The preservation of mineral assemblages that were¯uid-present during their prograde history is primarily related to the consumption of the¯uid by growth of more hydrous minerals as the retrograde history begins. The range of behaviour relating to the preservation of mineral assemblages is examined using calculated phase diagrams for¯uid-saturated conditions, contoured for the H 2 O content of the mineral assemblage. At equilibrium, as a mineral assemblage crosses contours of decreasing H 2 O content along a pressure±temperature path, it dehydrates, the¯uid being lost from the rock. If the assemblage crosses contours of increasing H 2 O content, the mineral assemblage starts to rehydrate using any¯uid on its grain boundaries. When the rock has consumed its¯uid, the resulting mineral assemblage is that preserved in the rock. Conditions relating to the preservation of mineral assemblages are discussed, 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.
In using calculated equilibria among minerals to understand metamorphic processes, there are usually "natural" choices of axes to use on phase diagrams, depending on the geological supposition of the processes operating in rocks. Thermodynamic variables come in conjugate pairs, intensive and extensive, and the natural choice of which to use as an axis of a phase diagram depends on which of the pair leads to paths for metamorphic processes that are easiest to visualize. An unnatural choice leads to diagrams on which paths are not intuitively obvious, and these diagrams usually have to be calculated. Ideas relating to conjugate pairs are illustrated using mineral equilibria for the upper amphibolite to granulite facies, calculated for an aluminous pelite composition in the system Na 2 O-CaO-K 2 O-FeO-MgO-Al 2 O 3-SiO 2-H 2 O (NCKFMASH). We look specifically at the pressurevolume conjugate pair and the H 2 O content-aH 2 O pair (i.e., the H 2 O-H2O pair). P-T versus V-T and T-M H2O versus T-a H2O diagrams are used to consider metamorphic processes. We show that the less obvious variable, for example V rather than P, may be more useful in considering certain situations, usually transient.
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