Tracking the transition from magmatic to post-crystallization environment in the Sesia Magmatic System (Italy) by coupling quartz OH-defects and trace element analyses

The production of a significant thermal and fluid anomaly during the assembly of large magmatic bodies inevitably leads to a prolonged, post-magmatic evolution of the system. To shed light on the transition between magmatic and post-crystallization environment, we investigated the variation of OH-defects and trace elements content in quartz from intrusive and eruptive products of the Sesia Magmatic System (SW Alps, Italy). Specifically, quartz crystals were sampled from a floor-to-roof section of the Valle Mosso Pluton and two rhyolitic units of the Sesia Caldera, which represents the crystallized remnants of a magmatic plumbing system beneath a large Permian caldera. A total of 120 quartz crystals were analysed using Fourier Transform InfraRed (FTIR) spectroscopy to investigate OH-defects both quantitatively and qualitatively and Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) to assess trace element abundances. Results indicate systematic variations: (1) intrusive quartz shows gradual decrease in Ti and increase in total defect water content (1–25 ppm), following the differentiation degree; (2) volcanic quartz displays variable Ti and water contents (2–14 ppm) comparable to those of the intrusive lithologies; (3) in both intrusive and eruptive units, lattice-bound Al-specific defects dominate over non-lattice bound Li-specific defects, except in a porphyritic dike showing evidence of fast cooling. Our findings suggest that, in absence of fast cooling, slow-diffusing elements and lattice-bound OHdefects (e.g. Al, Ti, AlOH) preserve primary magmatic signals, while fast-diffusing elements (e.g. Li) and interstitial defects (e.g. LiOH) provide insights into post-crystallization histories. The combined analysis of these features offers a powerful tool for reconstructing the thermal and chemical evolution of magmatic systems, from magma chamber processes to post-eruptive alteration. Moreover, it provides insights on the robustness of quartz OH-defects and trace element inventory as a tool for provenance indicator.