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Along-arc, inter-arc and arc-to-arc variations in volcanic gas CO 2 /S T ratios reveal dual source of carbon in arc volcanism
Language
English
Obiettivo Specifico
Status
Published
JCR Journal
JCR Journal
Peer review journal
Yes
Title of the book
Issue/vol(year)
/168 (2017)
Pages (printed)
24-47
Issued date
March 18, 2017
Abstract
Some 300–600 Tg of volatiles are globally vented each year by arc volcanism. Such arc gas emissions have contributed
to past and present-day evolution of the Earth atmosphere and climate by recycling mineral-bound volatiles
subducted along active slabs. Carbon dioxide (CO2) and total sulphur (ST) are, after water, the major
components of volcanic arc gases. Understanding their relative abundances (e.g., the CO2/ST ratio) in arc volcanic
gases is important to constrain origin and recycling efficiency of these volatiles along the subduction factory, and
to better constrain the global arc volcanic CO2 flux.Here,we review currently available information on global variations
of volcanic arc CO2/ST gas ratios. Weanalyse a dataset of N2000 published volcanic arc gasmeasurements
that comprise (i) low-temperature hydrothermal gas emissions, in which ST is dominated by hydrothermal
hydrogen sulphide (H2S), and (ii) high temperature “magmatic” gases rich in sulphur dioxide (SO2). We show
that the global CO2/ST population of hydrothermal gases is mainly controlled by S loss to hydrothermal fluids/
rocks.Wethen select a subset of high-temperature (≥450 °C) arc gaseswhich, being less affected by S hydrothermal
loss, can be used to infer the “deep” source of volatiles. Using a subset of time-averaged high-T gas
compositions for 56 arc volcanoes, we identify sizeable along-arc and inter-arc variations in the “magmatic”
arc gas CO2/ST ratio, which we ascribe to distinct volatile origins in the magma generation/storage zone. In the
attempt to resolve the slab vs. crustal contributions to arc gas budgets, we explore the global association between
volcanic gas CO2/ST ratios and non-volatile (trace elements) tracers in arc magmas. For the first time in a global
study,we find evidence for higher carbon output (CO2/ST) in arcswhere carbonate sediment subducts on the seafloor.
Indeed,most arc volcanoes exhibit gas vs. trace element relationships that are explained by addition of slabsediment
melts±fluids to themantlewedge.We also identify a subset of CO2-rich arc volcanoeswith unusually
high CO2/ST ratios (Etna, Stromboli, Vulcano Island, Popocatépetl, Soufriere of St Vincent, Bromo and Merapi),
which we interpret as the product of magma-limestone interactions in the upper crust. Evidence for this process
comes from carbonate xenoliths and/or carbonate basement that characterise these volcanic systems. Although
the mean global CO2/ST ratio of arc gas (~2.5) reflects a predominant source from subducted sediment,
limestone-assimilation-derived C may account for a substantial (~19–32%) fraction of the present-day global
arc budget, and may have contributed to elevated atmospheric CO2 levels and warmer climate in Earth's past.
Our global CO2/ST vs. trace element association paves the way to identifying the gas signature of volcanoes (or
arc segments) for which gas information is currently missing, and so improve our current global volcanic arc
CO2 flux inventory.
to past and present-day evolution of the Earth atmosphere and climate by recycling mineral-bound volatiles
subducted along active slabs. Carbon dioxide (CO2) and total sulphur (ST) are, after water, the major
components of volcanic arc gases. Understanding their relative abundances (e.g., the CO2/ST ratio) in arc volcanic
gases is important to constrain origin and recycling efficiency of these volatiles along the subduction factory, and
to better constrain the global arc volcanic CO2 flux.Here,we review currently available information on global variations
of volcanic arc CO2/ST gas ratios. Weanalyse a dataset of N2000 published volcanic arc gasmeasurements
that comprise (i) low-temperature hydrothermal gas emissions, in which ST is dominated by hydrothermal
hydrogen sulphide (H2S), and (ii) high temperature “magmatic” gases rich in sulphur dioxide (SO2). We show
that the global CO2/ST population of hydrothermal gases is mainly controlled by S loss to hydrothermal fluids/
rocks.Wethen select a subset of high-temperature (≥450 °C) arc gaseswhich, being less affected by S hydrothermal
loss, can be used to infer the “deep” source of volatiles. Using a subset of time-averaged high-T gas
compositions for 56 arc volcanoes, we identify sizeable along-arc and inter-arc variations in the “magmatic”
arc gas CO2/ST ratio, which we ascribe to distinct volatile origins in the magma generation/storage zone. In the
attempt to resolve the slab vs. crustal contributions to arc gas budgets, we explore the global association between
volcanic gas CO2/ST ratios and non-volatile (trace elements) tracers in arc magmas. For the first time in a global
study,we find evidence for higher carbon output (CO2/ST) in arcswhere carbonate sediment subducts on the seafloor.
Indeed,most arc volcanoes exhibit gas vs. trace element relationships that are explained by addition of slabsediment
melts±fluids to themantlewedge.We also identify a subset of CO2-rich arc volcanoeswith unusually
high CO2/ST ratios (Etna, Stromboli, Vulcano Island, Popocatépetl, Soufriere of St Vincent, Bromo and Merapi),
which we interpret as the product of magma-limestone interactions in the upper crust. Evidence for this process
comes from carbonate xenoliths and/or carbonate basement that characterise these volcanic systems. Although
the mean global CO2/ST ratio of arc gas (~2.5) reflects a predominant source from subducted sediment,
limestone-assimilation-derived C may account for a substantial (~19–32%) fraction of the present-day global
arc budget, and may have contributed to elevated atmospheric CO2 levels and warmer climate in Earth's past.
Our global CO2/ST vs. trace element association paves the way to identifying the gas signature of volcanoes (or
arc segments) for which gas information is currently missing, and so improve our current global volcanic arc
CO2 flux inventory.
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