Cooling rates and crystal residence times in plutonic rocks determined by diffusion chronometry (Adamello batholith, Italy)

Thomas Grocolas¹, Othmar Müntener¹, Anne-Sophie Bouvier¹

• Affiliations: ¹Institute of Earth Sciences, University of Lausanne, Switzerland

• Presentation type: Poster

• Presentation time: Tuesday 16:30 - 18:30, Room Poster Hall

• Poster Board Number: 269

• Programme No: 1.3.15

• Theme 1 > Session 3

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Abstract

Constraining the temperature evolution of felsic magma reservoirs through time constitutes an important scientific and societal challenge in order to mitigate future volcanic hazards. Diffusion chronometry 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 diffusion chronometry to plutonic rocks, their slow cooling prevented a robust interpretation of the retrieved timescales. Here we investigate the cooling rates and crystal-melt segregation timescales in the Western Adamello (WA), Italy. It is mainly composed of tonalite and volumetrically minor cumulative gabbro and leucotonalite having strongly zoned plagioclase crystals, and segregated granite. After determining the crystallisation temperature of plagioclase and the initial conditions prior to diffusion, cooling rates were inferred based on plagioclase mantle-to-rim profiles and 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 WA, and to the crystal residence times obtained on historical volcanic eruptions. 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.