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3.4 Metamorphism at Different Time Scales

In any dynamic system, such as the Earth, time is a crucial factor for our understanding of the contributions of various processes to the evolution of the system. In nature, timescales of interest range from milliseconds to seconds for impact events and earthquakes up to millions or even a few billions of years for the evolution of the core-mantle subsystem. For the experimentalist, both extremes represent a challenge because extrapolations are needed, and most of the concerned parameters are not state variables. A close look to Mother Nature can be helpful, however: Information on timescales and the nature of processes is preserved in textural and chemical disequilibria, and by reading this archive we can make comparisons to kinetic experimental data and reconstruct the dynamics of past events.

Here, we focus on two processes occurring at grossly different timescales, i.e. impact phenomena and regional metamorphism. It was recognized some 20 years ago that sudden mass extinctions may be closely related to catastrophic asteroid impacts, which lead to changes in the composition of the atmosphere and thus to abrupt changes in climate. For the materials-oriented geoscientist aiming at a quantitative understanding, important questions to address include: What impact energy is needed to produce these changes? What kind of target material would contribute to these effects, which temperature-pressure conditions have been reached for which durations, and what are the processes in detail? Close to the other extreme of timescales we have to deal with regional metamorphism. Here again, we see transient changes in pressure and temperature, and in the case of regional metamorphic rocks these changes are driven by plate tectonics. The aim of the metamorphic petrologist is to reconstruct the duration, magnitude, location and timing of plate tectonic processes by interpreting the information in mineral assemblages, mineral compositions and microstructures. Due to the frequent lack of equilibrium, more than one point on the pressure-temperature-time evolution may - at least in theory - be determined, and the evolution of an orogeny in space and time may finally be reconstructed.

Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Deutschland
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