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| Titel |
A relatively reduced Hadean continental crust |
| VerfasserIn |
Xiaozhi Yang, Fabrice Gaillard, Bruno Scaillet |
| 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 |
250088428
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| Publikation (Nr.) |
 EGU/EGU2014-2530.pdf |
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| Zusammenfassung |
| Among the physical and chemical parameters used to characterize the Earth, oxidation state,
as reflected by its prevailing oxygen fugacity (fO2), is a particularly important one. It
controls many physicochemical properties and geological processes of the Earth’s different
reservoirs, and affects the partitioning of elements between coexisting phases and the
speciation of degassed volatiles in melts. In the past decades, numerous studies have been
conducted to document the evolution of mantle and atmospheric oxidation state with time
and in particular the possible transition from an early reduced state to the present
oxidized conditions. So far, it has been established that the oxidation state of the
uppermost mantle is within ±2 log units of the quartz-fayalite-magnetite (QFM) buffer,
probably back to ~4.4 billion years ago (Ga) based on trace-elements studies of
mantle-derived komatiites, kimberlites, basalts, volcanics and zircons, and that the O2 levels
of atmosphere were initially low and rose markedly ~2.3 Ga known as the Great
Oxidation Event (GOE), progressively reaching its present oxidation state of ~10
log units above QFM. In contrast, the secular evolution of oxidation state of the
continental crust, an important boundary separating the underlying upper mantle from the
surrounding atmosphere and buffering the exchanges and interactions between the
Earth’s interior and exterior, has rarely been addressed, although the presence of
evolved crustal materials on the Earth can be traced back to ~4.4 Ga, e.g. by detrital
zircons.
Zircon is a common accessory mineral in nature, occurring in a wide variety of
igneous, sedimentary and metamorphic rocks, and is almost ubiquitous in crustal
rocks. The physical and chemical durability of zircons makes them widely used in
geochemical studies in terms of trace-elements, isotopes, ages and melt/mineral
inclusions; in particular, zircons are persistent under most crustal conditions and
can survive many secondary processes such as metamorphism, weathering and
erosion. Thus, zircons in granites of shallow crust may record the chemical/isotopic
composition of the deep crust that is otherwise inaccessible, and offer robust records of the
magmatic and crust-forming events preserved in the continental crust. In fact, due to the
absence of suitable rock records (in particular for periods older than ~4.0 Ga),
studies in recent years concerning the nature, composition, growth and evolution
of the continental crust, and especially the Hadean crust, have heavily relied on
inherited/detrital zircons. Natural igneous zircons incorporate rare-earth elements
(REE) and other trace elements in their structure at concentrations controlled by the
temperature, pressure, fO2 and composition of their crystallization environment.
Petrological observations and recent experiments have shown that the concentration of Ce
relative to other REE in igneous zircons can be used to constrain the fO2 during their
growth.
By combining available trace-elements data of igneous zircons of crustal origin, we show
that the Hadean continental crust was significantly more reduced than its modern counterpart
and experienced progressive oxidation till ~3.6 billions years ago. We suggest that the
increase in the oxidation state of the Hadean continental crust is related to the progressive
decline in the intensity of meteorite impacts during the late veneer. Impacts of carbon- and
hydrogen-rich materials during the formation of Hadean granitic crust must have
favoured strongly reduced magmatism. The conjunction of cold, wet and reduced
granitic magmatism during the Hadean implies the degassing of methane and water.
When impacts ended, magma produced by normal decompression melting of the
mantle imparted more oxidizing conditions to erupted lavas and the related crust. |
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