A trace element study of plagioclase and clinopyroxene phenocrysts in historical lavas from Mt. Etna, Sicily, by laser ablation ICP-MS Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/st74ct156

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  • Understanding magmatic processes occurring at depth in magmatic systems is fundamental for a more complete understanding of the variations sampled in lavas erupted from these systems on the Earth's surface. This thesis presents the results of detailed trace element analyses of plagioclase and clinopyroxene phenocrysts measured by laser ablation-inductively coupled plasma-mass spectrometry. The phenocrysts measured in this study represent the mineral constituents of 8 well-characterized historical lava flows (erupted between 1329-199 1 AD) on Mt. Etna, Sicily. Trace element analyses of phenocrysts, when combined with pre-existing whole rock major and trace element analyses, crystal size distribution data and petrographic analyses, lead to important constraints in the form of minerallmelt partition coefficients. This study focused on the development of laser ablation-inductively coupled plasma-mass spectrometry for the in situ analysis of mineral phases in volcanic rocks. Key elements measured by this technique were Sr and La-Gd in plagioclase and Sc, Y, Zr, Hf and all the naturally occurring REE in clinopyroxene. A typical analysis required both internal and external calibration and can be performed in approximately 3 minutes. There is a broader range of compositional variation in the rims of these phenocryst phases than in their cores. Rare earth elements in clinopyroxene form a set of parallel patterns that peak at neodymium, where they range in concentration from 15 ppm to 86 ppm. A mass balance calculation using previously measured whole rock concentrations and the proportion of phenocrysts was used to calculate the REE concentration of the liquid phase in equilibrium with the bulk phenocryst phases. These liquids were then used to estimate partition coefficients for the REE in plagioclase and clinopyroxene, which were found to compare very well with published values for similar alkaline lavas. Removing effects of fractional crystallization from these final melts reveals that there is a general trend towards lower REE abundances with time at Mt. Etna. The REE patterns of these fractionation-corrected magmas may be modeled as partial melts of a LREE-depleted spinel lherzolite source which interacts with a pyroxenite refining zone produced by crystal accumulation of earlier magmas that stagnated near the crustlmantle boundary.
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