metaminerals - Metamorphic Mineral Assemblages EENS 2120...

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This document last updated on 21-Mar-2011 EENS 2120 Petrology Prof. Stephen A. Nelson Tulane University Metamorphic Mineral Assemblages The mineral assemblages that occur in metamorphic rocks depend on four factors: z The bulk chemical composition of the original rock. z The pressure reached during metamorphism. z The temperature reached during metamorphism. z The composition of any fluid phase that was present during metamorphism. If a rock is taken to some higher pressure and temperature then the mineral assemblage that develops should represent stable chemical equilibrium if the conditions are held for a long enough period of time that equilibrium can be achieved. Since metamorphism usually involves long periods of geologic time, most metamorphic rocks represent an equilibrium mineral assemblage. The Phase Rule for Metamorphism Recall that the phase rule states that F = C + 2 - P where F = the variance of the system or number of degrees of freedom, C = the number of components in the system, and P = the number of phases present. the 2 stands for the two independent variables, Pressure and Temperature. If you think about it, in metamorphic rocks where temperature and pressure can both vary during metamorphism, the most likely case would be to find a divariant (F=2) assemblage of phases. A univariant assemblage (F=1) would be less likely to occur, and an invariant assemblage (F=0) would represent equilibrium at a fixed point in temperature and pressure, and would thus be even less likely to occur. So, for F=2, C=P, the number of phases present in a rock for the more common divariant assemblage will be equal to the number of components. If P is greater than C, then one of three possibilities exist for the mineral assemblage. Metamorphic Mineral Assemblages 3/21/2011 Page 1 of 8
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1. The assemblage represents a non-equilibrium assemblage (perhaps due to incomplete chemical reactions or due to the presence of retrograde minerals that developed during cooling, uplift, or unroofing of the metamorphic rock). 2. The assemblage represents univariant or invariant equilibrium, as discussed above. 3. The number of components have not been chosen correctly. If possibility (1) is the reason for the lack of correspondence with the phase rule, it can usually be determined by close inspection of the rock. Reaction textures present in the rock might indicate incomplete reaction. Known retrograde minerals, i.e. those stable at lower pressures and temperatures than the rest of the minerals in the rock, could be identified. These retrograde phases could then be subtracted from the number of phases being considered and the phase rule could be reapplied to only the phases known to be in equilibrium. (For example, the presence of chlorite in amphibolite and granulite facies rocks would be indicative that the chlorite is a retrograde mineral or mineral produced during weathering, and thus would not be considered in the application of the phase rule.) Possibility 2 could always occur, and if the number of components is chosen correctly and
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metaminerals - Metamorphic Mineral Assemblages EENS 2120...

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