Previous page     Contents     Next page

The rates of many tectonic processes can be inferred from constraints on the pressure-temperature-time (P-T-t) paths encountered by metamorphic rocks. One of the most important tasks of the petrologist, therefore, is to decipher the timescales of metamorphism using clues encoded in the chemistry and microstructures of the rocks in question. These timescales range from seconds (impact events) to thousands of years (contact metamorphism) to millions of years (regional metamorphism). A further complication is that metamorphism may occur on a punctuated time scale in response to short-term events, such as the influx of fluids or brief heating disturbances. In such cases it is common to see only a limited metamorphic imprint in the form of heterogeneously distributed, disequilibrium assemblages and microstructures.

Such heterogeneity is especially commonplace in rocks affected by extraterrestrial impacts because of the extremely short time scale of the event, even though ultra-high pressures and temperatures may be obtained. TEM is becoming an increasingly useful tool to characterise the phases and microstructures indicative of shock metamorphism and thus establish the origin of structures such as the Chicxulub and Sudbury craters, two of the largest suspected impact sites on the Earth. Because of its unique ability to image microstructures and analyse chemistry on a nanometer scale, TEM can also be used to examine short-range diffusive processes that take place at low temperatures or over short time periods. Application of this technique is yielding new experimental data on Fe-Mg diffusion in olivine at temperatures as low as 800°C, which is important for verifying whether diffusion coefficients measured at higher temperatures can be reliably extrapolated. This kind of experimental diffusion data is ultimately needed to interpret diffusion profiles measured in natural rocks, which under favourable circumstances, can be used to model cooling rates of minerals and hence the time scale of exhumation for metamorphic rocks. As shown below in a detailed petrographic study, however, the application of such modelling can be fraught with uncertainty when the assumption of fast grain boundary diffusion relative to volume diffusion turns out to be incorrect. The most likely cause for failure of this assumption is a lack of fluids, which are known to substantially increase grain boundary diffusion rates. It is becoming clear that a lack of fluids may also be the primary factor responsible for the preservation of metastable phases such as coesite, contained in "ultrahigh-pressure" rocks that have been subducted to depths greater than 100 km and subsequently exhumed to the surface. Study of natural samples using FTIR suggests that transformation of coesite is predominantly a "punctuated" event triggered by the influx of fluids at low temperatures.