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3.5 Timescales of Earth and planetary processes: constraints from natural samples

Terrestrial and extraterrestrial materials experience various physical and chemical conditions due to dynamic processes that act upon the Earth and bodies in the solar system. To better understand these processes we apply experimental results and equilibrium thermodynamics to evaluate the conditions under which natural materials formed. The dynamic nature of geologic processes and the slow rates of material and energy transport in rocks means that natural samples are often out of chemical or structural equilibrium. Features, such as chemical and isotopic compositional gradients, degree of crystallographic order, grain size, and distribution of defects can be used to investigate the timescales of geologic processes if we know the time and temperature dependence of kinetic processes such as diffusion, ordering, grain growth, or defect recovery. On one hand, we can compare the microstructures produced by kinetic processes in synthetic samples with those in natural samples to investigate processes and qualitatively constrain timescales. On the other hand, we can apply mathematical models with experimentally derived rate constants to the kinetic processes to quantitatively extract the timescale of the changing conditions required to produce a measured chemical distribution or microstructure.

The processes acting on natural samples with timescales ranging from tens of millions of years to microseconds can be investigated by applying the appropriate kinetic theory and rate data. In the first case, metamorphic processes and timescales can be examined to interpret tectonic processes and histories. In the second case, processes such as shock metamorphism can be investigated to better understand impact processes of planetary bodies.

Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany
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