metamorphreact

metamorphreact - Metamorphic Reactions EENS 212 Petrology...

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This document last updated on 22-Mar-2011 EENS 212 Petrology Prof. Stephen A. Nelson Tulane University Metamorphic Reactions, Isograds, and Reaction Mechanisms Types of Metamorphic Reactions Chemical reactions that take place during metamorphism produce mineral assemblages stable under the new conditions of temperature and pressure. Thus, in order to understand the mineral assemblages and what they mean in terms of the pressure and temperature of metamorphism, we must first explore the various types of metamorphic reactions. A metamorphic reaction is an expression of how the minerals got to their final state, but a reaction does not necessarily tell us the path that was actually taken to arrive at this state. Sometimes it is possible to deduce the path by means of a reaction mechanism. Thus, we will also explore reaction mechanisms. If we are considering a rock of fixed chemical composition, then a metamorphic reaction states the principles of equilibrium. In other words, if we can write a reaction expressing equilibrium between the minerals we see in the rock, we expect that the reaction must have been taking place during metamorphism. We will first look at various types of metamorphic reactions. Univariant Reactions For a given rock composition, a univariant reaction is one that plots as a line or curve on a pressure-temperature diagram. If all phases in the reaction are present in the rock, then we know that the rock must have been metamorphosed at some pressure and temperature along the reaction boundary Consider for example the simple Al 2 SiO 5 system with excess SiO 2 and H 2 O. In low grade metamorphic in this system, the reaction: Al 2 Si 4 O 10 (OH) 2 <=> Al 2 SiO 5 + 3SiO 2 + H 2 O Pyrophyllite Ky or Andal Qtz fluid defines a reaction boundary on a P-T diagram. This boundary can be determined experimentally or can be calculated using thermodynamic properties of the phases involved. If we find a rock that contains pyrophyllite, quartz, and an Al 2 SiO 5 mineral, then we know that metamorphism took place somewhere along the trajectory of the reaction boundary. Furthermore, combining this with the knowledge of the stability fields of the Al 2 SiO 5 minerals, we could place boundaries on the conditions of metamorphism. Metamorphic Reactions 3/22/2011 Page 1 of 10
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For example, if the mineral is andalusite, then we know the rock was metamorphosed at a pressure less than about 2.5 kilobars. If the mineral is kyanite, then we know that the pressure was greater than about 2.5 kilobars. Combinations of other such reactions could further constrain the pressure and temperature conditions of metamorphism. The example above, however, is probably too simple for a real rock. Although simple, we can use the
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metamorphreact - Metamorphic Reactions EENS 212 Petrology...

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