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.
Un ampio dataset costituito da crystalli di plagioclasio ed olivina relativo alle lave emesse durante l attività parossistica tra il 2011 e il 2013 al vulcano Etna, è stato utilizzato per definire le modalità e le scale temporali dei processi di residenza e trasferimento dei magmi in superficie. I cristalli di plagioclasio mostrano al nucleo e al bordo sia tessiture di equilibrio che di disequilibrio, le quali testimoniano storie complesse di crystallizzazione dei magmi in condizioni fisico-chimiche variabili. Cristalli con differenti caratteristiche tessiturali sono stati analizzati tenendo in considerazione le variazioni in elementi maggiori (An), minori (Fe e Mg) e in traccia (Sr/Ba) lungo l intero profilo composizionale. In particolare, il rapporto Sr/Ba in cristalli di plagioclasio caratterizzati da zonatura oscillatoria ha rivelato la presenza all interno del feeding system Etneo di volumi magmatici a basso contenuto in Sr, i quali preservano una signature geochimica simile a quella dei prodotti che hanno alimentato l attivita storica, pre-1971, dell Etna. L analisi della zonatura di Fe e Mg rispetto all anortite in corrispondenza delle tessiture di tipo sieve presenti ai bordi del plagioclasio indicano, inoltre, che processi di gas-flushing hanno avuto un ruolo dominante nell innesco delle eruzioni parossistiche, portando ad una rapida intensificazione dell intensità eruttive fino alla fase di fontanamento. Nel tentativo di interpretare il range di tessiture e composizioni osservate nei campioni naturali, sono stati eseguiti esperimenti che riproducono i processi di CO2-flushing a differenti livelli del plumbing system Etneo. Da un campione iniziale, cristallizzato in presenza di una fase fluida costituita unicamente da H2O, due set di esperimenti sono stati condotti a 1080°C e alla pressione di 300 Mpa e 100Mpa, utilizzando una miscela in volatili a diversa concentrazione di H2O e CO2. I risultati mostrano che in condizioni di alta pressione la migrazione di gas ricchi in CO2 favorisce la dissoluzione di plagioclasio e, pertanto, questo processo puo essere considerato uno dei principali meccanismi responsabili dello sviluppo di tessiture di disequilibrio al nuocleo dei plagioclasi. Lo stesso processo a pressione inferiore o uguale a 100 Mpa ha effetto principalmente sulla stabilità del clinopirosseno, e è causa principale del loro riassorbimento. La modellizzazione della diffusione dello stronzio in plagioclasio ha permesso di calcolare il tempo massimo di residenza dei magmi nel periodo 2011-2013. I tempi calcolati variano da 5 a 31 anni, suggerendo tempi di stazionamento brevi i e rapide dinamiche di risalita in superficie. Inoltre, attraverso la diffusione del Li in plagioclasio è stato possibile determinare le scale temporali di risalita e degassamento sin-eruttivo dei magmi. I risultati indicano tempi di risalita variabili tra 20 e 30, corrispondenti a velocità di 50-75 m/s per gli episodi parossistici del periodo 2011-2013. La zonatura compositionale delle olivine messo in evidenza numerosi processi di interazione di magmi compositionalmente divers che risiedono a differenti livelli crostali, i cui parametri termodinamici sono stati determinati attraverso modellizzazioni MELTS. Da un serbatoi profondo, localizzato a profondità pari a ~600 MPa, magma primitivo (M1) di composizione Fo84 risale in superficie intercettando inizialmente il magma M2 (Fo80-82) residente a ~390 MPa, e/o il magma M3 (Fo78; ~250 MPa) ed, in seguito, M4 (Fo75-76; ~140 MPa) ed M5 (Fo70-73; ~40 MPa). Modellizzando la diffusione della componente Fe-Mg dei cristalli di olivina sono stati definiti i tempi di trasferimento e residenza dei magmi a diversi livelli crostali. i quali variano da ~1 a 18 mesi. Correlando i tempi ottenuti con le profondità di stazionamento di ciascun ambiente magmatico appare chiaro come le dinamiche magmatiche che hanno preceduto i parossismi del 2011-2013 siano variabili, ma anche estremamente veloci.
MAGMA STORAGE, ASCENT AND DEGASSING HISTORIES TRACED BY TEXTURES AND CHEMICAL ZONING IN CRYSTALS: APPLICATION TO THE CO2-RICH BASALTIC SYSTEM OF MT. ETNA VOLCANO / Giuffrida, Marisa. - (2017 Jan 30).
MAGMA STORAGE, ASCENT AND DEGASSING HISTORIES TRACED BY TEXTURES AND CHEMICAL ZONING IN CRYSTALS: APPLICATION TO THE CO2-RICH BASALTIC SYSTEM OF MT. ETNA VOLCANO
GIUFFRIDA, MARISA
2017-01-30
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.File | Dimensione | Formato | |
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