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Unformatted text preview: 1 Estimation of P & T: Thermobarometry A- Geothermometry I- The petrogenetic grid approach: This approach relies on calculating petrogenetic grids for specific chemical systems, determining the P-T locations of the different metamorphic reactions, and estimating the P-T stability fields of the various mineral assemblages in that system. This is then followed by comparing the mineral assemblage in the studied rocks with the calculated grid, and estimating the P-T range over which the mineral assemblage in this rock is stable (a method similar to our lab exercise on metapelites). The problems with this approach include: (1) the lack of some thermodynamic data for some minerals, (2) unknown relations between activities and compositions of some minerals, which leads to assuming ideal mixing which may in turn lead to large errors in the calculated grid (3) lack of sufficient information about the composition of the fluid phase, and whether the composition of that fluid changed over time during the metamorphic event (4) insufficient knowledge of the composition of the fluid attending metamorphism (hence causing some shift and uncertainty about location of all dehydration reactions!). The major advantage of the petrogenetic grid approach is that metamorphic reactions deduced through a careful examination of the textural relations between the various minerals can be compared with the reactions calculated and plotted on the grid, which in turn allows us to deduce the P-T evolution (known as P-T path) of the rock examined. II- Geothermometers: Geothermometers are reactions that can be used to calculate or estimate T. Conditions necessary for a good geothermometer include: (1) Steep slopes in P-T space (obtained through a large entropy of reaction ( ∆ S r ) and a relatively small volume of reaction ∆ V r . for those with some background in thermo!) (2) The reaction should have been experimentally reversed at conditions comparable to those under which the rock was metamorphosed. (3) The reaction should be "insensitive to variations in bulk rock chemistry". A good geothermometer is one that is applicable to a variety of rock types with different compositions. (4) The reaction should be resistant to resetting under retrograde conditions. If this condition is not fulfilled, then the temperature calculated will be significantly lower than the "peak" temperature, and will instead represent a "closure" temperature. 2 Types of geothermometers: (1) Element exchange thermometers (e.g. Fe-Mg exchange between two ferromagnesian phases) (2) O 2 isotope geothermometers (3) Solvus geothermometers (4) Net transfer reactions (5) Fluid inclusions (6) Illite crystallinity (7) Vitrinite reflectance (8) Conodont colour index Note that the last three methods on this list are not considered reliable geothermometers, but could be useful in estimating temperature changes in a metamorphic terrain....
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This note was uploaded on 02/28/2012 for the course GLY 421 taught by Professor Staff during the Fall '11 term at Marshall.
- Fall '11