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Diagenesis of volcanogenic sandstones is characterized by the alteration of glass and feldspar (plagioclase) to carbonates, clays and zeolites. These alteration products are commonly distributed throughout the body in well defined zones, but the chemical and physical controls on the spatial location of these zones is poorly understood. A quantitative model describing these processes has been constructed by combining the equations describing chemical reaction with those governing mass transfer. This approach leads to a set of differential equations with associated boundary conditions constrained by a set of mass action equations. In this framework the identity and spatial distribution of the reaction products can be shown to originate as a consequence of counter-current mass flows in which different aqueous species diffuse in opposite directions due to imposed boundary conditions. In the case of opposing diffusion currents containing components which are capable of reacting to form a solid precipitate, a well-defined precipitation zone will form. The positions of these zones are a function of the boundary conditions and the strength of the sources and sinks.

The calculation emphasizes the importance of knowing whether or not the system was open or closed with respect to fluid flow and/or gas exchange and shows that the Peclet number is a key parameter as it not only affects the spatial distribution of the alteration products but also effects the magnitude of the precipitation flow.

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