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Metamorphic Reactions
CHAPTER 4
Metamorphic
Reactions
A R O C K undergoes structural and mineralogical (and chemical) changes during metamorphism in an attempt to reach equilibriiun with the new conditions. This generally results in recrystallization which is the reconstitution of existing stable phases, and crystallization (neomineralization) which is the formation of new phases resulting from the destruction of unstable ones. The evolution of textures in metamorphic rocks requires an understanding of changes in minerals regarded as units of crystal structure and not merely as chemical compoimds. Metamorphic processes will be regarded here as a series of structural transformations. Some mineralogical transformations may be comparatively simple, e.g. twinning or minor polymorphic change, or the recrystallization of, e.g., quartz. Others are more complex in that they involve considerable structural changes (e.g. andalusite to sillimanite) and many are much more profoimd in that they involve a change in chemical composition and structure (e.g. chlorite to garnet). These changes are regarded as transformations from an unstable or metastable phase to a stable phase in equilibrium with the new conditions; the changes must overcome a certain activation energy and take place at a certain rate under a certain driving force. Metamorphic reactions may be studied by either thermodynamic or kinetic methods. These may be very complicated but a great deal can be imderstood even with the few elementary principles outlined here. "Kinetics deals with the rate of reaction and with the explanation of the rate in terms of the reaction mechanism. Chemical kinetics with its dynamic viewpoint may be contrasted with thermodynamics with its static viewpoint. Thermodynamics is concerned only in the initial and final states of a system; the mechanism whereby one sj^tem is converted to another and the time required are of no importance. Time is not one of the thermodynamic variables. The most important subject in thermodynamics is the state of eqidlibriiun, and consequendy thermodynamics is the more powerful tool for investigating the conditions at equilibrium. Kinetics is concerned fimda47
48
Metamorphic Textures
mentally with the details of the process whereby a system gets from one state to another and with the time required for the transition. Equilibrium can also be treated in principle on the basis of kinetics as that situation in which the rates of forward and reverse reactions are equal" (Frost and Pearson, 1961). The kinetics and thermodynamics of metamorphic reactions have been discussed at length by Ramberg (1952), Fyfe, Turner and Verhoogen (1958) and Turner and Verhoogen (1960), but the student can proceed a long way by remembering three very simple concepts. (1) The lower the total energy of a system, the greater the stability, i.e. metamorphic transformations proceed so as to reduce the total energy. (2) Mineralogical transformations are impeded by an energy barrier which is called the activation energy. Energy must be available to surmount this barrier before the process takes place. The higher the activation energy, the slower the process. (3) Entropy is not a major factor in most metamorphic processes (see Fyfe et al.y 1958, p. 25). Entropy may be equated with atomic disorder and an increase in temperature increases the disorder. Reactions pro ceed in the direction of increased entropy.
Temperature FIG. 15. Energy levels of phases or assemblages (denoted by A and B) at different temperatures; t is the equilibrium temperature. A is more stable below and Β more stable above t
Figure 15 shows the relation between two materials A and Β which may interchange reversibly, i.e. A and Β can be polymorphs of one mineral, or a mixture A which can change isochemically into a mixture Β (e.g. A = cal cite + quartz, Β = woUastonite plus carbon dioxide). The energy of is less than that of Β below a temperature thus A is more stable. Above t, Β is more stable. The energy difference between A and Β depends on the tem perature; it is zero at / where A and Β are in equilibrium and becomes larger
Metamorphic Reactions
49
with departure (above or below) from t. T h e energy difference itself does not overcome the activation energy as this is achieved at local points by thermal fluctuations. T h e energy difference only indicates the direction of reaction— not the rate. MacKenzie (1965) emphasized the difference between equilibrium and metastability. Equilibriiun is synonymous with "reversible reaction", i.e. under certain conditions dG = 0 (at constant Ρ and T) or dF = 0 (at con stant V and T). A substance is stable if after any disturbance it tends to return to its original state. However, a substance can be in a condition of "false equilibriiun" or "metastable equilibriiun", if the velocity of reaction is zero. The mineral assemblage in a rock is considered to be "in equilibrium", i.e. the mineral phases are "in equilibrium" with each other, if they appear to be stable and not tending to break down individually or to react with each other. The assemblage need not, however, be in a lowest energy (most stable) condition but may be in a metastable equilibriiun. N o experimental, mineralogical or petrological criteria can tell unequivocally that an assem blage is stable, and metastable assemblages are known to persist throughout geological time because the reaction rates are infinitely slow and the activa tion energy barrier cannot be overcome. Metamorphic reactions are strongly time-dependent and the kinetics of reactions are extremely important.
Metamorphic
Reactions
A R O C K undergoes structural and mineralogical (and chemical) changes during metamorphism in an attempt to reach equilibriiun with the new conditions. This generally results in recrystallization which is the reconstitution of existing stable phases, and crystallization (neomineralization) which is the formation of new phases resulting from the destruction of unstable ones. The evolution of textures in metamorphic rocks requires an understanding of changes in minerals regarded as units of crystal structure and not merely as chemical compoimds. Metamorphic processes will be regarded here as a series of structural transformations. Some mineralogical transformations may be comparatively simple, e.g. twinning or minor polymorphic change, or the recrystallization of, e.g., quartz. Others are more complex in that they involve considerable structural changes (e.g. andalusite to sillimanite) and many are much more profoimd in that they involve a change in chemical composition and structure (e.g. chlorite to garnet). These changes are regarded as transformations from an unstable or metastable phase to a stable phase in equilibrium with the new conditions; the changes must overcome a certain activation energy and take place at a certain rate under a certain driving force. Metamorphic reactions may be studied by either thermodynamic or kinetic methods. These may be very complicated but a great deal can be imderstood even with the few elementary principles outlined here. "Kinetics deals with the rate of reaction and with the explanation of the rate in terms of the reaction mechanism. Chemical kinetics with its dynamic viewpoint may be contrasted with thermodynamics with its static viewpoint. Thermodynamics is concerned only in the initial and final states of a system; the mechanism whereby one sj^tem is converted to another and the time required are of no importance. Time is not one of the thermodynamic variables. The most important subject in thermodynamics is the state of eqidlibriiun, and consequendy thermodynamics is the more powerful tool for investigating the conditions at equilibrium. Kinetics is concerned fimda47
48
Metamorphic Textures
mentally with the details of the process whereby a system gets from one state to another and with the time required for the transition. Equilibrium can also be treated in principle on the basis of kinetics as that situation in which the rates of forward and reverse reactions are equal" (Frost and Pearson, 1961). The kinetics and thermodynamics of metamorphic reactions have been discussed at length by Ramberg (1952), Fyfe, Turner and Verhoogen (1958) and Turner and Verhoogen (1960), but the student can proceed a long way by remembering three very simple concepts. (1) The lower the total energy of a system, the greater the stability, i.e. metamorphic transformations proceed so as to reduce the total energy. (2) Mineralogical transformations are impeded by an energy barrier which is called the activation energy. Energy must be available to surmount this barrier before the process takes place. The higher the activation energy, the slower the process. (3) Entropy is not a major factor in most metamorphic processes (see Fyfe et al.y 1958, p. 25). Entropy may be equated with atomic disorder and an increase in temperature increases the disorder. Reactions pro ceed in the direction of increased entropy.
Temperature FIG. 15. Energy levels of phases or assemblages (denoted by A and B) at different temperatures; t is the equilibrium temperature. A is more stable below and Β more stable above t
Figure 15 shows the relation between two materials A and Β which may interchange reversibly, i.e. A and Β can be polymorphs of one mineral, or a mixture A which can change isochemically into a mixture Β (e.g. A = cal cite + quartz, Β = woUastonite plus carbon dioxide). The energy of is less than that of Β below a temperature thus A is more stable. Above t, Β is more stable. The energy difference between A and Β depends on the tem perature; it is zero at / where A and Β are in equilibrium and becomes larger
Metamorphic Reactions
49
with departure (above or below) from t. T h e energy difference itself does not overcome the activation energy as this is achieved at local points by thermal fluctuations. T h e energy difference only indicates the direction of reaction— not the rate. MacKenzie (1965) emphasized the difference between equilibrium and metastability. Equilibriiun is synonymous with "reversible reaction", i.e. under certain conditions dG = 0 (at constant Ρ and T) or dF = 0 (at con stant V and T). A substance is stable if after any disturbance it tends to return to its original state. However, a substance can be in a condition of "false equilibriiun" or "metastable equilibriiun", if the velocity of reaction is zero. The mineral assemblage in a rock is considered to be "in equilibrium", i.e. the mineral phases are "in equilibrium" with each other, if they appear to be stable and not tending to break down individually or to react with each other. The assemblage need not, however, be in a lowest energy (most stable) condition but may be in a metastable equilibriiun. N o experimental, mineralogical or petrological criteria can tell unequivocally that an assem blage is stable, and metastable assemblages are known to persist throughout geological time because the reaction rates are infinitely slow and the activa tion energy barrier cannot be overcome. Metamorphic reactions are strongly time-dependent and the kinetics of reactions are extremely important.