CRUSTAL ASSIMILATION IN THE RECENT MT. ETNA MAGMA PLUMBING SYSTEM: EVIDENCE FROM IN SITU PLAGIOCLASE TEXTURAL AND COMPOSITIONAL DATA

Mt. Etna is Europe’s largest and most active volcano, and as a result of its proximity to populated areas, understanding the structure of its magma plumbing system and the nature of its magmatic processes is essential for better predicting eruptive hazards. The aim of this study is to document core to rim textural, chemical, and isotopic variations in plagioclase, in order to investigate the structure of the subvolcanic magma system, and the processes by which magmas change composition. The complex textures of plagioclase crystals in six trachybasaltic samples from eruption years 1974, 1981, 2001, and 2004, were characterized using Nomarski Differential Interference Contrast imaging. Based on ~30 images per sample, 6 textural categories were defined by different combinations of monotonous, oscillatory, sieve, and patchy zoning. Core to rim electron microprobe analyses carried out at distinct textural boundaries revealed variable Anorthite (mol %) values ranging from 92 to 44. In most phenocrysts, An decreases unsystematically from core to rim, and simple correlations among An, FeO (wt. %), textural type, and eruption year are lacking. Core to rim profiles indicate intricate crystallization histories that likely reflect changing magma chamber conditions. Laser ablation ICPMS ⁸⁷Sr/⁸⁶Sr analyses were performed on cores and rims of selected crystals from each textural type within each sample. Crystal ⁸⁷Sr/⁸⁶Sr values ranged from ~0.70300 to 0.70370 (±.00002), and were significantly lower than preliminary groundmass ⁸⁷Sr/⁸⁶Sr values, which ranged from ~0.70466 to 0.70498. Whole-rock ⁸⁷Sr/⁸⁶Sr values are between groundmass and crystal values. The Δ⁸⁷Sr/⁸⁶Sr within each crystal, defined as core minus rim, varied from -0.00030 to +0.00011; while most crystals exhibit a core to rim increase, some showed a decrease and some had constant ⁸⁷Sr/⁸⁶Sr. Core to rim increases, combined with more radiogenic whole rock and preliminary groundmass ⁸⁷Sr/⁸⁶Sr, provide strong evidence of assimilation of a more radiogenic component over the timeframe of plagioclase crystallization. The absence of abundant evidence of multiple recharge events in individual crystals may suggest that the plumbing system is characterized by a plexus of dikes and sills.