Because rocks are good insulators, it is generally thought that temperatures deep in the crust evolve slowly, rising and falling over millions to tens of millions of years. Rapid pulses of fluid or the intrusion of hot magmas can heat or cool rocks more quickly, as can rapid uplift along a fault (which juxtaposes hot and cold rocks at a rate faster than heat conduction). Thus metamorphic processes are also thought to act over these time scales. Ague and Baxter challenge some of these notions in well-studied metamorphic rocks in Scotland, known as the Barrovian metamorphic belt and thought to represent burial and heating of rocks during continental collision. They show that concentrations of a trace element, strontium, across the mineral apatite are surprisingly variable. Laboratory data imply that if the minerals were at the temperatures inferred for the host rocks for even 1 million years, diffusion should have homogenized any gradients. Thus the authors infer that the rocks were heated and cooled in less time. This would seem to require rapid heat input by fluids and rapid exhumation, but at scales and rates that start to challenge what have been thought to be geologic limits. Stay tuned. — BH
Earth Planet. Sci. Lett. 261, 500 (2007).