DOCUMENTING THE ORIGIN OF COMPOSITIONAL DIVERSITY OF SUBDUCTION ZONE MAGMATISM, ALICUDI, AEOLIAN ARC (SOUTHERN TYRRHENIAN SEA, ITALY) USING IN SITU PLAGIOCLASE DATA

Documenting key magmatic processes that lead to compositional diversity at arc volcanoes can provide a framework for evaluating volcanic hazards. Alicudi, Aeolian Island Arc, Italy has a compositional range that is separated into 3 eruptive stages: stage 1 basalts; stage 2 basaltic andesites; stage 3 andesites (Peccerillo et al., 2004). The research objective is to identify the key processes causing compositional diversity at Alicudi by examining selected plagioclase from 10 samples that span the compositional and temporal range. The combination of plagioclase textural, major and trace element, and isotope data can be used to classify magma chamber processes. Through Nomarski Differential Interference Contrast imaging, 4 representative textural types were defined for each sample: simple oscillatory; complex oscillatory; sieved/patchy intermediate zone; sieve/patchy core. Andesite plagioclase commonly exhibits patchy/sieve textures, whereas basalt plagioclase commonly shows simple or complex oscillatory textures. Textural boundaries guided choices for electron microprobe analyses, and results reveal ranges of An₅₀-An₈₈ in basalts, An₄₂-An₉₃ in basaltic andesites, and An₄₅-An₈₈ in andesites. In most samples, increases of An₁₀ or greater are noted rimward of dissolution rims or within sieved/patchy zones, suggesting possible recharge events. The presence of An₈₈ cores in andesites may suggest some crystal inheritance from basalts. Laser Ablation Multi Collector Inductively Coupled Plasma Mass Spectrometry data documented ⁸⁷Sr/⁸⁶Sr of distinct textural types and where possible, core and rim variations. Basalt plagioclase has the highest ⁸⁷Sr/⁸⁶Sr range(0.70327-0.70375), whereas andesite plagioclase has the lowest (0.70284-0.70353), and crystals within all samples have heterogeneous ⁸⁷Sr/⁸⁶Sr signatures. Increases in ⁸⁷Sr/⁸⁶Sr from core to rim are observed for all stages. Collectively, the data suggest magma mixing occurred at Alicudi. Crustal assimilation is also likely, although more work is needed to identify and quantify the contaminant. In situ plagioclase data can provide time integrated information about petrogenetic processes that contribute to compositional diversity, thereby improving our ability to predict eruption style and to manage associated hazards.