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The genetic relationship between andesites and dacites at Tungurahua volcano, Ecuador
Language
English
Obiettivo Specifico
2V. Struttura e sistema di alimentazione dei vulcani
Status
Published
JCR Journal
JCR Journal
Peer review journal
Yes
Title of the book
Issue/vol(year)
/349 (2018)
Pages (printed)
283-297
Issued date
2018
Abstract
Volcanic eruptions of intermediary and silica-rich magmas (andesites, dacites and rhyolites) in convergent arc
settings generate voluminous and explosive eruptions that can strongly affect human activity and have significant
environmental impacts. It is therefore crucial to understand how these magmas are generated in order to
anticipate their potential impact. At convergent margins, primitive magmas (primitive basalts and/or andesites)
are derived from the mantle wedge and they are progressively modified by physical and chemical processes operating
between the melting zone and the surface to produce silica-rich magmas.
In order to elucidate the relationship between andesites and dacites, we focus on Tungurahua volcano, located in
the Ecuadorian Andes. We collected a set of samples comprising such lithologies that were erupted during the
last 3000 year BP. This relatively short period of time allows us to assume that the geodynamic parameters remain
constant. Petrology and major-trace element compositions of these lavas have already been examined,
and so we performed a complementary Pb-Sr isotope study in order to determine the nature and origin of the
components involved in andesite and dacite genesis. Sr isotopes range from 0.70417 to 0.70431, and Pb isotope
compositions range from 18.889 to 19.154 for 206Pb/204Pb, from 15.658 to 15.696 for 207Pb/204Pb, and from
38.752 to 38.918 for 208Pb/204Pb. Dacites display a remarkably homogeneous Pb isotopic composition, with
higher 206Pb/204Pb values for a given 207-208Pb/204Pb compared to andesites. Andesites show notable
207Pb/206Pb variations for a given SiO2 content, whereas dacites have lower and homogenous 207Pb/206Pb values.
Andesite and dacite altogether plot in a roughly triangular distribution, with dacitic magmas systematically plotting
at the high SiO2 and 87Sr/86Sr and low 207Pb/206Pb fields. Based on our new dataset, we show that at least 3
different components are required to explain the Tungurahua compositional and isotope variation: one corresponds
to the mantle, the second has a deep origin (slab component or lower crust), and a mixture between
these two components explains andesite heterogeneity. The third component is derived from the underlying
upper continental crust. While andesites are derived fromdeep components, dacites are derived from the andesitic
magmas that underwent an assimilation-fractional crystallization (AFC) process with incorporation of the
local metamorphic basement. Finally,we used the geochemical and isotopic data to produce a model of the magmatic
plumbing system beneath Tungurahua, consistent with geophysical and experimental petrology constraints.
We conclude that melt migration and storage in the upper crust appears to be a key parameter for
controlling volcanic behavior though time.
settings generate voluminous and explosive eruptions that can strongly affect human activity and have significant
environmental impacts. It is therefore crucial to understand how these magmas are generated in order to
anticipate their potential impact. At convergent margins, primitive magmas (primitive basalts and/or andesites)
are derived from the mantle wedge and they are progressively modified by physical and chemical processes operating
between the melting zone and the surface to produce silica-rich magmas.
In order to elucidate the relationship between andesites and dacites, we focus on Tungurahua volcano, located in
the Ecuadorian Andes. We collected a set of samples comprising such lithologies that were erupted during the
last 3000 year BP. This relatively short period of time allows us to assume that the geodynamic parameters remain
constant. Petrology and major-trace element compositions of these lavas have already been examined,
and so we performed a complementary Pb-Sr isotope study in order to determine the nature and origin of the
components involved in andesite and dacite genesis. Sr isotopes range from 0.70417 to 0.70431, and Pb isotope
compositions range from 18.889 to 19.154 for 206Pb/204Pb, from 15.658 to 15.696 for 207Pb/204Pb, and from
38.752 to 38.918 for 208Pb/204Pb. Dacites display a remarkably homogeneous Pb isotopic composition, with
higher 206Pb/204Pb values for a given 207-208Pb/204Pb compared to andesites. Andesites show notable
207Pb/206Pb variations for a given SiO2 content, whereas dacites have lower and homogenous 207Pb/206Pb values.
Andesite and dacite altogether plot in a roughly triangular distribution, with dacitic magmas systematically plotting
at the high SiO2 and 87Sr/86Sr and low 207Pb/206Pb fields. Based on our new dataset, we show that at least 3
different components are required to explain the Tungurahua compositional and isotope variation: one corresponds
to the mantle, the second has a deep origin (slab component or lower crust), and a mixture between
these two components explains andesite heterogeneity. The third component is derived from the underlying
upper continental crust. While andesites are derived fromdeep components, dacites are derived from the andesitic
magmas that underwent an assimilation-fractional crystallization (AFC) process with incorporation of the
local metamorphic basement. Finally,we used the geochemical and isotopic data to produce a model of the magmatic
plumbing system beneath Tungurahua, consistent with geophysical and experimental petrology constraints.
We conclude that melt migration and storage in the upper crust appears to be a key parameter for
controlling volcanic behavior though time.
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