Metamorphism buffer reactions

It is obvious that in the case of pressure gradients determined by the maximum controlled by buffer reactions in the pile of rocks, and by the minimum in fractures (in the case of open circulation Pf(n,in) - (hydr) of the column of fluid), mechanical movement of the fluid was in one direction — from rock to fracture. For movement in the opposite direction—from fracture to rock—it was necessary to create a corresponding pressure gradient. Such phenomena, in addition to diffusion along the concentration gradient, presumably have occurred in hydrothermal metamorphism with typical reactions of hydration and carbonation. However, for normal progressive metamorphism it is hard to imagine a mechanical model in which a fluid with a strictly constant value of / h,o " h,o introduced from... 

Such buffered assemblages are quite common in nature and make contact aureoles of impure carbonate rocks one of the best areas to study the evolution of metamorphic fluids, because fluid compositions are essentially controlled by the mineral reactions. Progressive metamorphism initially leads to a rapid enrichment in CO2/ because the talc and tremolite reactions consume H2O and release CO2. At higher temperatures, water enrichment can take place by the decomposition of tremolite to diopside and/or forsterite. Changes in the compositions of the solids will affect the fluid ratios, and in fig. 6 the compositional changes of coexisting calcite and dolomite as a function of temperature have been taken into account. 

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