RECONSTRUCTION OF PRE-ERUPTIVE MAGMAS FROM LASCAR VOLCANO (NE CHILE) USING APATITE AS COMPOSITIONAL PROXY

Apatite crystals provide a record of the pre-eruptive characteristics in igneous systems. Their trace element signatures record processes occurring in volcanic underpinnings, and their volatile element compositions record temporal variations in the volatile budget of magma. Recent advances have enhanced our understanding of the partitioning behavior of rare earth elements (REE) and volatiles between apatite and melts, providing insight into how magma storage depths and volatile budgets control eruptive style. Here, we present a comprehensive dataset of the concentration of volatiles and trace elements in apatite to model and reconstruct the pre-eruptive systems of the 26.5 ka Soncor Ignimbrite, the 9 ka Tumbres-Talabre eruption, and the 1993 Plinian eruption of Lascar volcano (NE Chile). We obtained in situ concentrations of S, Cl, F, and trace element contents of 64 apatite crystals (~20 per eruption). Using their volatile contents and existing thermodynamic models, we calculate the volatile budgets of the pre-eruptive magma.

Apatite crystal volatile contents were analyzed by electron probe microanalyzer (EPMA), and sensitive high-resolution ion microprobe - reverse geometry (SHRIMP-RG); and trace element contents were analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) and SHRIMP-RG. Absence of chemical zonation for most elements analyzed (REE, Cl, F) suggests these crystals experienced long residence times prior to eruption. Concentrations of S (503-2508 ppm), Cl (0.08–6.41 wt.%), and F (0.15–4.07 wt.%) show wide ranges between samples, while stoichiometrically determined water contents (OH) do not exceed 1.6 wt. %. Volatile and REE (La-Lu) compositional trends suggest that the chemistry of Lascar apatite is strongly controlled by magma mixing between three different melt sources. Fractional crystallization of plagioclase (Sr/Y=0.81–3.34; Eu/Eu*=0.27–0.60) also takes part in defining mineral compositions. Integrating this information and contextualizing it in a comprehensive model will enhance our knowledge about the understudied plumbing system of Lascar, provide further insight into the causes that unchain its explosive behavior, and highlight the importance of apatite as a versatile petrogenetic tool for the characterization of igneous systems.