Mt. Etna lies at the northern margin of the African plate, on the accretionary prism of the Africa-Europe subduction system. Differential roll-back of the Ionian oceanic lithosphere has created a vertical slab window through which the passive rise of asthenosphere causes partial melting and magma genesis. In this rapidly evolving geodynamic context, the increase of alkalinity in time, accompanied by variations of Sr and Nd isotopes, is a first order feature. Fluid mobile elements and B systematics reveal the fundamental role played by fluids released by the dehydrating oceanic lithosphere, even if they are added to the mantle source in amounts no larger than 1 wt.%. This induces relatively high H2O contents in Etna magmas that exert a strong control on phase relationships. Compositions of alkaline primary melts have been reconstructed and used to constrain the depth of origin and fractionation sequences at various pressures. It was found that phase relations at the crust-mantle boundary can only produce basaltic compositions akin to the less evolved sub-aphyric lavas, whereas trachybasaltic compositions are generated in the depth range 12-3 km together with cumulate bodies detected through geophysical investigations and here related to massive pyroxene and plagioclase fractionation. Detailed monitoring of Sr-isotope equilibrium between pyroxene and host rock revealed both the increase of 87Sr/86Sr over time and the occurrence of mixing processes between distinct batches of magma in the plumbing system. Sr, Nd and B systematics also reveal contamination effects due to interactions with lower crust and sporadic interaction with the sedimentary basement.

Genesis and evolution of Mt Etna alkaline lavas: petrological and Sr-Nd-B isotope constraints

ARMIENTI, PIETRO;D'ORAZIO, MASSIMO;INNOCENTI, FABRIZIO
2004

Abstract

Mt. Etna lies at the northern margin of the African plate, on the accretionary prism of the Africa-Europe subduction system. Differential roll-back of the Ionian oceanic lithosphere has created a vertical slab window through which the passive rise of asthenosphere causes partial melting and magma genesis. In this rapidly evolving geodynamic context, the increase of alkalinity in time, accompanied by variations of Sr and Nd isotopes, is a first order feature. Fluid mobile elements and B systematics reveal the fundamental role played by fluids released by the dehydrating oceanic lithosphere, even if they are added to the mantle source in amounts no larger than 1 wt.%. This induces relatively high H2O contents in Etna magmas that exert a strong control on phase relationships. Compositions of alkaline primary melts have been reconstructed and used to constrain the depth of origin and fractionation sequences at various pressures. It was found that phase relations at the crust-mantle boundary can only produce basaltic compositions akin to the less evolved sub-aphyric lavas, whereas trachybasaltic compositions are generated in the depth range 12-3 km together with cumulate bodies detected through geophysical investigations and here related to massive pyroxene and plagioclase fractionation. Detailed monitoring of Sr-isotope equilibrium between pyroxene and host rock revealed both the increase of 87Sr/86Sr over time and the occurrence of mixing processes between distinct batches of magma in the plumbing system. Sr, Nd and B systematics also reveal contamination effects due to interactions with lower crust and sporadic interaction with the sedimentary basement.
Armienti, Pietro; Tonarini, S.; D'Orazio, Massimo; Innocenti, Fabrizio
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11568/186217
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