Diffusion metamorphic 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... 

If two minerals contact with each other at constant the pressure-temperature condition where two minerals are unstable, reaction occurs between them to form stable mineral. The dominant rate limiting mechanisms are diffusion of aqueous species dissolved from minerals in fluid and dissolution and precipitation reactions. If fluid is not present, diffusion in solid phase occurs. But the rate of diffusion in solid phase is generally very slow. However, at very high temperature and pressure (metamorphic condition) the diffusion in solid phase may control the mass transfer. Reaction-diffusion model is able to be used to obtain the development of reaction zone between two minerals with time. 

Much work is needed to elucidate the effects on metamorphic differentiation of rock plastic flow and fluid infiltration (see section on skarns below). Reaction rates and diffusion coefficiently are largely unknown for geological systems under metamorphic conditions (see Brady [46]). Experimental generation of mineral bands out of initially uniform aggregates of two or more kinds of crystals (even if these are not rock-forming minerals but substances with faster dissolution/precipitation kinetics) should be attempted. 

The fact that most metamorphic inclusions are secondary has led to the diffuse opinion that they are useless. Notwithstanding the evidence that they are an intrinsic part of the rock and have consequently to be studied, I see the problem in an entirely different way primary and secondary inclusions are exactly alike neither is "a priori" secure as any is liable to have suffered one of the many processes which may alter the representativity of the enclosed fluid, such as selective trapping, leakage, necking down or reaction with the host mineral. Therefore any fluid inclusion data must be compared with results derived independently by other methods. This is the point at which we are now, as many of these data became available in metamorphic petrology from experimentation or theoretical models. Only when some limiting conditions have been fulfilled may additional data be obtained and it comes then... 

The relations and coefficients in Tables 1, 2 and 3 can be used to estimate the kinetics of many metamorphic processes, but the job will be much simpler if we make one additional assumption that metamorphic processes closely approximate a steady state. Fisher (1973, p. 910-911 1975, p. 115) showed that diffusion will automatically tend to shift potentials toward values such that the flux differences at every point in a structure just balance the local reactions, establishing a steady state. If this shift is rapid enough relative to growth of the overall structure, most of the diffusion will be driven by the steady-state potentials, and the small amount of mass transfer involved in attaining the steady state may be neglected. 

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