Geology

Stress Makes Dating Difficult

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Science  30 Aug 2002:
Vol. 297, Issue 5586, pp. 1449
DOI: 10.1126/science.297.5586.1449b

Fission track dating has become central to many tectonic studies, particularly for measuring rates at which rocks have been eroded and exposed in active mountain ranges such as the Himalayas and those of western North America. In theory, the abundance of fission tracks (trails of crystal damage left by alpha particles expelled from uranium atoms over time) in minerals such as zircon or apatite can be related to the time since the mineral cooled below its closure temperature—the point above which tracks become thermally healed, or annealed. That cooling age allows calculation of the rate at which rocks have been tectonically brought to the surface from deep in the crust, assuming there is a well-behaved temperature-depth relationship and that temperature is the dominant variable.

Wendt et al. report experiments that cast doubt on the latter assumption. Subjecting apatite grains to a variety of temperature, pressure, and loading conditions, they found that the annealing behavior of fission tracks in the material is extremely sensitive to pressure and differential stress, with variations in parameters such as closure age exceeding 100% in some cases. Thus, apatite fission track dating may be unreliable where pressure or differential stress has played a significant role in the geologic story, which includes most tectonically interesting settings. — SW

Earth Planet. Sci. Lett.201, 593 (2002).

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