dm75 starting material. Hot isostatic pressing conditions: Confining pressure 170 MPa; Temperature 700˚C; 4 hours. The a and b axes refer to the axes on the stereographic projections. The cold pressing direction is into the page for all images. A. Low magnification BSE image, with inset to scale. B. Calcite: Inverse pole figure (top); upper hemisphere contoured stereoplots (bottom) for the c, r, a, and f slip systems, clockwise from top left. C. Dolomite: Inverse pole figure (top); upper hemisphere contoured stereoplots (bottom) for the c, r, a, and f slip systems, clockwise from top left. Blackened portions of the IPFs are components of different phases, not indexed.  41  0.14  A  0.6  0.1  0.5  0.08  0.4  0.06  0.3  0.04  0.2  0.02 0 0 10  SM25 SM75  B  Area Fraction  Area Fraction  0.12  0.7  SM25 SM75  0.1 1  10  Grain Size (�m) μ  2  10  0 0 10  1  10  2  10  Grain Size (�m) μ  3  10  Figure 4.9 Area fraction grain size distributions of hot isostatically pressed starting material. A. Grain size distributions of calcite for SM25 and SM75. B. Grain size distributions of dolomite for SM25 and SM75.  42  Chapter 5: Results This chapter presents the experimental results and microstructural analysis of the run products. Optical microscopy, backscatter electron (BSE) imaging by scanning electron microscopy (SEM), and fabric analysis using electron backscatter diffraction (EBSD) were used to characterize the microstructure.  5.1 Sample Decarbonation All experiments were performed unvented and X-ray diffraction (XRD) analysis revealed no decarbonation products (i.e. periclase and/or lime; XRD patterns can be found in Appendix B). Under unvented conditions, decarbonation is a concern and a pore fluid pressure has been attributed to CO2 production in previous studies (Davis et al., 2008; Delle Piane et al., 2009a). However, the experiments performed in this study lie in the stability field of Cc and Dm (Figure 5.1) and as there is no empirical evidence for decarbonation, I have not corrected for a pore fluid pressure.  5.2 Mechanical Results A list of all experiments performed in this study can be found in Table 5.1 along with experimental conditions and sample compositions. When necessary, experiment numbers are referred to in conjunction with composition.  43  MgCa(CO3)2  MgO+CaCO3+CO2  300  250  P(CO2) (MPa)  200  CaCO3 + MgCa(CO3)2 Stability field  150  100  50  0 300  400  500  600  700  800  900  T (˚C) Figure 5.1 From Davis, 2005. Thermal disscoiation equilibrium of confined dolomite (Goldsmit and Newton, 1959). Dissociation begins at 560˚C. Unvented, jacketed, dolomite-bearing samples taken to ambient temperatures above this dissociation threshold will dissociate and generate a CO2 pore fluid pressure as indicated by the curve. The red dot is the location on the plot at which the experiments in this study sit in the Cc+Dm stability field, therefore no decarbonation is expected during the experiments. The red arrows indicate the P-T path of the sample before the experiment. The path was reversed at the end of the experiment, therefore no decarbonation is expected during the pump-up and heating or during the decompression and cooling of the samples.  44  Table 5.1 List of deformation experiments performed and results. Experiment  Dm  T  PC  !  !!"#  -1  (%)  (°C)  (MPa)  (s )  P1525  75  750  300  3x10-4  0.16  P1528  51  750  300  3x10-4  0.21  Yield Experiments  High Strain Experiments P1522  25  750  300  3x10-4  4.4  P1524  35  750  300  3x10-4  5  P1527  25  750  300  3x10-4  5.5  P1537  51  750  300  3x10-4  1.7  P1538  75  750  300  3x10-4  5.5  Low Strain Rate Experiments P1523  51  750  300  1x10-4  1.9  P1533  75  750  300  1x10-4  0.17  P1543  35  750  300  1x10-4  0.1  800  300  3x10-4  4.25  300  stepping  n.d.  High Temperature Experiment P1534  51  Strain Rate Stepping Experiment P1529  35  750  n.d. not determined, Dm – dolomite content (%), T – temperature (°C), PC – confining pressure (MPa), ! - shear strain rate (s-1), !!"# – maximum shear stress.  5.2.1 Strain Rate Stepping Experiment A strain rate stepping experiment was performed to determine the stress exponent value, n, that is required for processing the mechanical data collected during the torsion experiments (see Chapter 2). All mechanical data are fit to a power-law creep relationship of the form ! ∝ ! ! (after Paterson and Olgaard (2000)). For micritic calcite limestones (e.g. Solnhofen limestone) the n-value determined from strain rate stepping experiments suggests dominant deformation mechanisms. Based on the n-value, three deformation mechanism ‘regimes’ have been identified (Schmid et al., 1977). An exponential stress dependence of strain rate is characteristic of regime 1 (Schmid et al., 1977) and, therefore, n is not applicable. Regimes 2 and 3 are characterized by power-law creep where ! > 3 in regime 2 (!~4.7 for Solnhofen limestone) and 1 > ! > 3 in 45  regime 3 (!~1.7 for Solnhofen limestone) (Schmid et al., 1977). As mentioned in Chapter 2, regimes 1 and 2 are microstructurally characterized by intracrystalline deformation; regime 3 is characterized by superplastic flow (grain boundary sliding accommodated by diffusion creep and/or dislocation glide).  46  −2  Ln Shear Strain Rate  −4 −6 −8  n=3  n=2  −10 −12  n=1  −14 −16 1  1.5  2  2.5  Ln Torque  3  3.5  4  Figure 5.2 Log-log plot of shear strain rate vs. torque for the strain rate stepping experiment (P1529; Dm35). Conditions: T=750˚C; Pc = 300 MPa. The red line is the line of best fit through the mechanical data. The slope this line is the n-value for the given composition (n=2). n-values of 1 and 3 are shown for comparison.  47  A Dm35 sample was deformed in a strain rate stepping experiment, and from the procedure outlined in Chapter 3, n is empirically determined to be 2 (Figure 5.2). n = 2 was used to process all the mechanical data gathered in this study.  5.2.2 Yield Experiments Yield experiments were performed for Dm75 (P1525) and Dm51 (P1528), at a confining pressure of 300 MPa and temperature of 750°C using a strain rate of 3x10-4 s-1. Experiments were terminated after the mechanical yield point on the load-displacement curve so that the microstructure created during the first moments of inelastic deformation could be assessed. Yield shear stress is greater for Dm75 (~70 MPa) than for Dm51 (~48 MPa) (Figure 5.3A). I chose the yield point for all experiments to be the first deviation from the elastic portion of the deformation curve. The mechanical yield for both experiments occurs at !~0.025, suggesting that the slope of the elastic portion of both curves is the same. Following elastic deformation, both curves see a pronounced roll-over between 0.025 < ! < 0.075. Dm75 continued to strain harden until the experiment ended. Dm51 exhibited some strain hardening after roll-over but this is subtle and the curve is sub-horizontal. Both experiments were terminated by rapid decompression of the sample due to breaking of the iron jackets. This is a result of the inherent strength of the Dm51 and Dm75 materials with a diameter of 15 mm; the internal torque measured during deformation is proportional to the cube of the sample diameter (! ∝ ! ! ) (see Chapter 3, equation 2). Therefore, for the same shear stress, cores of smaller diameter can be deformed to lower internal torques, and are therefore more easily deformed.  5.2.3 High Strain Experiments High strain experiments were conducted for all four compositions (experiments P1524, P1527, P1537, and P1538) at T=750°C, Pc=300 MPa, and ! = 3×10!! ! !! . The maximum shear strain in each experiment exceeded ! = 5 (Figure 5.3B). Due to a heating coil malfunction within the sample furnace, the heating history during experiment P1537 (Dm51) is not confidently known beyond !~2; only the mechanical data up until the furnace malfunction is used. P1537 was not used for microstructural analysis because of its variable heating history. Yield strength of the synthetic composite samples increases with increasing dolomite content (Figure 5.3B). Dm25 and Dm35 are mechanically similar, both reaching a peak yield strength of ~79 MPa. Mechanical steady-state (~79 MPa) is established in both samples at γ < 0.1, with limited strain hardening in Dm25 at γ ~ 3.75. With increasing dolomite content, there is 48  a significant increase in strength; Dm51 and Dm75 have peak yield strengths of ~140 MPa and ~178 MPa, respectively. Dm51 reaches a tenuous steady-state at τ~130 MPa and γ~0.4. In particular, the evolution of the mechanical behaviour of the Dm75 sample is dynamic, with dramatic strain weakening after yield to a shear strain of !~1. Strain hardening and weakening are observed between 3 < ! < 4; this change in shear stress is considered to be real as there were no noted technical problems during the experiment.  5.2.4 High Temperature and Low Strain Rate Experiments High temperature and low strain rate experiments were performed on the high dolomite content materials with the aim of reducing the strength of the materials to prevent rupturing of the jackets. A high temperature experiment was performed on Dm51 which was deformed at T=800°C, Pc = 300 MPa, and ! = 3×10!! ! !! (P1534). At these conditions it was possible to deform Dm51 with a diameter of 15 mm. The mechanical data (Figure 5.3C) show a yield stress of ~67 MPa and a shear stress vs. shear strain curve similar to that of Dm25 shown in Figure 5.2B. Temporary mechanical steady state (τ~66 MPa) is achieved immediately after the yield point, followed by limited strain hardening beginning at !~2.75 to a maximum shear strain of τ~69 MPa and strain weakening at !~3.7. The experiment was halted by a catastrophic Mode I crack through the sample and jacket leading to rapid decompression. XRD analysis detected no decarbonation products and therefore the microstructure has been analyzed to investigate the microstructural response of the sample over large strains. Low strain rate experiments confirm that yield shear stress increases with increasing dolomite content (Figure 5.3D). Dm75 (P1533) was terminated by a jacket puncture due to the inherent strength of the Dm75 material. Dm51 (P1523) was deformed to !~1.9 and showed continued strain hardening after a yield stress of ~28 MPa to a maximum shear stress of ~36 MPa.  49  Yield Experiments  200  A  120  Shear stress (MPa)  Dm-75  80  Dm-75  100  60 Dm-51  40  Conditions: T = 750˚C; Strain rate = 3x10-4 s-1  20 0  −20 0  0.05  0.1  0.15  Shear strain  0.2  70  0.25  50 0  −50 0  Shear stress (MPa)  Shear stress (MPa)  80  Dm-51  50 40  20 10 0  −10 0  High Temperature Experiment Conditions: T = 800˚C; Strain rate = 3x10-4 s-1 1  2  3  Shear strain  Dm-51  Dm-35  Conditions: T = 750˚C; Strain rate = 3x10-4 s-1 1  2  3  Shear strain  4  5  Dm-75  60 40  5  Dm-51  Low Strain Rate Experiments  Dm-35  Conditions: T = 800˚C; Strain rate = 1x10-4 s-1  20  −20 0  6  D  0 4  Dm-25  100  C  60  30  B  High Strain Experiments  150  100  Shear stress (MPa)  140  0.5  1  Shear strain  1.5  2  Figure 5.3 Mechanical data for all the experiments performed in this study. Pc = 300MPa for all experiments. See table 5.1 for experimental conditions. A. Yield experiments: P1525 (Dm75) and P1528 (Dm51). B. High strain experiments: P1524 (Dm35), P1527 (Dm25), P1537 (Dm51), and P1538 (Dm75). C. High temperature experiment: P1534 (Dm51). D. Low strain rate experiments: P1523 (Dm51), P1533 (Dm75), and P1543 (Dm35).  50  5.3 Microstructure Six representative deformed samples were prepared for microstructural analysis (see Table 5.2). Table 5.2 List of sample slices used for microstructural analysis. Approximate shear strains for each slice are estimates based on the distance from the centre of the sample. All experiments were deformed using ! = 3×10!!   ! !! . See Chapter 3 for a description of sample preparation. Dolomite Content  Sample number  (%)  Pc  T  (MPa)  (°C)  Approx. γ  25%  P1527_1  300  750	
    5.5  25%  P1527_2  300  750  2.25  25%  P1527_3  300  750  0  35%  P1524_1  300  750  5  35%  P1524_2  300  750  2.5  51%  P1528_1  300  750  0.21  51%  P1534_1  300  800  4.25  51%  P1534_2  300  800  2.1  75%  P1538_1  300  750  5.5  75%  P1538_2  300  750  2.25  75%  P1538_3  300  750  0  75%  P1525_1  300  750  0.16  5.3.1 Yield Experiments Yield experiments were performed on Dm51 and Dm75 and pure calcite clumps are absent. Like the starting material compositions, the yield experiment microstructure is populated by equiaxed calcite with generally straight grain boundaries. Calcite grains are closely packed with triple junction grain boundaries (Figures 5.4). Dolomite grains are homogeneously distributed throughout both samples (Figures 5.4A and B). Dolomite grains are angular to subangular and are fractured; straight fractures appear to follow cleavage planes but curved fractures also exist. Fractures do not continue into the calcite matrix. Intergranular porosity is greatest at dolomite-calcite interfaces. There is no foliation evident in these samples in BSE imaging and all high aspect ratio dolomite grains are randomly oriented and do not define a fabric.  51  A  x z  B  x z  Dm  Cc  20 μm  100 μm  Figure 5.4 Scanning electron microscope (SEM) images of yield experiments. Dolomite C (Dm) is dark grey and calcite (Cc) is the light grey matrix. Dolomite grains are angular to subangular, and no rigid body rotation has taken place. See Table 5.1 for experimental conditions. Arrows indicate shear direction and the x-z coordinate systems show the orientation of the kinematic plane: x is the direction of maximum stretching and z is the direction of maximum shortening. A. Dm75 (P1525). Note Dm-Dm contacts. B. Dm51 (P1528).  52  A  x  B  x  x z  z  z  C crack in thin section  ψ  2.5mm  2mm  D  x  1.5mm  E  x  z  z  pyrite  pyrite c-slip 500 μm  phyllosilicate  500 μm  Figure 5.5 Deformed material. Dolomite is the larger, cream-coloured phase. Calcite is the dark matrix. Arrows indicate shear direction and the x-z coordinate systems show the orientation of the kinematic plane: x is the direction of maximum stretching and z is the direction of maximum shortening. A, B, and C. Core scans of Dm25 (P1527), Dm35 (P1524), and Dm75 (P1538), respectively, deformed to γ~5. Conditions: Pc=300 MPa, T=750˚C, and shear strain rate 3x10-4. Foliation is defined in A and B by elongate calcite clumps. Shear strain is calculated by γ=tanψ, as shown in A. The dark lines in C are cracks in the thin section glass and not experimentally induced. D. Plane polarized image of Dm25 run product. Foliation is delineated by c-slip surfaces and elongate pyrite grains. Dolomite grains are organized along foliation and are not obviously rounded. E. Plane polarized image of Dm75 run product. Foliation is delineated by elongate pyrite and phyllosilicates. Dolomite is obviously rounded, but not well organized along foliation.  53  5.3.2 High Strain Experiments Low dolomite content (Dm25 and Dm35): Core scans of Dm25, Dm35, and Dm75 samples deformed to high strain are shown in Figures 5.5A, 5.5B, and 5.5C. A bulk foliation is observed in Dm25 and Dm35, delineated by elongate, macroscopic zones of fine-grained calcite (Figure 5.5A and 5.5B). Dolomite grains are not obviously reduced in size or rounded, compared to the starting material (Figure 5.5D and 5.5E). Optical microscopy images of Dm25 show a reorganization of the calcite matrix along foliation, highlighted by elongate pyrites and dark, narrow bands along foliation, which appear to be c-slip surfaces (Figure 5.5D): surfaces of relatively higher shear strain. The calcite clumps identified in the starting material are sheared into thin bands (ellipses) of pure calcite with aspect ratios ranging from 0.2:4.5 and 0.4:7.5 (Figure 5.5A and5.5B). Assuming that these clumps were originally circular in cross section and that there was no loss of volume during deformation, the angular strain can be calculated ! = !"#$, where ! is the angle between the sample long axis and the long axis of the rotated clump (Figure 5.5A). These clumps record shear strains of 4 and 5.7 for Dm25 and Dm35, respectively. The calcite grains comprising the layers are equiaxed, with straight grain boundaries exhibiting triple junctions. Compositional layering of the samples is defined by these thin layers of pure calcite, alternating with composite mixtures (Figure 5.6A). Calcite layers are inclined along the direction of maximum stretching, while the surrounding dolomite-calcite material foliation is less inclined, resulting in two foliations (Figure 5.6B). This results in deflected foliations throughout Dm25 and Dm35 samples. Within the Dm-Cc bulk matrix, calcite grains are closely packed with straight grain boundaries forming triple junctions (Figure 5.6C). Grain boundaries are most defined approximately parallel to the direction of maximum stretching (Figure 5.6C), suggesting some grain elongation. Contributing to the foliation is the rigid body rotation of high aspect ratio dolomite grains (aspect ratios > 1; Figure 5.6B and 5.6D). Dolomite grains do not appear to have sustained any additional brittle fracture, as fracture density is qualitatively the same as in the starting material. While there is observed rigid body rotation, there does not appear to be significant rounding of grains above ~100 µm. Dolomite grains <100 µm show some rounding (Figure 5.6D). Accessory minerals, in particular pyrite and phyllosilicates, act as strain markers. Pyrite is significantly elongated and boudinaged throughout Dm25 and Dm35 samples and defines foliation along with c-slip surfaces and elongate calcite clumps. 54  A  B  x  x  z  z  200 μm  C  x z  20 μm  D  x z  20 μm  Figure 5.6 High strain deformed material: Dm25; P1527; γ~5; 750˚C; 3x10-4 s-1. A. Deformed Cc clumps, circles are sheared to ellipses (dashed line). C-slip surfaces are along present near these features (white arrow). Plane polarized optical photograph. B. Deflected foliation from bulk Cc-Dm into more deformed pure-Cc band. Dashed white line delineates the bulk foliation within the sample. Dashed orange line delineates the orientation of the boundaries of the pure-Cc band. C. Closely packed, equiaxed to tabular Cc-grains. Grain boundaries form triple junctions and are generally straight. Note significant isolated porosity. D. Dm35 (P1524). Localized porosity developing at boundaries of coarse grained dolomite, along foliation.  55  A  x  B  z  z  200 μm C  x z  200 μm  x  200 μm D  x z  20 μm  Figure 5.7 High strain deformed material: Dm75. P1538; γ~5; 750˚C; 3x10-4 s-1. A and B. Plain polarized light (ppl) optical images. A. Evidence of brittle fracture during deformation. C-slip surfaces are traced by white lines. Mode I cracks in dolomite are identified by the black arrow. The white ellipse highlights a dolomite grain that has fragmented by shear. B. Evidence of ductile deformation. The white line defined the local foliation developed within the calcite matrix. The foliation is deflected around large dolomite grains that cannot be incorporated into the foliation. A deformed pyrite is identified by the white ellipse. C. SEM image showing patchy foliation development in Dm75, (see the centre of the image where a narrow flow band occurs). The opaque mineral in top left hand corner (white) has been boudined and deformed around the more rigid dolomite grain. D. Closely packed, equiaxed to elongate cc-grains. Grain boundaries form triple junctions and are generally straight. Straight calcite grain boundaries and high aspect ratio dolomite grains define foliation. Stretched, boudined phyllosilicate, centre-top.  56  A  x  B  x  z  z  porosity  200 μm  20 μm  Figure 5.8 Dm51 deformed at high temperature; P1534; γ~4; 800˚C; 3x10-4 s-1. A. Foliation defined by dolomite grains with high aspect ratios (i.e. aspect ratios >1). Foliation deflected around a coarse grained, fractured dolomite. Dashed white line shows foliation. B. Closely packed, equiaxed to tabular cc-grains. Grain boundaries form triple junctions and are generally straight. Straight grain boundaries are oriented along foliation. Note significant isolated porosity (small, black features) in A and B.  A  57  While porosity is prevalent at calcite grain boundaries, porosity is visibly reduced with respect to the starting material. Porosity is more homogeneously distributed at triple junctions in the calcite matrix than in the starting material. Intracrystalline porosity (both pores and fractures) in dolomite remains largely unchanged and there is no evidence of fracture healing. Locally, there are regions of higher porosity within the calcite matrix aligned along foliation (Figure 5.5E and 5.5F). These regions form in pressure shadow-like geometries on the peripheries of some dolomite grains > 70 µm (Figure 5.6D). High dolomite content (Dm75): There is a poorly defined foliation in Dm75 defined by crude variation in grain size and fine-grained, high aspect ratio dolomite. On the sample scale, foliation is highlighted by deformed phyllosilicates and pyrites (Figure 5.5E). Dolomite grains are visibly rounded at this scale. There is evidence of brittle fracture in Dm75 including discontinuous slip surfaces sub-parallel to the shear zone boundary (as in Dm25; Figure 5.7A), Mode I cracks in individual dolomite grains (Figure 5.7A), and R1 Riedel shear fractures resulting in grain size reduction of dolomite (Figure 5.7A). However, narrow bands of deformed calcite deflected around more rigid dolomite attest to ductile deformation in the calcite aggregates (Figure 5.7B, 5.7C). Similar to Dm25 and Dm35, calcite grains are locally equiaxed to elongate, bounded by straight grain boundaries, which meet neighbouring calcite grains at triple junctions (Figure 5.7D). Straight grain boundaries are oriented along foliation. In particular, in narrow regions between dolomite grains, calcite grain boundaries are oriented parallel to the dolomite grain boundaries, irrespective of the global direction of maximum stretching (Figure 5.7D). While there are Mode I fractures and brittle deformation of dolomite, no fractures are observed propagating from dolomite and into the calcite matrix. Locally, a lineation is defined by reorientation of dolomite <20 µm. In particular, local foliation only incorporates dolomite grains <20 µm and wraps around coarser dolomite grains (Figure 5.7B and 5.7C), resulting in ribbons of foliation surrounding areas of coarse-grained dolomite (Figure 5.7C). Twinning is noted in some dolomite grains but this is relict from the starting material. When present, accessory pyrite and phyllosilicates, derived from the Badshot marble, are stretched and boudinaged reflecting the extension associated with shear strain (Figure 5.7D, centre-top). Intergranular porosity still exists but appears significantly reduced compared to the starting material. In contrast to low dolomite samples, there do not appear to be regions of localized porosity forming along foliation. 58  5.3.3 High Temperature Experiments At high temperature (800°C), calcite grains are closely packed and equiaxed to elongated. Straight grain boundaries parallel to the direction of shear are more defined than grain boundaries outside of this orientation. No local calcite clumps exist and there is no observed compositional banding. Foliation is more easily identified in the Dm51 than in the Dm75 sample deformed at 750°C due to the higher fraction of calcite. Bulk foliation is defined by high aspect ratio dolomite grains of all scales rotated into the direction of maximum elongation. Dolomite grains <50 µm define local foliation and are deflected around coarse-grained dolomite (Figure 5.8A). Dolomite grains < 50 µm are rounded. Intercrystalline porosity is present (Figure 5.8B), but as with deformed samples of Dm25, Dm35, and Dm75, it is reduced when compared to the starting material. Porosity is greatest at dolomite grain boundaries, but local concentrations of porosity along foliation are absent.  5.4 Texture Analyses 5.4.1 ! = ! Shear strain equal to 0 strain sample cuts of the deformed Dm25 (P1527) and Dm35 (P1524) samples were examined (see Figure 3.5). These sections intersect the centre of the deformed cores where there is zero rotation and therefore no shear strain (Paterson and Olgaard, 2000). However, these sections were exposed to the same experimental pressure and temperature conditions. In both the Dm25 (Figure 5.9) and Dm75 (Figure 5.10) there is a diffuse to poorly developed c-axis CPO developed in calcite grains. This may have developed during cold pressing of the starting powders (as suggested by Figures 4.7 and 4.8), or it is possible that the ! = 0 slices were offset from the sample centres and therefore record some strain.  5.4.2 Yield Experiments Yield experiments (Dm75 and Dm51) show non-existent to weak CPO development of the c-axis in calcite. CPO of the c-axis in calcite is diffuse for Dm51 (Figure 5.11), but locally developed for Dm75 (compare Figures 5.12B and 5.13B). The EBSD analysis of these deformed samples show little to no intracrystalline textural development (e.g. no twinning, no dislocation glide); in this regard (e.g., EBSD results) they are similar to the undeformed material. Calcite  59  grains are equiaxed and lack intracrystalline deformation. Similarly, dolomite grains show little intracrystalline deformation and porosity remains isolated and concentrated at grain boundaries.  5.4.3 High Strain Experiments All compositions taken to high strain at 750°C (Dm25, Dm35, and Dm75) show welldefined activation of the c-axis and a-axis slip systems in calcite (Figures 5.14B, 5.15B, and 5.16B). All stereonets have been corrected for the inclination of foliation to the shear boundary, and therefore all stereonets are displayed in the true kinematic plane of the sample. The c-axes girdles are well defined and symmetric along the direction of maximum shortening (the z-axis in the kinematic plane), indicating basal slip activation. With increasing dolomite content, the stereonet patterns become more diffuse, but remain well defined. In all cases, the c-axis system patterns are most pronounced, followed by the a-axis, r-axis, and f-axis systems. As in the c-axis system, the a-axis girdles are well defined but are symmetric about the direction of maximum stretching (x-axis). Two-dimensional grain size distributions from P1527 (Dm25) and P1538 (Dm75) (Figure 5.17) suggest possible calcite grain growth in Dm25 (from 5.5 to 7.5 µm) and possible calcite grain size reduction (from 4.5 to 3.5 µm) in Dm75. These changes in grain size are considered to be too small to have been mechanically induced and I conclude that there is neither grain growth nor grain size reduction in the deformed samples. None of the high strain experiments display a significant activation of any of the dolomite slip systems. This is confirmed by the absence of undulose extinction in optical thin section. Inverse pole maps for experiment P1538 (large strain, Dm75; Figure 5.16C) show minor undulosity in coarse dolomite grains, but most grains are composed of one even colour indicating no intracrystalline deformation. All deformed samples show shape preferred orientation (SPO), indicating rigid body rotation of high aspect ratio dolomite grains. As described above, this foliation development becomes difficult to identify in Dm75, but is nonetheless present. SPO demonstrates that in low dolomite content samples Dm25 (Figure 5.14) and Dm35 (Figure 5.15), high aspect ratio dolomite grains have become oriented with their long axes parallel to the direction of maximum stretching.  5.4.4 High Temperature Experiment At high temperature (experiment P1534), Dm51 shows a well-developed CPO in calcite along the c-axis perpendicular to foliation, suggesting the activation of basal slip (Figure 5.18B). 60  As with the high strain experiments carried out at 750°C, a defined CPO along the a-axis is also developed. The r- and f- patterns are diffuse and show no CPO development along those slip system.  5.4.5 Calcite Clumps For low dolomite content samples (i.e. Dm25 and Dm35), the pure calcite layers (an artifact of the sample preparation, see Figure 4.6) have the strongest crystallographic preferred orientations. This is illustrated in Figure 5.19 (a compilation of four EBSD scans across a calcite layer). It is clear that the calcite layer has a more defined CPO than the surrounding calcitedolomite composite (Figure 5.19B). Figure 5.19C shows a region scanned ~600 µm away from the calcite layer. The stereographic projections of the calcite slip systems are identical to the general observations above. Figure 5.19D is of an area adjacent to the calcite band margin (~100 µm away from the calcite band), which still contains dolomite grains. The calcite is evidently more texturally evolved than that found apart from the band; the calcite slip system girdles are more defined and asymmetric, indicating the sense of shear. Finally, Figure 5.19E lies within the calcite band. The slip system girdles are well defined and asymmetrical with respect to the maximum shortening direction. In particular, the c-axis girdle is oriented such that it indicates the sinistral shear sense used in the experiments.  61  A  SZB  z  25%-Dm - Experiment P1527 Conditions: T = 750˚C Strain rate: 0 s-1 γ=0 20 μm  20 μm  B  Calcite  C  Dolomite  SZB  SZB  z  z  40 μm  40 μm  z  z  SZB  z  z  z  SZB  z SZB  z  z  SZB  Figure 5.9 EBSD analaysis of 0-strain, Dm25. A. Experimental conditions; Pc = 300 MPa. Low magnification BSE image, with inset of the EBSD scan area to scale. The sample is oriented such that the shear zone boundary (SZB) (i.e. the top and bottom of the deformed samples) is horizontal and the z is the direction of axial load in the deformation rig. This convention differs from the coordinate system used in deformed samples as the sample has not been strained. B. Calcite: Inverse pole figure (top); upper hemisphere contoured stereoplots (bottom) for the c, r, a, and f slip systems, clockwise from top left. C. Dolomite: Inverse pole figure (top); upper hemisphere contoured stereoplots (bottom) for the c, r, a, and f slip systems, clockwise from top left. Blackened portions of the IPFs are components of different phases, not indexed. See text for details.  62  A  75%-Dm - Experiment P1538 Conditions: T = 750˚C Strain rate: 0 s-1 γ=0  SZB  z  20 μm 100 μm  B  Calcite  C SZB  SZB  z  z  40 μm  40 μm  z x  z  z  x  x  x  z x  z  z  z  z x  Dolomite  x  x  Figure 5.10 EBSD analaysis of 0-strain, Dm75. A. Experimental conditions; Pc = 300 MPa. Low magnification BSE image, with inset of the EBSD scan area to scale. The sample is oriented such that the shear zone boundary (SZB) (i.e. the top and bottom of the deformed samples) is horizontal and the z is the direction of axial load in the deformation rig. This convention differs from the coordinate system used in deformed samples as the sample has not been strained. B. Calcite: Inverse pole figure (top); upper hemisphere contoured stereoplots (bottom) for the c, r, a, and f slip systems, clockwise from top left. C. Dolomite: Inverse pole figure (top); upper hemisphere contoured stereoplots (bottom) for the c, r, a, and f slip systems, clockwise from top left. Blackened portions of the IPFs are components of different phases, not indexed. See text for details. 63  A  51%-Dm - Experiment P1528 Conditions: T = 750˚C Strain rate: 3x10-4 s-1 γ = 0.21  x  x  z  z  20 μm  100 μm  B  Calcite  C  Dolomite  x  x  z  z  70 μm  z x  z  z x  x  x  z x  z  z  z  z x  70 μm  x  x  Figure 5.11 EBSD analaysis of yield experiments, Dm51. A. Experimental conditions; Pc = 300 MPa. Low magnification BSE image, with inset of the EBSD scan area to scale. The cooridante system refers to the orientation of the kinematic plane, where x is the direction of maximum stretching and z is the direction of maximum shortening. B. Calcite: Inverse pole figure (top); upper hemisphere contoured stereoplots (bottom) for the c, r, a, and f slip systems, clockwise from top left. C. Dolomite: Inverse pole figure (top); upper hemisphere contoured stereoplots (bottom) for the c, r, a, and f slip systems, clockwise from top left. Blackened portions of the IPFs are components of different phases, not indexed. See text for details.  64  A  75%-Dm - Experiment P1525 Conditions: T = 750˚C Strain rate: 3x10-4 s-1 γ = 0.16  x z  100 μm  100 μm  B  Calcite  C  Dolomite x  x z  z  200 μm  200 μm  z x  x  x  x  z x  z  z  z  z x  z  z  x  x  Figure 5.12 EBSD analysis of yield experiment, Dm75. A. Experimental conditions; Pc = 300 MPa. Low magnification BSE image, with inset of the EBSD scan area to scale. The coordinate system refers to the orientation of the kinematic plane, where x is the direction of maximum stretching and z is the direction of maximum shortening. B. Calcite: Inverse pole figure (top); upper hemisphere contoured stereoplots (bottom) for the c, r, a, and f slip systems, clockwise from top left. C. Dolomite: Inverse pole figure (top); upper hemisphere contoured stereoplots (bottom) for the c, r, a, and f slip systems, clockwise from top left. Blackened portions of the IPFs are components of different phases, not indexed. See text for details.  65  A  75%-Dm - Experiment P1525 Conditions: T = 750˚C Strain rate: 3x10-4 s-1 γ = 0.16  x z  20 μm  20 μm  B  Calcite  C  Dolomite x  x z  z  45 μm  z  x  z  z  x  x  x  z x  z  z  z  z x  45 μm  x  x  Figure 5.13 EBSD analysis of yield experiment, Dm75. A. Experimental conditions; Pc = 300 MPa. Low magnification BSE image, with inset of the EBSD scan area to scale. The coordinate system refers to the orientation of the kinematic plane, where x is the direction of maximum stretching and z is the direction of maximum shortening. B. Calcite: Inverse pole figure (top); upper hemisphere contoured stereoplots (bottom) for the c, r, a, and f slip systems, clockwise from top left. C. Dolomite: Inverse pole figure (top); upper hemisphere contoured stereoplots (bottom) for the c, r, a, and f slip systems, clockwise from top left. Blackened portions of the IPFs are components of different phases, not indexed. See text for details. 66  A 25%-Dm - Experiment P1527 Conditions: T = 750˚C Strain rate: 3x10-4 s-1 γ~5  x z  100 μm 100 μm  B  x  Calcite  C  x  Dolomite  z  z  100 μm  100 μm  z x  x  x  x  z x  z  z  z  z x  z  z  x  x  Figure 5.14 EBSD analysis of high strain experiment, Dm25. A. Experimental conditions; Pc = 300 MPa. Low magnification BSE image, with inset of the EBSD scan area to scale. The coordinate system refers to the orientation of the kinematic plane, where x is the direction of maximum stretching and z is the direction of maximum shortening. B. Calcite: Inverse pole figure (top); upper hemisphere contoured stereoplots (bottom) for the c, r, a, and f slip systems, clockwise from top left. C. Dolomite: Inverse pole figure (top); upper hemisphere contoured stereoplots (bottom) for the c, r, a, and f slip systems, clockwise from top left. Blackened portions of the IPFs are components of different phases, not indexed. See text for details.  67  A  x z  x z  35%-Dm - Experiment P1524 Conditions: T = 750˚C Strain rate: 3x10-4 s-1 γ~5  100 μm 100 μm  B  x  Calcite  x  Dolomite  z  z  100 μm  100 μm  z z  x  z  z  x  x  x  z  x  z  z  z  x  C  x  z  x  Figure 5.15 EBSD analysis of high strain experiment, Dm35. A. Experimental conditions; Pc = 300 MPa. Low magnification BSE image, with inset of the EBSD scan area to scale. The coordinate system refers to the orientation of the kinematic plane, where x is the direction of maximum stretching and z is the direction of maximum shortening. B. Calcite: Inverse pole figure (top); upper hemisphere contoured stereoplots (bottom) for the c, r, a, and f slip systems, clockwise from top left. C. Dolomite: Inverse pole figure (top); upper hemisphere contoured stereoplots (bottom) for the c, r, a, and f slip systems, clockwise from top left. Blackened portions of the IPFs are components of different phases, not indexed. See text for details.  68  A  75%-Dm - Experiment P1538 Conditions: T = 750˚C Strain rate: 3x10-4 s-1 γ~5 x z  100 μm  100 μm  B  Calcite  C  Dolomite x  x z  z  200 μm  200 μm  z x  x  x  x  z x  z  z  z  z x  z  z  x  x  Figure 5.16 EBSD analysis of high strain experiment, Dm75. A. Experimental conditions; Pc = 300 MPa. Low magnification BSE image, with inset of the EBSD scan area to scale. The coordinate system refers to the orientation of the kinematic plane, where x is the direction of maximum stretching and z is the direction of maximum shortening. B. Calcite: Inverse pole figure (top); upper hemisphere contoured stereoplots (bottom) for the c, r, a, and f slip systems, clockwise from top left. C. Dolomite: Inverse pole figure (top); upper hemisphere contoured stereoplots (bottom) for the c, r, a, and f slip systems, clockwise from top left. Blackened portions of the IPFs are components of different phases, not indexed. See text for details. 69  0.16 0.14  0.7  Dm−25 Dm−75  A  0.6 0.5  Area Fraction  Area Fraction  0.12 0.1  0.4  0.08  0.3  0.06  0.2  0.04  0.1  0.02 0 0 10  Dm−25 Dm−75  B  1  10  Grain Size (�m) μ  2  10  0 0 10  1  10  2  10  Grain Size (�m) μ  3  10  Figure 5.17 Area fraction grain size distributions of high strain experiments. A. Grain size distributions of calcite for Dm25 (P1527) and Dm75 (P1538). B. Grain size distributions of dolomite for Dm25 (P1527) and Dm75 (P1538).  70  A  51%-Dm - Experiment P1534 Conditions: T = 800˚C Strain rate: 3x10-4 s-1 γ~4  x z  100 μm  20 μm  B  Calcite  C  Dolomite x  x z  z  100 μm  100 μm  z x  x  x  x  z x  z  z  z  z x  z  z  x  x  Figure 5.18 EBSD analysis of high temperature experiment, Dm51. A. Experimental conditions; Pc = 300 MPa. Low magnification BSE image, with inset of the EBSD scan area to scale. The coordinate system refers to the orientation of the kinematic plane, where x is the direction of maximum stretching and z is the direction of maximum shortening. B. Calcite: Inverse pole figure (top); upper hemisphere contoured stereoplots (bottom) for the c, r, a, and f slip systems, clockwise from top left. C. Dolomite: Inverse pole figure (top); upper hemisphere contoured stereoplots (bottom) for the c, r, a, and f slip systems, clockwise from top left. Blackened portions of the IPFs are components of different phases, not indexed. See text for details.  71  A  x  B  Figure 5.19 Crystallographic preferred orientation development near calcite clumps. A. BSE image of Dm25 sample deformed to γ~5.5 (see Table 5.1; P1527). A deformed calcite aggregate is outlined in red. B. IPF and upper hemisphere stereonet projection of the major slip systems in calcite across the deformed calcite band. C. IPF and steronet projection of calcite in a region removed from the calcite band. D. IPF and stereonet projections of a region adjacent to the calcite band, but including dolomite grains. E. IPF and stereonet projection of a region within the calcite band.  25%-Dm γ~5.5  z  B E D 200 μm  z x  z x  z  C  x  z x  100um  D  C  E  50 μm  70 μm 100 μm  z x  z x  z x  z x  z x  x  x z  x  z  z  x z  z x  z  x  z x  72  5.5 Chemical Changes Attending Deformation 5.5.1 Energy-dispersive X-ray Spectroscopy Energy-dispersive X-ray spectroscopy (EDS) analysis used to assist EBSD analysis of the calcite and dolomite phases highlights slight changes in composition with increasing strain. In particular, the Mg content of calcite increases with increasing strain. This is most pronounced in calcite grains proximal to dolomite phases. Figure 5.20 demonstrates the evolution of magnesium transfer from γ=0 to the largest strains for Dm-25 (!~5.5). Magnesium is restricted to dolomite grains at γ=0. With increasing strain magnesium becomes more easily mobilized and is observed in calcite proximal to dolomite grains.  5.5.2 Microprobe I performed microprobe analysis on Dm25 and Dm75 samples deformed to high strain (experiments P1527 and P1538) to confirm EDX observations of Mg2+ migration from dolomite to calcite during deformation (Figure 5.21). Microprobe analysis confirms a depletion of Mg2+ in fine-grained dolomite proximal to calcite in Dm25, however, calcite removed from dolomite grain boundaries is not enriched (yellow points). Mg-enrichment of calcite is pervasive in Dm75, owing to the abundance of dolomite throughout the system. Mg-enrichment is observed throughout the sample, regardless of proximity to thin ribbons of plastically deformed calcite.  73  γ~0  γ~2.25  γ~5.5 x  x z  z  200 μm  200 μm  100 μm  0  186  0  201  0  144  Figure 5.20 Energy-dispersive X-ray spectroscopy (EDS) maps of magnesium concentration for experiment P1527. Dm25 deformed in torsion at Pc=300MPa, T=750˚C, and strain rate 3*x0-4 s-1. Dark green grains are dolomite. The calcite matrix contains very little magnesium and, therefore, is light green. For γ=0, there is little magnesium contained within the matrix. With increasing strain, Mg concentrations increase in the matrix. This only occurs in regions of the sample with locally significant dolomite content. With increasing strain, Mg becomes more concentrated along the developing foliation of the sample.  74  A  25%-Dm; P1527  x  Dolomite Calcite Mg-calcite  z  20 μm  B  75%-Dm; P1538  x z  2  100 μm Figure 5.21 Microprobe analysis maps. Data from Table A.D1 is plotted as follows: yellow points represent Cc (0.90<xCa<1.00); green points represent Dm (0.50<xCa<0.55); red points represent Mg-enriched calcite (0.55<xCa<0.90). A. Microprobe analysis map for Dm25, P1527. B. Microprobe analysis map for Dm75, P1538.  75  Chapter 6: Discussion 6.1 Overview of Results 6.1.1 Mechanical Data The mechanical strength of calcite-dolomite (Cc-Dm) composites increases with dolomite content under high pressure-temperature conditions. As mentioned in Chapter 2, the stress exponent, n, is an unknown, empirical parameter diagnostic of deformation mechanism. In this study, a power law creep relationship is used to describe the mechanical data: !!  ! = !! ! ! !" ,  (1)  where ! is the shear strain rate, A is a constant, n is an empirically-derived stress exponent, Q is the activation energy, T is the temperature, and R is the gas constant (Paterson and Olgaard, 2000). The stress exponent, n = 2, determined from the strain rate stepping experiment of Dm35 suggests the influence of more than one deformation mechanism. When of the form ! ∝ ! ! , a stress exponent of 1 < ! < 3 indicates that the material has deformed by superplastic flow; this requires a combination of crystal plasticity and/or diffusion creep, with grain boundary sliding processes (Olgaard, 1990; Schmid et al., 1977). In this study, this power law creep flow law is appropriate for low dolomite experiments (Dm25 and Dm35) whose mechanical response is dominated by calcite deformation. Run products from dolomite-rich experiments show a mix of brittle deformation in dolomite and ductile flow of calcite. Therefore, the rheological behaviour of the Dm75 and Dm51 experiments are better described by a composite flow law involving Mohr-Coulomb behaviour and power law creep; a power law rheology is not appropriate in describing these experiments. The dolomite-poor composites (Dm25 and Dm35) attain a mechanical steady state immediately following attainment of yield shear stress, indicating that there is no change in deformation mechanism (or combination of mechanisms) during the course of the experiments (Barnhoorn et al., 2003). Dm75 is approximately twice as strong as Dm25 and Dm35. In contrast to the dolomite-poor experiments, Dm75 experiences significant strain weakening (~16%).  76  A  B  80  60 50 40 30 20 10 0  −10 0  C  z  1  2  3  Shear strain  4  5  60  x  50 40 30 20 10  −10 0  6  D z  1  2  3  Shear strain  4  5  6  200  z  x  150  x  50  Shear stress (MPa)  60  Shear stress (MPa)  z  x  0  x  70  40  100  30 20 10 0  −10 0  z  70  x  x  Shear stress (MPa)  Shear stress (MPa)  70  80  z  z  z  z  x  z  x  x  50 z  0 x  1  2  3  Shear strain  4  5  −50 0  1  2  3  Shear strain  4  5  6  Figure 6.1 Evolution of crystallographic preferred oreintation (CPO) of the c-axis in calcite with strain. Stress-strain curves represent the stress-strain conditions at the sample edge. Stereonets are c-axis orientation: x refers to the direction of maximum stretching; z is the direction of maximum compression. Red dots indicate the approximate point on the stress strain curve from which the stereonets are taken. In particular, CPO becomes less diffuse with increasing strain (i.e. the c-axis girdle becomes more narrow with increasing strain). A. Dm25 (P1527). B. Dm35 (P1524). C. Dm51 (P1534). D. Dm75 (P1538).  77  6.1.2 Microstructure and Texture: Calcite Intracrystalline strain within the calcite matrix of run products is rare. Calcite grains are generally polygonal and equiaxed to tabular. Grain boundaries are straight and meet at triple junctions. Calcite grain size distribution is conserved between the starting and deformed materials (+/- 1 µm), suggesting no grain growth or grain size reduction. Locally, a weak foliation in calcite layers is defined by straight grain boundaries oriented parallel to the shear direction, but more commonly, there is no foliation in the matrix calcite. Strong c- and a-axis CPOs developed in all high strain experiments. These are dominant slip systems activated in calcite under these experimental conditions (Barber et al., 2007; Barnhoorn et al., 2005a; De Bresser and Spiers, 1997; Delle Piane et al., 2009a). With increasing strain, calcite CPO becomes defined for all high strain experiments (Figure 6.1). C-axis CPOs are symmetrical and oriented perpendicular to the direction of maximum stretching for all samples but become increasingly diffuse with increasing dolomite content. Figure 6.1 summarizes the c-axis CPOs and mechanical responses of experiments P1527 (Dm25), P1524 (Dm35), P1534 (Dm51), and P1538 (Dm75) (see Table 5.1 for experimental conditions). The shear strains calculated for the calcite clumps are similar to the shear strains determined from the stress-strain curves (γ = ~5). Notably, the CPOs of these calcite clumps are better developed than the surrounding Dm-Cc bulk material.  6.1.3 Microstructure and Texture: Dolomite Dolomite grains with high aspect ratios (i.e. aspect ratios > 1) are rotated such that their long axes help to define a foliation. In dolomite-poor run products, dolomite grains show no change in grain size, nor is there any qualitative increase in crack density within the grains. These observations indicate that brittle deformation (e.g. microcracking and shear fracturing) has not accommodated significant strain. Rounding of dolomite grains <~ 50 µm in diameter is observed in all dolomite-poor deformed samples. In contrast, Dm75 run products contain abundant Mode I cracks and shear fractures within dolomite. All dolomite grains are visibly sub-rounded to rounded and some dolomite grains are offset by R1 Riedel shears, resulting in grain size reduction. There is no well-developed CPO in dolomite from deformed samples, nor is there pervasive undulose extinction, although Dm75 shows minor undulose extinction in coarse dolomite. Therefore, dolomite did not accommodate strain by intracrystalline plasticity.  78  Diffusion processes were active, as indicated by the movement of Mg2+ from dolomite to calcite. Mg2+ enrichment in calcite is localized around rounded dolomite grains < ~50 µm suggesting solid-state diffusive processes must be operative.  6.2 Deformation Mechanisms 6.2.1 Dislocation Creep with Dynamic Recrystallization The development of crystallographic preferred orientations (CPOs) in geologic materials requires intracrystalline deformation and is expected to develop under a) low homologous temperatures (T/Tm, where Tm is the absolute melting temperature of the phase) and/or high strain rates, or b) conditions where recrystallization is dominant (Wenk et al., 1990). The microstructure associated with a CPO typically shows evidence of extensive undulose extinction, subgrain development, and the generation of dynamically recrystallized grains. My experiments operated at high homologous temperatures for calcite and moderate strain rates. The calcite matrices comprise small, equidimensional, polygonal, strain-free calcite grains, which are not typically associated with dislocation creep. Additionally, there is no evidence of dynamic recrystallization; grain size distributions of the starting and deformed materials show no change in calcite grain size. Thus, microstructural observations suggest that steady state was not achieved in my experiments by dislocation creep and associated dynamic recrystallization. Rather, the microstructures are indicative of those formed by grain boundary sliding processes (Schmid et al., 1977). However, intracrystalline deformation mechanisms (i.e. dislocation glide +/- climb or cross slip) must be active in calcite to generate slip on the calcite slip systems, as shown by EBSD data (Rutter et al., 1994; Schmid et al., 1987). Despite this, there is no evidence to suggest that these mechanisms are dominant. The mechanical and microstructural evidence suggests superplastic flow accommodated by intracrystalline plasticity as the dominant deformation behaviour, as discussed in the following section.  6.2.2 Superplastic Flow and Calcite Superplasticity, or superplastic flow, is a metallurgic term that refers to exceptional ductility in tensile tests, where elongation exceeds 1000% without necking and subsequent failure (Langdon, 2006). It is a grain size sensitive mechanism – that is, it favours small grain sizes. Superplasticity is characterized by nearly Newtonian flow (n = 1) and is observed for temperatures >0.5 the homologous temperature. Superplasticity is characterized by n-values 79  between 1 and 3 for constitutive equations of the form ! ∝ ! ! , and microstructurally, grains are equiaxed, polygonal, strain free, and generally less than 10 microns in diameter. Superplasticity does not specify a deformation mechanism, but rather refers to a macroscopic behavior. Grain boundary sliding (GBS) is a dominant mechanism in superplastic flow (Edington et al., 1976; Schmid et al., 1977). Grain boundary sliding refers to the movement of grains at or in the vicinity of their interfaces in response to an external stress (Langdon, 2006). It cannot accommodate displacement on its own because of irregularities in the grain boundaries and, especially, at the junctions where more than two grains meet (Schmid et al., 1977). Typically, GBS occurs with 1) the diffusion of material as required for accommodation (including grain boundary diffusion; Ashby and Verrall (1973)); or 2) plastic deformation of grains (i.e. glide of dislocations; Mukherjee (1975)). Grain boundary sliding accommodated by dislocation creep or glide is known as Rachinger sliding (Langdon, 2006). Rashinger sliding requires grain sizes below 10 µm and temperature greater than 0.5T/Tm. This process results in strain free, equiaxed grains with a developed CPO due to dislocation creep accommodating deformation in polycrystalline materials. Grain boundary sliding accommodated by diffusion, either grain boundary diffusion or intragranular diffusion, is known as Lifshitz sliding (Langdon, 2006). This process results in elongate grains due to mass transfer from areas of high stress to areas of low stress. Grain boundary sliding also encourages chemical exchange between phases, since neighbouring grains are constantly moving past one another (Herwegh et al., 2003). The microstructure of calcite in all run products in this study supports superplastic flow of calcite aggregates. Grain boundary diffusion is required to explain the subtle elongation of calcite grains and the solid-state diffusion processes that accommodate Mg2+ movement from dolomite into calcite. Dislocation glide is required to explain the well-developed CPO of c- and a-slip. There is scarce work published on the deformation behaviour of Cc-Dm composites. Delle Piane et al. (2009a) used the same torsion rig and experimental conditions (i.e. Pc, T, strain rate) used in this study to deform Cc-Dm mixtures with ~ 9 %-Dm and 38 %-Dm. The major difference between the two studies, is that dolomite and calcite in Delle Piane et al. (2009a) have grain sizes of ~ 10 µm. Delle Piane et al. (2009) report near-Newtonian creep at 700°C with a stress exponent of n~1.7 and peak shear stresses of ~90 MPa and ~140 MPa for 9%-Dm and 38%-Dm, respectively. Both calcite and dolomite phases have strong c-axis preferred orientation 80  inclined to the shear direction. Shape preferred orientation was not found to be significantly dependent on strain. Individual grains are internally strain free and grain boundaries are straight and aligned. They interpret that grain boundary sliding and diffusion processes were dominant and accommodated, in part, by dislocation activity. Thus, at fine grain sizes, calcite and dolomite have similar strengths when deforming by superplastic flow. The Delle Piane et al. (2009a) experiments support the observations of diffusion creep in fine-grained dolomite in this study.  6.2.3 Calcite Clumps: Analogues for Veins in Nature? Compositionally homogeneous calcite bands accumulated the same calculated shear strain as the surrounding Cc-Dm host. However, EBSD analysis showing stronger CPOs in these regions and the presence of deflected foliations suggest strain partitioning. Strain partitioning may occur because compositionally homogeneous regions are more easily deformed as grain boundary pinning is not encouraged (Olgaard, 1990), resulting in maintaining the initial compositional zoning of the samples. Areas rich in dolomite possibly accommodated less displacement (i.e. are less sheared) than monomineralic layers of calcite. While these calcite regions are artifacts of the sample preparation, they provide an interesting analog for calcite veins in nature that are observed to absorb more strain than surrounding material (Kennedy and White, 2001). Low chemical potential gradients between single phase grains inhibit diffusion processes, leading to the activation of dislocation glide and, ultimately, back-stressing from the pileup of dislocations at grain boundaries, resulting in a population of strain free grains with similar CPO. This effect is more pronounced in pure calcite regions of Dm25 and Dm35 because the chemical potential gradients between grains are such that diffusion processes are curtailed (Kennedy and White, 2001).  6.2.4 The Role of Dolomite: Brittle and Ductile Behaviour Small quantities of Mg2+ in calcite limit grain growth, thereby keeping grain size sensitive diffusion creep and grain boundary sliding operative during deformation (Herwegh et al. (2003). Herwegh et al. (2003) found that calcite grain size is inversely proportional to Mgcontent, resulting in an extrinsic control on strength as calcite grain growth is inhibited. In my experiments, Mg2+ migration from dolomite to calcite confirms that diffusion creep processes occurred during deformation, and this may have contributed to the maintenance of small grain size throughout the duration of the experiments. With respect to quartz-calcite composites, the addition of quartz significantly increases the flow stress needed for steady state deformation 81  (Rybacki et al., 2003). That study suggests that the incorporation of Si into the dislocation cores of calcite is responsible for the increase in flow strength of calcite. It is unknown if a similar driving force exists in the Cc-Dm system, though this cannot be excluded due to the evidence for Mg2+ migration during deformation. In samples containing < 51% dolomite, the coarse grained dolomite grains show no evidence of extensive brittle deformation or intracrystalline deformation. Finer grained dolomite (< 50 µm) that have aspect ratios >1 are rotated into the foliation, indicating their active role as rotating rigid bodies. Although the calcite deforms by superplastic flow and accommodates most strain, these composite rocks are stronger than 100% calcite of the same grain size and deformed under similar experimental conditions, see Figure 6.2 (Schmid et al., 1987). I speculate that although the dolomite grains are not deformed, the distributed presence of these rigid bodies acts to create anastomosing, connected networks of calcite grains. In effect, the dispersed dolomite grains provide local resistance to grain boundary sliding and this resistance results in an increase in the flow stress necessary for steady state deformation. Strength increases with Dm51, yet only minor brittle deformation of dolomite is observed. However, there is a marked increase in strength in Dm75. In Dm75, dolomite forms connected grain networks that support most of the load. I interpret that the high yield stress of Dm75 is a result of Dm-Dm contact initially supporting the load. Subsequent reorganization of Dm-Dm contacts by Mode I and shear fractures leads to weakening and the establishment of flow networks in the fine-grained calcite. Steady state is only achieved when the calcite and finegrained dolomite establish a grain boundary network for grain boundary sliding to occur. The increase in shear stress at high strain may result from the inhibition of these flow networks and brittle deformation of dolomite. The re-establishment of the calcite networks leads to the final strain weakening (refer to Figure 6.1D). Similar behaviour is proposed in the strong phase of other multi-phase systems (e.g. quartz-calcite; Rybacki et al. (2003)). Dm75 composites are weaker than 100% dolomite deformed under similar conditions (Figure 6.3), attesting to the role of the calcite networks in weakening the rocks. Thus, with increasing dolomite content, the composites are stronger. There are two distinctly different types of deformation mechanisms occurring in parallel (or in series, it is difficult to assess) during shearing: brittle failure of dolomite by Mode I cracks, shear fractures, and subsequent grain size reduction (all of which are pressure-dependent mechanisms) and superplastic flow of calcite (which is more temperature- and grain size-dependent).  82  Regime 1 Regime 2  2  Regime 3 60  1.5  0˚C 70  1  0˚C 0˚C  80  0.5 0  0˚C  90  log10 Differential stress (MPa)  2.5  −0.5 −1 0  2  4  6 −1  −log10 Strain rate (s )  8  Figure 6.2 Comparison of study data with the reported deformation behaviour of Solnhofen limestone. Log-log plot of the differential stress vs. strain rate for compression deformation experiments on Solnhofen limestone (Schmid et al., 1977). Regime 1: Exponential relationship between strain rate and stress; Regime 2: Power-law creep; Regime 3: Superplasticity. Regimes 1 and 2 are characterized in the microstructure by dislocation glide and/or dislocation creep. Regime 3 is characterized in the microstructure by grain boundary sliding. Red stars indicate data from this study. With increasing differential stress, these stars represent the peak strengths of Dm25, Dm35, Dm51, and Dm75. The green and blue stars indicate the peak stress for Solnhofen limestone deformed by torsion to high strains at 700˚C and 800˚C, respectively (Schmid et al., 1987).  83  -1 1000  Crystal Plasticity and Twinning  10-14 s-1 σ1-σ3 (MPa)  100 10 1  10-5 s-1 10-6 s-1  Dislocation Creep  10-8 s-1 Diffusion Creep  10-14 s-1  0.1 100  300  500  700  10-10 s-1 900  T (˚C) Figure 6.3 Comparison of study data with reported deformation behaviour of Madoc dolomite. Deformation map for 100 μm dolomite. The stress-temperature field for synthetic dolomite experiments is shown by the shaded box. (Davis et al., 2008). Experiment P1538 (Dm75, taken to high strain) is plotted as a red star and lies in the diffusion creep field. The green star represents the differential stress of Madoc dolomite (grain size = 240 μm) at 700˚C (Davis et al., 2008).  84  My data suggest that for dolomite contents below a minimum of 35%, dolomite does not actively deform, but its presence is rate-controlling given the strength of the composites compared to micritic limestone (Figure 6.1). Only when dolomite is present in sufficient quantities (>51 %) to inhibit the flow of calcite and/or restrict calcite flow to narrow, localized bands does brittle fracture of dolomite grains become mechanically significant in accommodating strain.  6.3 Application to Natural Systems Calcite and dolomite coexist in many fault systems (e.g. the Apennines, Italy and the Canadian Rocky Mountains, Canada). Generally, at greenschist facies and lower conditions, dolomite is coarser grained and is found within variably dolomitized fine-grained micritic limestones (Figure 6.4). Only recently has a global interest in the role of dolomite on the mechanical behaviour of these systems arisen, particularly with respect to fault strength. The Portoro horizon in the Gulf of La Spezia (Isola Palmaria), Italy, is composed of a micritic limestone that has been partially dolomitized and crosscut by calcite and dolomite grains (Figure 6.4). Dolomitized layers impede propagation of deformation into the protolith, and deformation is restricted to the micritic mylonite (Taini, 2003). I conclude from this study that coarse grained dolomite in a micritic calcite matrix has a profound effect on the strength of composite materials, even at low concentrations (i.e. 25%Dm). The strengths of low-dolomite (Dm25 and Dm35) samples in this study are far stronger than 100% micritic calcite, since dolomite grains inhibit the superplastic flow of calcite aggregates. In natural systems, this may result in the locking of high-dolomite content faults, and stress being transferred to other faults with minimal dolomite content.  6.4 Summary Strain localization in this study results from the contribution of different deformation mechanisms in calcite and dolomite. In particular, grain boundary sliding assisted by dislocation and diffusion creep are the primary mechanisms accommodating strain in the low dolomite content Cc-Dm composites; brittle deformation processes become increasingly important with increasing dolomite content.  85  20 cm Figure 6.4 Field examples of coexistant micrite and dolomite, Toscana Strata of the Apennines in the Gulf of La Spezia. Dark, fine grained bands are micrite. Dolomite is light coloured. The dolomite is relatively coarsegrained (between 60 μm and 150 μm). Dolomotization is irregular. (Taini, 2003).  86  Chapter 7: Conclusions In this study strain localization in calcite-dolomite (Cc-Dm) composites is established by the operation of different deformation mechanisms in calcite and dolomite. This disparate behaviour is fundamentally related to dolomite grain size, where fine-grained calcite and dolomite deform by grain boundary sliding assisted by diffusion and dislocation creep and coarse grained dolomite deforms by brittle processes. In this system, especially in high dolomite content samples (Dm75), grain boundary sliding of fine-grained calcite and dolomite is periodically inhibited by clusters of coarse-grained dolomite contact; this leads to strain hardening. Upon fracture and reorganization of dolomite grains, grain boundary sliding networks in calcite are re-established and the rocks experience strain weakening as displacement is by calcite, rather than by brittle deformation of strong dolomite. Even with low Dm% content, the presence of dolomite grains acts to increase the strength of shear zones, presumably because the material must flow around rigid bodies. The main conclusions of this study are listed below:  •  Dolomite inherently changes the strength of carbonate composites – as dolomite content increases, the mechanical strength of the composites increases. o Peak yield strengths for Dm25 and Dm35 were ~79 MPa. o Peak yield strengths of ~140 MPa and ~178 MPa were observed for Dm51 and Dm75, respectively.  •  Calcite absorbs most of the deformation by grain boundary sliding assisted by dislocation and diffusion creep. !!  •  A stress exponent, n, (from the power law creep law: ! = !! ! ! !" ) of 2 was determined from a shear strain rate experiment conducted on Dm35; given the similar behaviour of Dm25 and Dm35, the n-value is assumed to be appropriate for both compositions.  •  The interplay between superplastic flow in calcite and brittle failure in dolomite means that any calculated stress exponent, n, for Dm75 is invalid as deformation of Dm75 requires both brittle and ductile deformation processes.  •  The mechanical data suggest that a mechanical threshold, defined by composition, exists in Cc-Dm composites. 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M., 1997, The role of veining and dissolution in the evolution of fine-grained mylonites: the McConnell thrust, Alberta: Journal of Structural Geology, v. 19, no. 6, p. 785-797. Kennedy, L. A., and White, J. C., 2001, Low-temperature recrystallization in calcite: Mechnisms and consequences: Geology, v. 29, no. 11, p. 1027-1030. Kern, H., and Wenk, H. R., 1983, Calcite Texture Development in Experimentally Induced Ductile Shear Zones: Contributions to Mineralogy and Petrology, no. 83, p. 231-236. Langdon, T. G., 2006, Grain boundary sliding revisited: Developments in sliding over four decades: Journal of Material Sciences, no. 41, p. 597-609. Leiss, B., and Barber, D. J., 1998, Mechanisms of dynamic recrystallization in naturally deformed dolomite inferred from EBSP analyses: Tectonophysics, no. 303, p. 51-69. Leiss, B., and Molli, G., 2002, 'High-temperature' texture in naturally deformed Carrara marble from the Alpi Apuane, Italy: Journal of Structural Geology, no. 25, p. 649-658. Maitra, S., Choudhury, A., Das, H. S., and Pramanik, J., 2005, Effect of compaction on the kinetics of thermal decomposition of dolomite under non-isothermal condition: Journal of Material Science, v. 40, p. 4749-4751. Mao, X.-B., Zhang, L.-Y., Li, T.-Z., and Liu, H.-S., 2009, Properties of failure mode and thermal damage for limestone at high temperatures: Mining Science and Technology, no. 19, p. 290-294. McIntosh, R. M., Sharp, J. H., and Wilburn, F. W., 1990, The thermal decomposition of dolomite: Thermochimica Acta, v. 165, p. 281-296. Middleton, G. V., and Wilcock, P. R., 1994, Mechanics in the Earth and Environmental Sciences, Great Britain, Cambridge University Press. Miller, J. A., Viola, G., and Mancktelow, S., 2008, Oxygen, carbon and strontium isotope constraints on the mechanisms of nappe emplacement and fluid-rock interaction along the subhorizontal Naukluft Thrust, central Namibia: Journal of the Geological Society, v. 165, p. 739-753. Molli, G., White, J. C., Kennedy, L. A., and Taini, V., 2011, Low-temperature deformation of limestone, Isola Palmaria, northern Apennine, Italy - The role of primary textures, precursory veins and intracryatlline deformation in localization: Journal of Structural Geology, no. 33, p. 225-270. Mukherjee, A. K., 1975, High-temperature creep, New York, Academic Press, Treatise on Materials Science and Technology.  91  Newman, J., and Mitra, G., 1994, Fluid-influenced deformation and recrystallization of dolomite at low temperatures along a natural fault zone, Mountain City window, Tennessee: Geological Society of America Bulletin, v. 106, p. 1267-1280. Oesterling, N., Heilbronner, R., Stunitz, H., Barnhoorn, A., and Molli, G., 2006, Strain dependent variation of microstructure and texture in naturally deformed Carrara marble: Journal of Structural Geology, no. 29, p. 681-696. Olgaard, D. L., 1990, The role of second phase in localizing deformation, in Knipe, R. J., and Rutter, E. H., eds., Deformation Mechanisms, Rheology and Tectonics, Volume 54, Geological Society Special Publication, p. 175-181. Olgaard, D. L., and Fitz Gerald, J. D., 1993, Evolution of pore microstructures during healing of grain boundaries in synthetic calcite rocks: Contributions to Mineralogy and Petrology, no. 115, p. 1380154. Passchier, C. W., and Trouw, R. A. J., 2005, Microtectonics, Germany, Springer. Paterson, M. S., and Olgaard, D. L., 2000, Rock deformation tests to large shear strains in torsion: Journal of Structural Geology, v. 22, p. 1341-1358. Paterson, M. S., and Turner, F. J., 1970, Experimental deformation of constrained crystals of calcite in extension, Berlin, Springer, Experimental and Natural Rock Deformation. Proceedings International Symposium, Darmstadt. Paterson, M. S., and Wong, T.-F., 2005, Experimental Rock Deformation - The Brittle Field, New York, Springer. Pieri, M., Burlini, L., Kunze, K., Stretton, I., and Olgaard, D. L., 2000a, Rheological and microstructural evolution of Carrara marble with high shear strain: results from high temperature torsion experiments: Journal of Structural Geology, no. 23, p. 1393-1413. Pieri, M., Kunze, K., Burlini, L., Stretton, I., Olgaard, D. L., Burg, J. P., and Wenk, H. R., 2000b, Texture development of calcite by deformation and dynamic recrystallization at 1000K during torsion experiments of marble to large strains: Tectonophysics, no. 330, p. 119-140. Pouchou, J. L., and Pichoir, F., 1985, PAP ϕ(ρZ) procedure for improved quantitative microanalysis: Microbeam Analysis, p. 104-106. Raudsepp, M., Pani, E., and Dipple, G. M., 1999, Measureing mineral abundance in skarn. I. The Rietveld method using X-ray powder diffraction data: Canadian Mineralogist, v. 37, no. 1, p. 1-15. Rutter, E., 1986, On the nomenclature of mode of failure transitions in rocks: Tectonophysics, v. 122, no. 3-4, p. 381-387. Rutter, E., Casey, M., and Burlini, L., 1993, Preferred crystallographic orientation development during the plastic and superplastic flow of calcite rocks: Journal of Structural Geology, v. 16, no. 10, p. 1431-1446. Rutter, E. H., 1972, The Effects of Strain-Rate Changes on the Strength and Ductility of Solenhofen Limestone at Low Temperatures and Confining Pressures: International Journal of Rock Mechanics and Mining Sciences and Geomechanics Abstracts, v. 1972, p. 183-189. -, 1995, Experimental study of the influence of stress, temperature, and strain on the dynamic recrystallization of Carrara marble: Journal of Geophysical Research, v. 100, No. B12, p. 24651-24663. Rutter, E. H., Casey, M., Burlini, L., and 1994, Preferred crystallographic orientation development during the plastic and superplastic flow of calcite rocks: Journal of Structural Geology, v. 16, p. 1431-1446.  92  Rybacki, E., Paterson, M. S., Wirth, R., and Dresen, G., 2003, Rheology of calcite-quartz aggregates deformed to large strain in torsion: Journal of Geophysical Research, v. 108, no. B2, p. 2089. Samtani, M., Dollomore, D., and Alexander, K. S., 2002, Comparison of dolomite decomposition kinetics with related carbonates and the effect of procedural variables on its kinetic parameters: Thermochimica Acta, v. 392-393, p. 135-145. Schmid, S. M., 1976, Rheological evidence for changes in the deformation mechanism of Solnhofen limestone towards low stress: Tectonophysics, v. 31, p. T21-T28. Schmid, S. M., Boland, J. N., and Paterson, M. S., 1977, Superplastic flow in Finegrained limestone: Tectonophysics, no. 43, p. 257-291. Schmid, S. M., Panozzo, R., and Bauer, S., 1987, Simple shear experiments on calcite rocks: rheology and microfabric: Journal of Structural Geology, v. 9, no. 5/6, p. 747-778. Schmid, S. M., Paterson, M. S., and Boland, J. N., 1980, High temperature flow and dynamic recrystallization in Carrara marble Tectonophysics, v. 65, p. 245-280. Siddiqi, G., 1997, Transport properties and mechanical behaviour of synthetic calcite-quartz aggregates [Ph.D.: Massachusetts Institute of Technology. Smith, S., Holdsworth, R. E., Collettini, C., and Imber, J., 2007, Footwall extension during lowangle normal faulting; controls on fault zone structure and fault rock distribution: Abstracts with Programs - Geological Society of America, v. 39, no. 6, p. 51. Spiers, C. J., and Wenk, H. R., 1980, Evidence for slip on r and f in the positive sense in deformed calcite single crystals: EOS. Trans. Am. Geophys. Union, v. 61, p. 1128. Taini, V., 2003, Strutture e processi di deformazione di bassa temperatura in rocce carbonatiche: esempio della falda toscana nella zona di La Spezia. : University of Pisa. Turner, F. J., Griggs, D. T., and Heard, H., 1954, Experimental deformation of calcite crystals: Bulletin of the Geological Society of America, v. 86, p. 883-934. Turner, F. J., and Heard, H., 1965, Deformation in calcite crystals at different strain rates: Univ. Calif. Publ. Geol. Sci., v. 46, p. 103-126. Turner, F. J., and Orozco, M., 1976, Crystal bending in metomorphic calcite, and its relations to associated twinning: Contributions to Mineralogy and Petrology, no. 57 p. 83-97. Viola, G., Mancktelow, N. S., and Miller, J. A., 2006, Cyclic frictional-viscous slip oscillations along the base of an advancing nappe complex: Insights into brittle-ductile nappe emplacement mechanisms from the Naukluft Nappe Complex, central Namibia: Tectonics, v. 25, no. TC3016. Walker, A. N., Rutter, E. H., and Brodie, K. H., 1990, Experimental study of grain-size sensitive flow of synthetic, hot pressed calcite rocks, in Knipe, R. J., and Rutter, E. H., eds., Deformation Mechanisms, Rheology and Tectonics, Volume 54, Geologic Socitey Special Publications, p. 259-284. Weiss, L. E., and Turner, F. J., 1972, Some observations on translation gliding and kinking in experimentally deformed calcite and dolomite: American Geophysical Union, Geophys. Monograph, no. 16, p. 95-107. Wenk, H. R., Barber, D. J., and Reeder, R. J., 1983, Microstructures in Carbonates, Mineralogical Society of America, Carbonates: Mineralogy and Chemistry. Woodward, N. B., Wojtal, S., Paul, J. B., and Zadins, Z. Z., 1988, Partitioning of deformation within several external thrust zones of the Appalachian orogen: J. Geol., v. 96, p. 351361.  93  Appendix A: Matlab Code A.1 Grain Size Analysis %% Processes data from Mastersizer 2000 grain size analysis close all clear all %% load vol% grain size analysis file A = importdata('Kushnir_Calcite_Pure_for_Matlab.txt') %% assign grain size analysis data data_grains = A; bins = data_grains(1,:) %% converting from vol% to number% vol_percent_cc = mean(data_grains(2:end,:)); radius = bins/2; volume = (4/3)*pi*radius.^3; n = vol_percent_cc./volume; tot = sum(n); num_percent_cc = n/tot*100; %% plot vol% and num% save('Cc','vol_percent_cc','num_percent_cc') %plot(bins,vol_percent) % plotyy(bins,vol_percent,bins,num_percent) %axis([0 50 0 10])  A.2 Paterson Deformation Analysis A.2.1 Torsion_data_processing_n_factor.m %% Code loads strain rate stepping experiment and plots pre-defined strain %% rates and the internal torques of Dm35 once the sample has reached %% steady state at a given strain rate % Load torsion data and plot desired parts of curve. close all A = importdata('P1529_D_35_C_4_stepping_expt_only.txt','\t') data = A.data; time = data(:,1); %Paterson time (seconds from start of day) time_real = (time-time(1))*86400; %convert to real time (s) time_hours = time_real./60./60; %convert to hours time_zero = time_hours - time_hours(6000); temp = data(:,2); %degC Pc = data(:,3); %MPa intForce= data(:,4); torsionPos = data(:,5); intTorque_un_zero = data(:,6);  94  intTorque = intTorque_un_zero - intTorque_un_zero(6300); extTorque = data(:,7); intAxialPos = data(:,8); extAxialPos = data(:,9); extForce = data(:,10); %% define constants w = 219; % revolutions/min d = 15.00; % diameter (mm) l = 10.00; % sample length (mm) %gammaDot = 3*10^-4; % sample shear rate (/s) - at outer edge of sample % strain rates gammaDot = [1*10^-5 5*10^-5 7*10^-5 1*10^-4 3*10^-4]; % time at which strain rates were set/changed Time = [0 46 126 151 169 186 226] ./ 60; %plot time vs. internal torque plot(time_zero(6000:20000),intTorque(6000:20000)) % internal torque recorded as steady state is reached at each strain rate Mint = [intTorque(2337) intTorque(8552) intTorque(9690) intTorque(10618) intTorque(12056)]; figure(2) plot(log(Mint),log(gammaDot),'.') %loglog(log(Mint),log(gammaDot),'.') %plot(log10(gammaDot),log10(Mint),'.') %% User must fit the log-log plot of strain rate vs. internal torque; the %% slope of the linear fit is the stress exponent, n. For Dm35, n=2.  A.2.2 Torsion_load_data.m function [time_real,temp,Pc,intForce,torsionPos,intTorque,extTorque,intAxialPos,extAxi alPos,extForce] = torsion_load_data %% tosion_load_data.m - script for user-controlled loading of raw torsion %% data %% load RAW file [filename, pathname, filterindex] = uigetfile('*.*', 'Pick RAW file'); full_name = fullfile(pathname,filename); uiimport(full_name); display('Press any key to continue script.'); pause; data_tor = data; %% Assigning variables time = data_tor(:,1); %Paterson time (seconds from start of day) temp = data_tor(:,2); %degC Pc = data_tor(:,3); %MPa intForce= data_tor(:,4); torsionPos = data_tor(:,5); intTorque = data_tor(:,6);  95  extTorque = data_tor(:,7); intAxialPos = data_tor(:,8); extAxialPos = data_tor(:,9); extForce = data_tor(:,10); %% Converting Paterson time to real time in seconds from start of %% experiment time_real = (time-time(1))*86400; %convert to real time (s)  A.2.3 Torsion.m function torsion(filename) %% function loads torsion raw data and converts internal torque to stress %% using a power-law creep equation from Paterson and Olgaard, 2000 %% load RAW file [time_real,temp,Pc,intForce,torsionPos,intTorque,extTorque,intAxialPos,extAxi alPos,extForce] = torsion_load_data %% load sample dimensions and experimental strain rate diam = input('Core diameter (in mm):'); d = diam/1000; %in m len = input('Core length (in mm):'); strnrt = input('Strain rate (in 1/s):'); tor = input('Sample diamter ~15mm? - 1 = yes; 2 = no'); %% convert time to hours, if you care time_hours = time_real./60./60; %convert to hours %% Define n for power law n = 2; % from stepping experiment... %% zero plot %plot(time_hours,intTorque) %[time_0,x,y] = selectdata('selectionmode','closest','verify','on') time_zero = time_real - time_real(1); intTorque_zero = intTorque - intTorque(1); plot(time_zero,intTorque_zero,'.r') %% Iron jacket correction from Barnhoorn PhD thesis 2003 (Appendix C) % parameters for iron jacket at 750C 3*10^-4 1/s % if tor == 1 % intJacketTorque = 2.8; % else % intJacketTorque = 1; % end %parameters for iron jacket at 750C 1*10^-4 1/s if tor == 1 intJacketTorque = 2.4; else  96  intJacketTorque = 0.9; end % parameters for iron jacket at 800C 3*10^-4 1/s % if tor == 1 % intJacketTorque = 1.4; % else % intJacketTorque = NaN; % end %jacket correction intTorque_corr = intTorque_zero - intJacketTorque; %% process for strain strain = strnrt*time_zero; shear_stress = (4*intTorque_corr*(3+(1/n))) ./ (pi * d^3); semilogy(strain, shear_stress) xlabel('Strain') ylabel('Shear Stress') save(filename,'strain','shear_stress')  A.3 Microprobe Analysis %% Microprobe anaylsis for Mg content of carbonates %load datafile from Edith %% load RAW file [filename, pathname, filterindex] = uigetfile('*.*', 'Pick RAW file'); full_name = fullfile(pathname,filename); uiimport(full_name); display('Press any key to continue script.'); pause; %% data_micro = data; data_micro_oxide = data(:,[2 4:end]); % C O Mg Ca Mn Fe b = ones(length(data_micro_oxide),8); %% OxMolWeight = [44.00964 40.32 56.08 70.94 71.85]; % CO2 MgO CaO MnO FeO catNumPerOx = [1 1 1 1 1]; % CO2 MgO CaO MnO FeO anNumPerCat = [2 1 1 1 1]; % CO2 MgO CaO MnO FeO for i = 1 : length (data_micro_oxide); % calculate cation proportions catProp = data_micro_oxide(i,:) .* catNumPerOx ./ OxMolWeight; % cation norm (4) catNorm = catProp .* 4 ./ sum(catProp); % predicted anion total  97  anionTot = catNorm .* anNumPerCat; % anion sum anionSum = sum(anionTot); % sum of x-site cations catSum = sum(anionTot(2:end)); % Ca fraction xCa = catNorm(3) / catSum; %Mg fraction xMg = catNorm(2) / catSum; b(i,:) = [anionTot catSum xCa xMg]; end %% text = textdata(2:end,1); P1527 = b(3:44,:); P1538 = b(45:165,:); standards = b([1,2,166:169],:); figure(1) plot(standards(:,2),'g.') hold on plot(P1527(:,2),'b.') title('P1527 Mg Content') figure(2) plot(standards(:,2),'g.') hold on plot(P1538(:,2),'b.') title('P1538 Mg Content') %%  save file  save('Microprobe.txt','text','b','-ascii', '-tabs' )  98  Appendix B: X-ray Diffractograms All XRD data are presented here, including the Rietveld analysis of the starting powders, Rietveld analysis of the starting HIP materials, and smear mount XRD of the deformed run products. All analyses confirm that decarbonation did not produce enough periclase (MgO) or lime (CaO) to be detected.  99  B.1 Starting Powders 33,000 32,500 32,000 31,500 31,000 30,500 30,000 29,500 29,000 28,500 28,000 27,500 27,000 26,500 26,000 25,500 25,000 24,500 24,000 23,500 23,000 22,500 22,000 21,500 21,000 20,500 20,000 19,500 19,000 18,500 18,000 17,500 17,000 16,500 16,000 15,500 15,000 14,500 14,000 13,500 13,000 12,500 12,000 11,500 11,000 10,500 10,000 9,500 9,000 8,500 8,000 7,500 7,000 6,500 6,000 5,500 5,000 4,500 4,000 3,500 3,000 2,500 2,000 1,500 1,000 500 0 -500 -1,000  Calcite 0.09 % Dolomite 99.76 % Quartz 0.15 %  4  6  8  10  12  14  16  18  20  22  24  26  28  30  32  34  36  38  40  42  44  46  48  50  52  54  56  58  60  62  64  66  68  70  72  74  76  78  80  Dolomite Powder: 0.09% calcite; 99.76% dolomite  Calcite 99.04 % Dolomite 0.87 % Quartz 0.10 %  22,000 21,500 21,000 20,500 20,000 19,500 19,000 18,500 18,000 17,500 17,000 16,500 16,000 15,500 15,000 14,500 14,000 13,500 13,000 12,500 12,000 11,500 11,000 10,500 10,000 9,500 9,000 8,500 8,000 7,500 7,000 6,500 6,000 5,500 5,000 4,500 4,000 3,500 3,000 2,500 2,000 1,500 1,000 500 0 -500 4  6  8  10  12  14  16  18  20  22  24  26  28  30  32  34  36  38  40  42  44  46  48  50  52  54  56  58  60  62  64  66  68  70  72  74  76  78  80  Calcite Powder: 99.04% calcite; 0.87% dolomite  100  B.2 Hot Isostatic Pressing Product  AK_D_25_1_E.raw  Calcite 74.47 % Dolomite 25.53 %  10,000  5,000  0 5  10  15  20  25  30  35  40  45  50  55  60  65  70  75  25%-Dolomite: 74.47% calcite; 25.53% dolomite  10,000  AK_D35_2.raw_1  Calcite 64.82 % Dolomite 35.18 %  5,000  0 5  10  15  20  25  30  35  40  45  50  55  60  65  70  75  35% - Dolomite: 64.82% calcite; 35.18% dolomite  101  AK_D50-C3.raw  Calcite 48.86 % Dolomite 51.50 %  5,000  0 5  10  15  20  25  30  35  40  45  50  55  60  65  70  75  51% -Dolomite: 48.86% calcite; 51.50% dolomite  AK_D75_A2.raw  Calcite 24.73 % Dolomite 75.54 %  10,000  5,000  0 5  10  15  20  25  30  35  40  45  50  55  60  65  70  75  75% - Dolomite: 24.73% calcite; 75.54% dolomite  102  B.3 Deformed Run Product  Lin (Counts)  20000  0 3  10  20  30  40  50  60  70  80  2-Theta - Scale BadShot - File: BadShot.raw - Start: 3.000 ° - End: 80.088 ° - Step: 0.039 ° - Step time: 71.5 s Y + 10.0 mm - D_25_3_C - File: D_25_3_C.raw - Start: 3.000 ° - End: 80.088 ° - Step: 0.039 ° - Step time: 114.4 s Y + 20.0 mm - D_25_3_C_3_P1527 - File: D_25_3_C_3_P1527.raw - Start: 3.000 ° - End: 80.088 ° - Step: 0.039 ° - St Y + 30.0 mm - D_35_3_C - File: D_35_3_C.raw - Start: 3.000 ° - End: 80.088 ° - Step: 0.039 ° - Step time: 114.4 s Y + 40.0 mm - D_35_C_1_P1524 - File: D_35_C_1_P1524.raw - Start: 3.000 ° - End: 80.088 ° - Step: 0.039 ° - Step ti Y + 50.0 mm - D_50_3_B - File: D_50_3_B.raw - Start: 3.000 ° - End: 80.088 ° - Step: 0.039 ° - Step time: 114.4 s  Y + 60.0 mm - D_50_3_B_3_P1528 - File: D_50_3_B_3_P1528.raw - Start: 3.000 ° - End: 80.088 ° - Step: 0.039 ° - St Y + 70.0 mm - D_50_B_4_P1534 - File: D_50_B_4_P1534.raw - Start: 3.000 ° - End: 80.088 ° - Step: 0.039 ° - Step ti Y + 80.0 mm - D_75_1_A - File: D_75_1_A.raw - Start: 3.000 ° - End: 80.088 ° - Step: 0.039 ° - Step time: 114.4 s Y + 90.0 mm - D_75_1_A_1 - File: D_75_1_A_1.raw - Start: 3.000 ° - End: 80.088 ° - Step: 0.039 ° - Step time: 114.4 s Y + 100.0 mm - D_75_3_A - File: D_75_3_A.raw - Start: 3.000 ° - End: 80.088 ° - Step: 0.039 ° - Step time: 114.4 s  103  Appendix C: Microprobe Data Microprobe analysis was performed on Dm25 and Dm75 deformed to high strain at 750°C (experiments P1527 and P1538). Figure C.1 is a map of all data points collected. Table C.1 gives xCa and xMg values at each point.  104  A  21  x  20  z  18  19 4  1 2  27  3 5  31  16  15 24  6  30  14  23  22 7  17  13  11  8 9  25  12  10  29 26  28  20 μm  B  x  z  49 35 36  21  33  46 45 43 47 48 44  56  57 55  51  32 39 37 40 41 11 20 42 19 10 16 9 17 14 5 4 18 15 8 3 13 12 6 7 2  50  30 31  38  53  54 52  22 23 24 28 25 29 27 26 1  100 μm  Figure C.1 Microprobe analysis maps. Data from Table C.1 is plotted as follows: yellow points represent Cc (0.90<xCa<1.00); green points represent Dm (0.50<xCa<0.55); red points represent Mg-enriched calcite (0.55<xCa<0.90); magenta points represent excessive Mg-enrichments (xCa<0.50). A. Microprobe analysis map for Dm25, P1527. Numbers are suffixes to prefix P1527_Scan3_ in Table C.1. B. Microprobe analysis map for Dm75, P1538. Numbers are suffixes to prefix P1538_Scan4_ in Table C.1.  105  Measurement	
  Point  106  anion	
  CO2 anion	
  MgO  anion	
  CaO anion	
  MnO  anion	
  FeO  sum	
  of	
  x-­‐ site	
  cation  xCa  xMg  	
  	
  	
  	
  'P1527_1_Scan3_1'  3.9768  0.1  1.9074  0.0027  0.0015  2.0116  0.9482  0.0497  	
  	
  	
  	
  'P1527_1_Scan3_2'  4.0527  0.4492  1.5184  0.0005  0.0056  1.9737  0.7693  0.2276  	
  	
  	
  	
  'P1527_1_Scan3_3'  4.0919  0.9298  1.0071  0.0012  0.0159  1.954  0.5154  0.4758  	
  	
  	
  	
  'P1527_1_Scan3_4'  4.0552  0.9527  1.0045  0.0013  0.0139  1.9724  0.5093  0.483  	
  	
  	
  	
  'P1527_1_Scan3_5'  4.048  0.0934  1.8816  0  0.001  1.976  0.9522  0.0472  	
  	
  	
  	
  'P1527_1_Scan3_6'  4.0326  0.1243  1.8564  0  0.0031  1.9837  0.9358  0.0627  	
  	
  	
  	
  'P1527_1_Scan3_7'  3.9979  0.1025  1.8959  0.0007  0.002  2.001  0.9474  0.0512  	
  	
  	
  	
  'P1527_1_Scan3_8'  4.0148  0.5163  1.4735  0.0002  0.0026  1.9926  0.7395  0.2591  	
  	
  	
  	
  'P1527_1_Scan3_9'  4.0552  0.0909  1.8771  0.002  0.0023  1.9724  0.9517  0.0461  	
  	
  	
  	
  'P1527_1_Scan3_10'  4.003  0.16  1.8335  0.0007  0.0043  1.9985  0.9174  0.0801  	
  	
  	
  	
  'P1527_1_Scan3_11'  4.0707  0.1161  1.8472  0  0.0013  1.9646  0.9402  0.0591  	
  	
  	
  	
  'P1527_1_Scan3_12'  3.9834  0.666  1.3355  0.0004  0.0064  2.0083  0.665  0.3316  	
  	
  	
  	
  'P1527_1_Scan3_13'  4.0165  0.5344  1.4495  0  0.0078  1.9917  0.7277  0.2683  	
  	
  	
  	
  'P1527_1_Scan3_14'  4.0305  0.9482  1.0176  0.0014  0.0175  1.9848  0.5127  0.4777  	
  	
  	
  	
  'P1527_1_Scan3_15'  4.04  0.9505  1.0105  0  0.019  1.98  0.5103  0.4801  	
  	
  	
  	
  'P1527_1_Scan3_16'  4.0309  0.121  1.8537  0  0.0099  1.9845  0.9341  0.0609  	
  	
  	
  	
  'P1527_1_Scan3_17'  4.0154  0.1114  1.8787  0.0005  0.0018  1.9923  0.943  0.0559  	
  	
  	
  	
  'P1527_1_Scan3_18'  4.1112  0.8974  1.0354  0.0008  0.0108  1.9444  0.5325  0.4615  Measurement	
  Point  107  anion	
  CO2 anion	
  MgO  anion	
  CaO anion	
  MnO  anion	
  FeO  sum	
  of	
  x-­‐ site	
  cation  xCa  xMg  	
  	
  	
  	
  'P1527_1_Scan3_19'  4.058  0.087  1.8828  0.0001  0.0011  1.971  0.9553  0.0442  	
  	
  	
  	
  'P1527_1_Scan3_20'  4.0394  0.7412  1.2265  0.0017  0.0109  1.9803  0.6193  0.3743  	
  	
  	
  	
  'P1527_1_Scan3_21'  4.0517  0.9514  1.0116  0.0014  0.0098  1.9742  0.5124  0.4819  	
  	
  	
  	
  'P1527_1_Scan3_22'  4.0916  0.1267  1.8251  0  0.0024  1.9542  0.9339  0.0648  	
  	
  	
  	
  'P1527_1_Scan3_23'  4.0508  0.0924  1.8812  0  0.001  1.9746  0.9527  0.0468  	
  	
  	
  	
  'P1527_1_Scan3_24'  4.0139  0.0698  1.9231  0.0001  0  1.9931  0.9649  0.035  	
  	
  	
  	
  'P1527_1_Scan3_25'  4.0043  0.0768  1.9209  0  0.0001  1.9978  0.9615  0.0385  	
  	
  	
  	
  'P1527_1_Scan3_26'  4.1012  0.9245  1.0035  0.0014  0.02  1.9494  0.5148  0.4742  	
  	
  	
  	
  'P1527_1_Scan3_27'  4.0163  0.9699  1.0104  0  0.0116  1.9919  0.5073  0.4869  	
  	
  	
  	
  'P1527_1_Scan3_28'  4.0299  0.1447  1.8372  0.0002  0.003  1.9851  0.9255  0.0729  	
  	
  	
  	
  'P1527_1_Scan3_29'  4.0392  0.8591  1.1071  0.0002  0.0141  1.9804  0.559  0.4338  	
  	
  	
  	
  'P1527_1_Scan3_30'  4.0826  0.0943  1.8626  0.0004  0.0015  1.9587  0.9509  0.0481  	
  	
  	
  	
  'P1527_1_Scan3_31'  4.0299  0.0824  1.9027  0  0  1.9851  0.9585  0.0415  	
  	
  	
  	
  'P1538_1_Scan4_1'  4.0297  0.9638  1.0036  0.0004  0.0174  1.9851  0.5056  0.4855  	
  	
  	
  	
  'P1538_1_Scan4_2'  4.0541  0.8177  1.1431  0.0002  0.012  1.9729  0.5794  0.4144  	
  	
  	
  	
  'P1538_1_Scan4_3'  4.0866  0.5664  1.3755  0.0014  0.0135  1.9567  0.703  0.2894  	
  	
  	
  	
  'P1538_1_Scan4_4'  4.052  0.9458  1.0066  0.0027  0.0189  1.974  0.5099  0.4791  	
  	
  	
  	
  'P1538_1_Scan4_5'  5.4704  1.0689  0.1651  0  0.0308  1.2648  0.1305  0.8451  Measurement	
  Point  108  anion	
  CO2 anion	
  MgO  anion	
  CaO anion	
  MnO  anion	
  FeO  sum	
  of	
  x-­‐ site	
  cation  xCa  xMg  	
  	
  	
  	
  'P1538_1_Scan4_6'  4.0593  0.8242  1.1338  0.0004  0.012  1.9704  0.5754  0.4183  	
  	
  	
  	
  'P1538_1_Scan4_7'  4.0453  0.4146  1.5548  0.0017  0.0064  1.9774  0.7863  0.2097  	
  	
  	
  	
  'P1538_1_Scan4_8'  5.3765  1.0137  0.2661  0  0.0319  1.3117  0.2029  0.7728  	
  	
  	
  	
  'P1538_1_Scan4_9'  4.0208  0.244  1.7439  0  0.0017  1.9896  0.8765  0.1226  	
  	
  	
  	
  'P1538_1_Scan4_10'  4.0303  0.2526  1.7291  0  0.0031  1.9848  0.8712  0.1273  	
  	
  	
  	
  'P1538_1_Scan4_11'  3.9954  0.6116  1.3775  0  0.0132  2.0023  0.688  0.3054  	
  	
  	
  	
  'P1538_1_Scan4_12'  4.005  0.954  1.0225  0  0.0209  1.9975  0.5119  0.4776  	
  	
  	
  	
  'P1538_1_Scan4_13'  4.0703  0.3188  1.6397  0.0003  0.0061  1.9649  0.8345  0.1623  	
  	
  	
  	
  'P1538_1_Scan4_14'  3.9921  0.2485  1.7506  0  0.0049  2.004  0.8736  0.124  	
  	
  	
  	
  'P1538_1_Scan4_15'  5.3297  1  0.3035  0.0004  0.0313  1.3351  0.2273  0.749  	
  	
  	
  	
  'P1538_1_Scan4_16'  4.0275  0.2997  1.6831  0.0004  0.0031  1.9863  0.8474  0.1509  	
  	
  	
  	
  'P1538_1_Scan4_17'  4.0487  0.2886  1.683  0.0001  0.004  1.9757  0.8519  0.1461  	
  	
  	
  	
  'P1538_1_Scan4_18'  4.0191  0.8969  1.0764  0.0023  0.0149  1.9905  0.5408  0.4506  	
  	
  	
  	
  'P1538_1_Scan4_19'  4.0512  0.2244  1.744  0  0.006  1.9744  0.8833  0.1136  	
  	
  	
  	
  'P1538_1_Scan4_20'  4.0398  0.2275  1.7464  0  0.0062  1.9801  0.882  0.1149  	
  	
  	
  	
  'P1538_1_Scan4_21'  4.0644  0.9409  1.0083  0.0006  0.018  1.9678  0.5124  0.4781  	
  	
  	
  	
  'P1538_1_Scan4_22'  4.0083  0.9745  1.0127  0  0.0087  1.9959  0.5074  0.4882  	
  	
  	
  	
  'P1538_1_Scan4_23'  4.0366  0.8325  1.1393  0.0012  0.0086  1.9817  0.5749  0.4201  Measurement	
  Point  109  anion	
  CO2 anion	
  MgO  anion	
  CaO anion	
  MnO  anion	
  FeO  sum	
  of	
  x-­‐ site	
  cation  xCa  xMg  	
  	
  	
  	
  'P1538_1_Scan4_24'  3.9877  0.5287  1.4693  0.0016  0.0066  2.0061  0.7324  0.2635  	
  	
  	
  	
  'P1538_1_Scan4_25'  4.051  0.3577  1.611  0  0.0058  1.9745  0.8159  0.1812  	
  	
  	
  	
  'P1538_1_Scan4_26'  4.0422  0.958  1.0063  0.0014  0.0132  1.9789  0.5085  0.4841  	
  	
  	
  	
  'P1538_1_Scan4_27'  4.0321  0.9484  1.0224  0.0005  0.0126  1.9839  0.5153  0.478  	
  	
  	
  	
  'P1538_1_Scan4_28'  4.0206  0.3519  1.6311  0.0003  0.0065  1.9897  0.8198  0.1768  	
  	
  	
  	
  'P1538_1_Scan4_29'  4.0119  0.2797  1.7101  0  0.0042  1.994  0.8576  0.1403  	
  	
  	
  	
  'P1538_1_Scan4_30'  4.0343  0.3951  1.5802  0.0005  0.0071  1.9829  0.7969  0.1992  	
  	
  	
  	
  'P1538_1_Scan4_31'  4.0021  0.5661  1.4204  0  0.0124  1.9989  0.7106  0.2832  	
  	
  	
  	
  'P1538_1_Scan4_32'  4.0717  0.8993  1.0524  0  0.0125  1.9641  0.5358  0.4578  	
  	
  	
  	
  'P1538_1_Scan4_33'  4.0094  0.306  1.6804  0.0011  0.0077  1.9953  0.8422  0.1534  	
  	
  	
  	
  'P1538_1_Scan4_35'  4.063  0.9209  1.0279  0.0002  0.0195  1.9685  0.5222  0.4678  	
  	
  	
  	
  'P1538_1_Scan4_36'  4.0375  0.2436  1.7313  0.0002  0.0061  1.9812  0.8739  0.1229  	
  	
  	
  	
  'P1538_1_Scan4_37'  4.0097  0.2306  1.7608  0.0015  0.0022  1.9951  0.8826  0.1156  	
  	
  	
  	
  'P1538_1_Scan4_38'  3.9981  0.9458  1.0373  0.0004  0.0175  2.001  0.5184  0.4727  	
  	
  	
  	
  'P1538_1_Scan4_39'  4.006  0.797  1.1846  0.0008  0.0146  1.997  0.5932  0.3991  	
  	
  	
  	
  'P1538_1_Scan4_40'  3.9713  0.9674  1.0289  0.0019  0.0162  2.0144  0.5108  0.4803  	
  	
  	
  	
  'P1538_1_Scan4_41'  3.9748  0.9435  1.0524  0.0003  0.0165  2.0126  0.5229  0.4688  	
  	
  	
  	
  'P1538_1_Scan4_42'  4.0177  0.2656  1.7218  0  0.0038  1.9912  0.8647  0.1334  Measurement	
  Point  110  anion	
  CO2 anion	
  MgO  anion	
  CaO anion	
  MnO  anion	
  FeO  sum	
  of	
  x-­‐ site	
  cation  xCa  xMg  	
  	
  	
  	
  'P1538_1_Scan4_43'  4.0178  0.9636  1.0125  0.0008  0.0143  1.9911  0.5085  0.4839  	
  	
  	
  	
  'P1538_1_Scan4_44'  4.012  0.9644  1.0157  0.0008  0.0131  1.994  0.5094  0.4836  	
  	
  	
  	
  'P1538_1_Scan4_45'  4.0054  0.9189  1.0637  0.0012  0.0136  1.9973  0.5326  0.4601  	
  	
  	
  	
  'P1538_1_Scan4_46'  4.0048  0.9705  1.0132  0.0006  0.0133  1.9976  0.5072  0.4858  	
  	
  	
  	
  'P1538_1_Scan4_47'  4.0462  0.9375  1.0264  0.0017  0.0113  1.9769  0.5192  0.4742  	
  	
  	
  	
  'P1538_1_Scan4_48'  3.9847  0.9823  1.01  0  0.0154  2.0077  0.5031  0.4893  	
  	
  	
  	
  'P1538_1_Scan4_49'  4.0671  0.2927  1.6676  0  0.0062  1.9665  0.848  0.1488  	
  	
  	
  	
  'P1538_1_Scan4_50'  4.0803  0.9191  1.0234  0.0009  0.0165  1.9599  0.5222  0.469  	
  	
  	
  	
  'P1538_1_Scan4_51'  3.9947  0.9127  1.0772  0.0002  0.0125  2.0027  0.5379  0.4558  	
  	
  	
  	
  'P1538_1_Scan4_52'  4.0099  0.2625  1.7285  0  0.004  1.9951  0.8664  0.1316  	
  	
  	
  	
  'P1538_1_Scan4_53'  4.0202  0.9551  1.018  0.0018  0.015  1.9899  0.5116  0.48  	
  	
  	
  	
  'P1538_1_Scan4_54'  4.026  0.7788  1.196  0.0005  0.0116  1.987  0.6019  0.3919  	
  	
  	
  	
  'P1538_1_Scan4_55'  4.0276  0.2161  1.7636  0.0006  0.0059  1.9862  0.8879  0.1088  	
  	
  	
  	
  'P1538_1_Scan4_56'  3.9652  0.7217  1.2812  0.0015  0.013  2.0174  0.6351  0.3577  	
  	
  	
  	
  'P1538_1_Scan4_57'  4.0506  0.2421  1.7277  0  0.0049  1.9747  0.8749  0.1226  

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