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Paul Dennis edited The_two_component_mixing_model__.tex
over 8 years ago
Commit id: 33722cc74e78a1f36d535b1148dd797e50b36eee
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The two-component mixing model provides a process to account for the repeated cycles of isotope change seen along the growth axis of the vein, notably for transect DLR7, Figure 5. Here we see a repeated pattern of warming accompanied by a decrease in the oxygen isotope composition of the precipitated calcite. This trend is abruptly terminated with a return to cooler conditions accompanying calcite growth. This could be caused by the upward flow of formation water along a high permeability fault plane. During ascent the fluid will mix with the local groundwater. The variable mixing ratio is determined by the relative fluxes of the two components. The flow of formation waters would be driven by overpressure in the deep fault-bounded sedimentary basins. Moreover, such pressures may also intiate fault activity through reduction of the normal stresses across slip planes. Increasing rates of formation water release, leading to increased temperatures of precipitation, could be related to a pore fluid pressure sensitivity of shale permeability (add references). As the pore pressure increases the permeability increases leading to higher flow rates. At a critical pressure the fault ruptures, pore fluid pressure dissipates and the efflux of formation water abruptly decreases.
Such a process is closely alligned to the seismic valve model for faulting and fluid release described by Sibson (1981).