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Guiraud et al., 2001

Guiraud et al., 2001 - H2O in metamorphism and the...

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H 2 O in metamorphism and the preservation of metamorphic mineral assemblages Michel Guiraud 1 , Roger Powell 2 and Gisella Rebay 3 1 Laboratoire de Min´ eralogie, URA CNRS N736, Mus´ eum National d’Histoire Naturelle, 61 rue Buffon, 75005 Paris France 2 School of Earth Sciences, The University of Melbourne, Parkville, Victoria 3052, Australia 3 Dipartimento di Scienze della Terra, Universita di Pavia, via Ferrata, I-27100 Pavia, Italy Abstract The preservation of mineral assemblages that were fluid-present during their prograde history is primarily related to the consumption of the fluid by growth of more hydrous minerals in the transition to the retrograde history. The range of behaviour relating to the preservation of mineral assemblages is examined using calculated phase diagrams for fluid-saturated conditions, contoured for the H 2 O-content of the mineral assemblage. At equilibrium, along a pressure-temperature path on such a diagram, while a mineral assemblage crosses contours of decreasing H 2 O-content, the mineral assemblage evolves via dehydration, the fluid being lost from the rock. Once the assemblage crosses contours of increasing H 2 O-content, the mineral assemblage starts to rehydrate using the fluid in its porosity. When the rock has consumed its fluid, it becomes fluid-absent, and the mineral assemblage at this point tends to be the one preserved in the rock. Conditions relating to the preservation of mineral assemblages are considered, and examples of the consequences of different pressure-temperature paths on preservation in a metapelitic and a metabasic rock composition are considered on phase diagrams calculated with thermocalc . Introduction It is often considered that metamorphic mineral assemblages reflect the conditions associated with the maximum temperature reached along the pressure-temperature ( PT ) 1
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path that they followed. For example, this assumption is used explicitly in geothermometry and geobarometry, when the mineral compositions in an assemblage are used to calculate the PT of that part of the PT path. The systematic results gained with such an approach might suggest that the assumption is a reasonable one. It is a commonplace to justify the assumption in the case of originally hydrous-mineral-bearing, fluid-present mineral assemblages in terms of a prograde history involving dehydration and fluid loss, followed by a retrograde history in which little happens because the necessary H 2 O is not available to reverse the prograde reactions. The “metamorphic peak”, from where mineral assemblages are effectively preserved, is considered to correspond to the end of dehydration, at this transition from the prograde to the retrograde history. If H 2 O is added during the retrograde history, then rehydration is observed, the severity of the retrogression depending on the amount of H 2 O added.
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