268-5 MAGMA RESIDENCE AND STORAGE CONDITIONS AT LARGE SILICIC DOMES IN THE ANDEAN CENTRAL VOLCANIC ZONE, CHILE

Session: Old and the New, Long and the Short: Perspectives on Integration of Timescales of Magmatic Processes: Special Session Related to MGPV Awards to Madison Myers and Anita Grunder (Posters)

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Understanding the timescales and physical conditions of magma storage is essential for interpreting the thermal and eruptive histories of silicic volcanic systems. This study investigates magma residence timescales and crystallization conditions in large silicic domes—Chao, Chillahuita, and Tocopuri—located in the Central Volcanic Zone (CVZ) of the Andes in northern Chile. We employ Sr and Mg diffusion geospeedometry of plagioclase phenocrysts to quantify crystal-scale timescales and assess magmatic processes prior to eruption. A total of 55 plagioclase phenocrysts were analyzed: 25 from Tocopuri, 20 from Chillahuita, and 10 from Chao for major element compositions by Electron Probe Microanalysis. High spatial resolution core-to-rim trace element profiles were obtained using Laser Ablation Inductively Coupled Plasma Mass Spectrometry. These data were modeled using a forward diffusion model based on the finite difference method, incorporating compositionally dependent diffusion coefficients. Anorthite (%An) content ranges from 0.30 to 0.65 wt% across Chao, Chillahuita, and Tocopuri domes, with core enrichment and interquartile ranges of 0.05586, 0.0658, and 0.08013, respectively, while Mg ranges from 7 to 66,720 ppm, Fe from 0 to 93,571 ppm, and Sr from 281 to 2,742 ppm, reflecting diverse magmatic histories. All domes show %An, Mg, Sr, and Fe core enrichment in plagioclases, consistent with open-system magmatic evolution and complex thermal histories. Thermodynamic constraints on crystallization temperatures and pressures were obtained through amphibole and feldspar geothermobarometry, yielding average crystallization temperatures of 939 °C for Chao, 931 °C for Chillahuita, and 929 °C for Tocopuri. Preliminary modeling results indicate substantial pre-eruptive residence times, with Mg diffusion ages of approximately 17.3 ka for Chao, 59.0 ka for Chillahuita, and 16.8 ka for Tocopuri. These long storage durations suggest that silicic magmas can reside in the crust for tens of thousands of years before eruption, undergoing protracted thermal evolution.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7722

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