Abstract
In this communication we present a simple microdynamic model which can explain the beginning of the zebra pattern formation in rocks. The two dimensional model consists of two main processes, mineral replacement along a reaction front, and grain boundary migration affected by impurities. In the numerical model we assume that an initial distribution of second-phase particles is present due to sedimentary layering. The reaction front percolates the model and redistributes second-phase particles by shifting them until the front is saturated and drops the particles again. This produces and enhances initial layering. Grain growth is hindered in layers with high second-phase particle concentrations whereas layers with low concentrations coarsen. Due to the grain growth activity in layers with low second-phase particle concentrations these impurities are collected at grain boundaries and the crystals become very clean. Therefore the white layers in the pattern contain large grains with low concentration of second-phase particles, whereas the dark layers contain small grains with a large second-phase particle concentration.
Highlights
The zebra texture is a periodic pattern which is encountered worldwide in geological formations and is often associated with hydrothermal mineralization
Due to the layered distribution of second-phase particle densities, which were superimposed by the dolomitization front, high grain boundary migration rates only occur in regions with low impurity densities
A combination between second-phase particle redistribution by a replacement front and subsequent grain growth can generate patterns similar to zebra textures observed in natural samples
Summary
The zebra texture is a periodic pattern which is encountered worldwide in geological formations and is often associated with hydrothermal mineralization. This approach can generate a periodic layered distribution of impurities similar to mud bands in snow sediments or the opaque second-phase material in the fine grained layers of the zebra dolomite Even though this theory does not account for the difference in grain size between the dark and light regions, it may be regarded as a possible process which triggers the formation of the zebra pattern. That self-organization occurs in the type of hydrological system studied in this communication was already proposed by Ortoleva et al [18], where banded Pb-Zn mineralization was linked to geochemical self-organization In this communication we present a simple generic model which can explain the differences in color and grain size in the layers of the zebra pattern. As already pointed out by Fontbote [7] the host rock and the infiltrating dolomitizing fluid will account for the “out-ofequilibrium condition.” These are general principles which are present during self-organized pattern formation [17, 18]
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