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Titel |
The oxidation state of primary MOR-Basalts before degassing of C-H-S-O species indicates an oxidized source regions buffered by sulphur-sulphate equilibrium |
VerfasserIn |
Fabrice Gaillard, Giada Iacono-Marziano, Yann Morizet, Yves Marrocchi |
Konferenz |
EGU General Assembly 2014
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 16 (2014) |
Datensatznummer |
250089319
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Publikation (Nr.) |
EGU/EGU2014-3519.pdf |
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Zusammenfassung |
The Earth’s mantle redox state regulates the igneous inputs of volcanic gases in the
atmosphere and probably controls the depth of mantle weakening by redox melting.
Capturing the processes that control basalt oxidation state and how the latter is
connected to their mantle sources is a central topic in planetary sciences. Mid-Ocean
Ridge Basalts, constituting 80% of the extrusive rocks, are widely used for this
purpose.
But MOR-basalts are considerably degassed. In particular, they have lost most of their
primary CO2 by open system degassing and it is unclear if this can importantly affect their
redox state. In order to reconstruct their primary volatile contents and their primary oxidation
states, we simulate here their degassing considering primary undegassed MORB being
variably enriched in H2O and CO2. Our results indicate that clear relationships between
MORB oxidation state and their volatile enrichments are triggered by degassing:
Volatile-rich melts degas more CO2, H2O and SO2, which triggers a more pronounced
decrease in Fe3+/Fetot. The reduction associated to degassing is relatively small, but it
explains well recently reported geochemical observations relating geochemical
enrichments and MORBs oxidation state. The oxidation state of MORBs as retrieved
from their post-eruption Fe3+/Fetot therefore underestimates the oxidation state
of their source regions by about 0.3-0.8 log-units; the degree of underestimation
correlating with the amount of initial CO2 and H2O. The source regions of MORBs
must buffer fO2s that are centred on FMQ+1 just after melt extraction from the
mantle.
This fO2 range corresponds to the region where the sulphide-sulphate equilibrium in
basalts is the dominant redox buffer, and we therefore propose that the fact most basalts
erupting on Earth since the Archean have a constant fO2 of FMQ reflects the redox buffering
by sulphur during partial melting. Prior to melting, the deep subsolidus mantle must be
equilibrated with higher fO2 as oxidized species (Fe3+, S6+) are all incompatible. The
mantle sourcing MORBs is more oxidized than previously established and is rather
controlled by sulphide-sulphate equilibriums than buffered by graphite-CO2 equilibrium |
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