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Issue Date: 14-Mar-2018
Authors: Giuffrida, Marisa
Title: Magma storage, ascent and degassing histories traced by textures and chemical zoning in crystals: application to the C02-Rich basaltic system of Mt. Etna Volcano
Abstract: An extensive compositional dataset of plagioclase and olivine crystals from lavas emitted between the 2011 and 2013 at Mt. Etna has been used to constrain modes and timescales of magma storage and transfer to the surface. Plagioclase crystals display either near-equilibrium or disequilibrium textures at the core and rim that indicate complex histories of magma crystallization under variable chemical and physical conditions. Crystals with different textures have been characterized for major (An), minor (Fe and Mg) and trace elements (Sr/Ba). The Sr /Ba ratio in oscillatory-zoned plagioclase revealed the presence into the plumbing system of low-Sr magma volumes that preserve a geochemical signature similar to that of magmas feeding the historic, pre-1971 activity. Fe and Mg zoning vs anorthite in correspondence of plagioclase sieve textures also suggest that processes of gas-flushing have had a dominant role in triggering the paroxysmal eruption, determining a sudden intensification of the eruption intensity up to the fountaining phase. In order to interpret the range of crystal textures and compositions that may form in CO2-rich systems, a series of experiments was conducted to reproduce processes of CO2-flushing at distinct sections of the plumbing system. Starting from an initial material containing a pure-H2O fluid, experiments were performed at temperature of 1080°C and at two distinct pressures of 100 MPa and 300 MPa, using volatile mixtures at variable H2O+CO2 concentrations. Results show that CO2-flushing at high pressure greatly promote plagioclase destabilization during their early grow history, and is therefore one of the main mechanism responsible for the spectrum of disequilibrium textures at the plagioclase cores. The transfer and injection of prevalent CO2-rich gases at shallow (~100MPa) depth mostly reduce the clinopyroxene stability, causing severe destabilization of their rims. Through Sr-diffusion modelling in plagioclase the maximum time of magma storage during the considered eruptive period has been evaluated. Timescales of crystal residence in the plumbing system are short (five years to three decades), suggesting limited magma storage and faster transfer dynamics to the surface. The investigation of Li diffusion in plagioclase allow a direct quantification of magma ascent and vesiculation upon eruption. Li diffusion calculations yield timescales of sin-eruptive magma ascent between 20 and 30 sec, corresponding to rates of 50-75 m/s. Chemical zoning of olivine crystals highlights processes of multi-step magma transfer and residence at different levels of the plumbing system. The migration of magmas to the surface occurred primarily stepwise through multiple episode of injection and mixing between five compositionally-distinct magmatic environments (Mi), whose P-T- O2 characteristics and concentrations in dissolved volatiles were constrain by thermodynamic modeling. From a deepest reservoir, located at depth of ~600 MPa, the most primitive magma M1 (Fo84) moved along dominant pathways, intercepting the M2 (Fo80-82) at ~390 MPa and/or M3 (Fo78; 250 MPa), M4 (Fo75; ~140 MPa) and finally the shallow M5 (Fo70-73; ~40 MPa) storage zone. For some eruptive episodes, olivine zonings highlight a preferential route of transfer, connecting the M1 and M5 storage zones that facilitated the migration of primitive magma at shallow depth. Fe-Mg diffusion modelling on olivine normal and reverse zoning defines the timescales of magma transfer and storage across these magmatic environments, which vary from ~1 to 18 months, whereas intrusion and mixing by more basic magma into the shallowest reservoir occurred always within 5 months before eruption. Relevance of this study mainly relies on the quantification of volcanic processes at depth that may have considerable consequences in development of unusual, high-energy eruptions at basaltic volcanoes, generally acknowledged for their weak to mild explosive activity.
Appears in Collections:Area 04 - Scienze della terra

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