S-CH PERTURBATION · Stress and chemical perturbation around mineral inclusions

Stress and chemical perturbation around mineral inclusions

A key to understanding the processes operating in the outer part of the Earth is to look at the metamorphic rocks produced in orogenic belts. These rocks now exhumed to the Earth’s surface provide a record of what they experienced, if only they can be correctly interpreted. Mineral reactions in metamorphic rocks have mechanical effects that may result in the development of pressure variations even on a hand specimen or grain scale and thus are critical for interpreting microstructural and mineral composition observations. The recent use of high resolution devices has revealed the three-dimensional size, shape, composition and distribution of microstructural features in metamorphic rocks down to the nanometre-scale. The new observations show that mechanically maintained pressure variations can be significant (~1 GPa) even on a micro-scale. Such pressure variations are however not considered in the conventional petrology approaches, even though pressure is an important parameter for correct description and understanding of the global geodynamic processes. We questioned the current petrology approach and the conventional quantification of the petrographic observations. Our analysis indicated that the current conventional quantification approaches based on constant pressure assumption are not appropriate and the development of a new approach is therefore necessary. Pressure variations associated with coesite inclusions in various host minerals were discussed. Then possible mechanical models relevant to a polycrystalline material composed of strong single crystals and weak grain boundaries were outlined. In a mechanical context, the development of coherent exsolution lamellae in feldspars and chemical zoning in reaction rims were discussed. It was then argued that the existence of grain-scale pressure gradients combined with diffusional equilibrium may explain chemical zoning of minerals in high grade metamorphic rocks. A method was proposed by which this zoning can be used to infer pressure based on equilibrium thermodynamics and on the assumption of zero net flux across the microstructure. Such an approach obviates the necessity of invoking commonly used kinetic factors to explain preserved mineral composition zoning preserved in rocks metamorphosed at high grade. The results obtained during the Marie Curie fellowship served as key arguments in the ERC starting grant proposal of Lucie Tajcmanova which has been successfully funded. The ERC project will start in October 2013.

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