2019 | Oscillatory zoned minerals in magmatic rocks: chrono- or rheometers?

Understanding the geomaterials at microscale is essential to comprehending Earth’s history and making quantitative predictions about the evolution of magmatic systems. Chemical diffu-sion in minerals is an important tool that is commonly applied for rate estimates (geospeedome-try) in geology. However, the classical geospeedometry approach does not consider the me-chanical effects caused by the density changes that occur during the diffusion of species within crystals. Recently, we have developed a thermodynamically-consistent, coupled model for chemical dif-fusion and mechanical deformation (Zhong et al., 2017; Earth and Planetary Science Letters, v.474). The new approach allows distinguishing the two controlling mechanisms that are re-sponsible for the preservation of intercrystalline chemical zonation, i.e. diffusion controlled, and mechanically controlled. Using thermodynamic data, experimentally derived diffusion coeffi-cients, and viscous flow laws for creep, we can numerically simulate the effect of chemical dif-fusion on the development of stress gradients across a mineral grain and vice versa. Due to the mechanical-chemical coupling, the stress variations can maintain the chemical zonation longer than predicted by the classical diffusion model. The fully coupled mechanical-chemical model provides an alternative explanation for the preservation of chemically zoned minerals, and may contribute to a better understanding of processes in the deep Earth interior. The new tool opens up new research questions on how well we understand diffusion in miner-als as well as how well the commonly used chemical diffusion geospeedometry predicts appro-priate timescales for natural processes. A major implication of this work is that it provides a framework to couple chemical diffusion and mechanical deformation for future studies on the chemo-mechanical processes in magmatic rocks. We will focus on common oscillatory zoned minerals in magmatic rocks such as olivine, plagio-clase and pyroxene. To better understand how the sharp intercrystalline chemical zoning can be preserved on geological time scales, we apply two different diffusion modelling approaches – the classical and the newly developed coupled approach. In order to test the quantitative esti-mates and constrain the theoretical models that are proposed, we will combine high-resolution analytical techniques (TEM) and electron back scattered diffraction (EBSD). The newly devel-oped technique of HR-EBSD offers not only the possibility of extracting crystal-orientation data, but it makes possible to estimate intercrystalline stresses that are still preserved within the minerals. Our plan is investigate whether the preserved chemical zoning is chemically con-trolled and thus suitable for rate estimates or mechanically-controlled and thus can tell us something about stress state which is preserved in magmatic rocks on geological time scales. The proposed work include the most advanced quantification tools with cutting edge analytical measurements. Especially HR-EBSD is newly introduced to geosciences and both PI’s contrib-ute to its development and adjustment to geomaterials.