Brief thermal pulses during mountain building recorded by Sr diffusion in apatite and multicomponent diffusion in garnet

We use the records of Sr diffusion preserved in apatite and corroborating multicomponent Fe-Mg-Ca-Mn diffusion preserved in garnet from the classic Barrovian metamorphic zones (Scotland) to quantify the timescale for the thermal peak of crustal heating c. 465 Ma. We show that Sr diffusion in apatite is a powerful means to determine thermal timescales, and provide the first set of diffusion-based timescale estimates across a wide range of metamorphic grades for Barrovian metamorphism. The results demonstrate that the thermal peak was extremely brief and lasted a few hundred thousand years. This timescale is one to two orders of magnitude shorter than peak timescales predicted by conventional models based on conductive relaxation of overthickened crust. The short peak thermal pulse or pulses probably involved advective heat transfer driven by magmas and associated fluid flow, followed by rapid exhumation. Peak thermal pulses may represent a very short part (< 1 Ma) of overall mountain building cycles lasting ∼ 107 yr or more, but play a dominant role in determining mineralogy, geochemical fluxes, and fluid production in mountain belts.