Point defect chemistry of minerals under a hydrothermal environment

Abstract

The kinetics of rock/water interactions are sufficiently rapid that most hydrothermal systems in nature will be in equilibrium with the adjacent rock mass. The bulk rock chemistry buffers the fugacity of oxygen, which in turn fixes the fugacities of water and of hydrogen for a given pressure and temperature. Systems in which only water, oxygen, and hydrogen are present as fluid phases are considered here. Variations in the fugacity of oxygen by several orders of magnitude are possible locally, controlled by variations in local rock chemistry; these lead to relative small variations in the fugacity of water. Incorporation of a hydrogen defect that is capable of acting as an acceptor into silicates leads to a strong dependence of point defect chemistry upon the fugacities of both water and oxygen. The strong dependence on the fugacity of water is capable of explaining the hydrolytic weakening effect, but in view of the strong dependence on oxygen fugacity, the question should also be raised whether it is an oxygen effect that is observed in the classical hydrolytic weakening process or solely a dependence on changes in the fugacity of water. Examples are given for impure natural quartz, olivine, and albite with trace amounts of calcium.

Water contents and deformation mechanism in ductile shear zone of middle crust along the Red River fault in southwestern China

“…Many experiments [3][4][5][6]19,28,32,34,35,38,[44][45][46][47][48][49][50][51][52][53][54][55][56] demonstrated that trace amount of water can weaken the creep strength for quartz and feldspar by one order of magnitude. Water enhances dislocation climb, recrystallization, and grain size reduction, which are considered to be the major deformation mechanism for developing ductile shear zones [19,37,38,51,54,55] .…”

“…The content of trace amount of water in minerals decreases with increasing metamorphic grade [13,[39][40][41][42][43] , suggesting decreasing water contents with increasing depth (temperature and pressure). Experiments at high temperature and high pressure indicate that trace amount of water can markedly weaken the creep strength of quartz [3][4][5][6]28,32,34,35,[44][45][46][47][48][49][50] , feldspar [19,38,51 -56] , feldspar-pyroxene hot-press aggregates [54] and granite [47] . The manifestation of the weakening effect on rock deformation includes enhanced strain rate, or lower creep strength and lower activation energy for higher water contents.…”

Effects of chemical environment on the solubility and incorporation mechanism for hydrogen in olivine

“…If hydrogen diffuses into the olivine lattice as a positively charged ion (or proton), it is expected to become associated with negatively charged point defects such as cation vacancies or anion interstitials (Hobbs 1984;Mackwell and Kohlstedt 1990). Hence, the solubility and speciation of hydrogen in a quasiternary mineral such as olivine should depend on the concentrations and the types of the point defects with which hydrogen ions become associated and, in turn, should be governed by the four independent thermodynamic parameters -temperature (T), total pressure (P), oxygen fugacity (fo2) and orthopyroxene activity (aopx) -for (Mg, Fe)2SiO4 olivine at fixed Mg:Fe ratio (Bai et al 1991).…”

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“…Es el denominado "debilitamiento hidrolítico" (Griggs, 1967;Blacic y Christie, 1984;Hobbs, 1984;Kronenberg et al, 1986). Los experimentos recientes demuestran que los medios hidrotermales son vías muy favo-rabIes para la difusión en el cuarzo de muy diversos tipos de defectos que rebajarían la resistencia de este mineral (Dennis, 1984;Etheridge et al, 1984;Kronenberg et al, 1986; entre otros).…”