Document Type

Thesis

Date of Degree Completion

Spring 2011

Degree Name

Bachelor of Science

Department

Geological Sciences

Committee Chair

Dr. Wendy A. Bohrson, Geological Sciences

Second Committee Member

Dr. Christopher Mattinson, Geological Sciences

Third Committee Member

Dr. Audrey D. Huerta, Director Science Honors Research Program

Abstract

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. Nomarski Differential Interference Contrast (NDIC) imaging was used to characterize the complex textures of plagioclase crystals in six trachybasaltic samples from eruption years 1974, 1981, 2001, and 2004. Approximately 30 NDIC images per sample revealed 6 textural categories defined by combinations of monotonous, oscillatory, sieve, and patchy zoning. Core to rim electron microprobe analyses carried out at distinct textural boundaries revealed variable anorthite (An) (mol %) values ranging from 92 to 44. In most phenocrysts, An decreases non-monotonically from core to rim, and simple correlations among An, FeO (wt. %), textural type, and eruption year are lacking, indicating intricate crystallization histories that likely reflect changing magma chamber conditions. Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LA-ICPMS) 87Sr/86Sr analyses were performed on cores and rims of selected crystals from each textural type within each sample. Phenocryst 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 D.. 87 Sr/86Sr within each crystal, defined as rim minus core, varied from -0.00030 to +0.00011; while most crystals exhibit a core to rim increase, some showed a decrease and some had constant 87 Sr/86Sr. The prevalence of core to rim increases, combined with whole rock and preliminary groundmass 87Sr/86Sr that are more radiogenic than plagioclase phenocrysts, provide strong evidence of assimilation of a more radiogenic component over the timeframe of plagioclase crystallization. Additionally, large (> 10) abrupt increases in An within many phenocrysts and the existence of 6 plagioclase crystals with an increase in87Sr/86Sr from core to rim is evidence that magma recharge also plays a significant role in plagioclase crystallization histories. Petrologic, textural, compositional, and isotopic heterogeneity among samples that represent eruptions only years apart may be evidence for distinct plagioclase crystallization histories within a plexus of dikes and sills.

Comments

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