Cooling rates and melt extraction timescales determined by diffusion chronometry on shallow crustal plutonic rocks.

Abstract

Diffusion chronometry has emerged as a valuable tool to track the timescales of magmatic processes and is now routinely applied on erupted volcanic products to infer crystal residence times and mixing-to-eruption timescales. Despite some attempts to apply such a technique to plutonic rocks, slow cooling in these systems complicates the interpretation of the retrieved timescales. Here we investigate the cooling rates and crystal-melt segregation timescales in the Western Adamello and Re di Castello units of the Adamello batholith (Italy). The Western Adamello is mainly composed of tonalite and volumetrically minor cumulative gabbro and leucotonalite, and segregated granite. The studied area from the Re di Castello exhibits a concentric structure mainly composed of tonalite, granodiorite, and aplitic and pegmatitic granite. Strongly zoned plagioclase crystals occur in both leucotonalite and granodiorite samples, while quartz crystals displaying normal Ti zoning are only observed in some granodiorites. After determining the crystallisation temperatures of plagioclase mantle and rim and quartz, as well as the initial conditions prior to diffusion, cooling rates were estimated based on plagioclase mantle-to-rim and quartz profiles. The retrieved cooling paths correspond, within uncertainty, to cooling rates calculated using thermal modelling and obtained by previous studies using ³⁹Ar/⁴⁰Ar dating. Crystal-melt segregation timescales were then calculated based on the diffusion modelling of plagioclase core-to-mantle profiles and the retrieved cooling rates. The calculated timescales (~ 10⁴-10⁵ yr) likely correspond to the plagioclase core-to-mantle residence time before crystallisation of the rim. Interestingly, these crystal residence times are similar to the zircon crystallisation timespan recorded in the Western Adamello and Re di Castello units, and to the crystal residence times obtained on historical volcanic eruptions (~ 10⁴-10⁶ yr). Overall, these findings highlight that plutonic systems can be used to reconstruct magmatic timescales and support the hypothesis of a close connection between plutonism and volcanism.

Supplementary information: The online version contains supplementary material available at 10.1007/s00410-025-02238-0.