NOISE AND OSCILLATORY ZONING OF MINERALS

Abstract

Oscillatory mineral zonation is usually associated with crystal growth in an open system, either a hydrothermal system or a melt after a period of magma mixing or degassing. Such systems may be driven sufficiently far from thermodynamic equilibrium to produce autonomous patterns by geochemical self-organization. The resulting zonation patterns will be the result of coupling between the generally nonlinear crystal growth dynamics and the boundary conditions imposed by externally controlled fluctuations. We examine the effects of noisy boundary conditions on four different crystal growth models. These are models for plagioclase growth in magmatic systems and , for carbonates in sedimentary systems (Wang and Merino, 1992), for garnets in hydrothermal systems (Jamtveit, 1991), and for silicate growth from a melt (Wang and Wu, 1995). The plagioclase model is sensitive to noise, even to low amplitude noise, implying that an observed zonation pattern will be significantly affected by processes other than local growth and transport processes. For the garnet model, fluctuations in the external environment may cause synchronization, so that different crystals develop similar zonation patterns, even if the formation of zonation patterns is a consequence of nonlinear local dynamics. This implies that similarity in intracrystalline zonation does not necessarily imply that the zonation pattern was produced by changes in the external (environmental) conditions. The formation of a zonation pattern may be a consequence of local nonlinearities in the growth process, but the pattern details may be strongly affected by subtle changes in the external environment.