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Clinopyroxene-melt element partitioning during interaction between trachybasaltic magma and siliceous crust: Clues from quartzite enclaves at Mt. Etna volcano
Author(s)
Language
English
Obiettivo Specifico
Status
Published
JCR Journal
JCR Journal
Title of the book
Issue/vol(year)
/284-285 (2017)
Pages (printed)
447-461
Issued date
2017
Abstract
A peculiar characteristic of the paroxysmal sequence that occurred on March 16, 2013 at the New South East
Crater of Mt. Etna volcano (eastern Sicily, Italy) was the eruption of siliceous crustal xenoliths representative of
the sedimentary basement beneath the volcanic edifice. These xenoliths are quartzites that occur as subspherical
bombs enclosed in a thin trachybasaltic lava envelope. At the quartzite-magma interface a reaction corona develops
due to the interaction between the Etnean trachybasaltic magma and the partially melted quartzite.
Three distinct domains are observed: (i) the trachybasaltic lava itself (Zone 1), including Al-rich clinopyroxene
phenocrysts dispersed in a matrix glass, (ii) the hybrid melt (Zone 2), developing at the quartzite-magma interface
and feeding the growth of newly-formed Al-poor clinopyroxenes, and (iii) the partially melted quartzite
(Zone 3), producing abundant siliceous melt. These features makes it possible to quantify the effect of magma
contamination by siliceous crust in terms of clinopyroxene-melt element partitioning. Major and trace element
partition coefficients have been calculated using the compositions of clinopyroxene rims and glasses next to
the crystal surface. Zone 1 and Zone 2 partition coefficients correspond to, respectively, the chemical analyses
of Al-rich phenocrysts andmatrix glasses, and the chemical analyses of newly-formed Al-poor crystals and hybrid
glasses. For clinopyroxenes fromboth the hybrid layer and the lava flow expected relationships are observed between
the partition coefficient, the valence of the element, and the ionic radius. However,with respect to Zone 1
partition coefficients, values of Zone 2 partition coefficients show a net decrease for transition metals (TE), highfield
strength elements (HFSE) and rare earth elements including yttrium(REE+Y), and an increase for large ion
lithophile elements (LILE). This variation is associated with coupled substitutions on theM1, M2and T sites of the
type M1(Al, Fe3+)+TAl=M2(Mg, Fe2+)+TSi. The different incorporation of trace elements into clinopyroxenes
of hybrid origin is controlled by cation substitution reactions reflecting local charge-balance requirements. According
to the lattice strain theory, simultaneous cation exchanges across the M1,M2, and T sites have profound
effects on REE+Y and HFSE partitioning. Conversely, both temperature and melt composition have only a minor
effect when the thermal path of magma is restricted to ~70 °C and the value of non-bridging oxygens per tetrahedral
cations (NBO/T) shifts moderately from 0.31 to 0.43. As a consequence, Zone 2 partition coefficients for
REE+Y and HFSE diverge significantly fromthose derived for Zone 1, accounting for limited cation incorporation
into the newly-formed clinopyroxenes at the quartzite-magma interface.
Crater of Mt. Etna volcano (eastern Sicily, Italy) was the eruption of siliceous crustal xenoliths representative of
the sedimentary basement beneath the volcanic edifice. These xenoliths are quartzites that occur as subspherical
bombs enclosed in a thin trachybasaltic lava envelope. At the quartzite-magma interface a reaction corona develops
due to the interaction between the Etnean trachybasaltic magma and the partially melted quartzite.
Three distinct domains are observed: (i) the trachybasaltic lava itself (Zone 1), including Al-rich clinopyroxene
phenocrysts dispersed in a matrix glass, (ii) the hybrid melt (Zone 2), developing at the quartzite-magma interface
and feeding the growth of newly-formed Al-poor clinopyroxenes, and (iii) the partially melted quartzite
(Zone 3), producing abundant siliceous melt. These features makes it possible to quantify the effect of magma
contamination by siliceous crust in terms of clinopyroxene-melt element partitioning. Major and trace element
partition coefficients have been calculated using the compositions of clinopyroxene rims and glasses next to
the crystal surface. Zone 1 and Zone 2 partition coefficients correspond to, respectively, the chemical analyses
of Al-rich phenocrysts andmatrix glasses, and the chemical analyses of newly-formed Al-poor crystals and hybrid
glasses. For clinopyroxenes fromboth the hybrid layer and the lava flow expected relationships are observed between
the partition coefficient, the valence of the element, and the ionic radius. However,with respect to Zone 1
partition coefficients, values of Zone 2 partition coefficients show a net decrease for transition metals (TE), highfield
strength elements (HFSE) and rare earth elements including yttrium(REE+Y), and an increase for large ion
lithophile elements (LILE). This variation is associated with coupled substitutions on theM1, M2and T sites of the
type M1(Al, Fe3+)+TAl=M2(Mg, Fe2+)+TSi. The different incorporation of trace elements into clinopyroxenes
of hybrid origin is controlled by cation substitution reactions reflecting local charge-balance requirements. According
to the lattice strain theory, simultaneous cation exchanges across the M1,M2, and T sites have profound
effects on REE+Y and HFSE partitioning. Conversely, both temperature and melt composition have only a minor
effect when the thermal path of magma is restricted to ~70 °C and the value of non-bridging oxygens per tetrahedral
cations (NBO/T) shifts moderately from 0.31 to 0.43. As a consequence, Zone 2 partition coefficients for
REE+Y and HFSE diverge significantly fromthose derived for Zone 1, accounting for limited cation incorporation
into the newly-formed clinopyroxenes at the quartzite-magma interface.
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