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| Titel |
Towards the bulk carbon content of Earth; new metal carbide geobarometer in high pressure diamond. |
| VerfasserIn |
A. P. Jones, D. Dobson, H. J. Milledge |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250028263
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| Zusammenfassung |
| Formation of the metallic core in Earth (and other terrestrial planets) is not thought to have
completely removed metallic iron from the lower mantle, where metallic iron might therefore
be expected to occur as a widespread minor component [1]. We provide a new interpretation
of metallic carbide inclusions in some diamond, which support a very high pressure origin
from the lower mantle. Unlike rare carbides reported from diamonds previously without
silicon [2], some diamonds from Jagersfontein coexist with iron-rich carbides which
do contain significant silicon and oxygen, including in some cases their partial
exsolution products. Based on an experimental calibration for liquid iron coexisting with
lower mantle perovskite [3], we are able to show that some carbides were likely
derived from pressures of approximately 45 GPa, or depths of >1100 km. This
potential geobarometer has not been corrected for the behaviour of carbon in the liquid
iron system, which might be an important experimental goal. The recognition of
this independent carbide geobarometer offers an important new tool to confirm
the superdeep origin of some diamond. The carbide-bearing diamonds are from
a group whose charcteristics have recently been described [5]. Their distinctive
light carbon isotopic signature (13δC ~ 17 ) coupled with evidence for very low
contents nitrogen which is nonetheless highly aggregated, might be interpreted as
indicative of subducted carbon. However, we are also open to the possibility that
the bulk carbon isotopic composition of the Earth might also be different from
the normal mantle value (13δC ~ 6 ), in which case the potential 0.4 wt% C in
the Earth’s core could also be isotopically very light, as suggested by Grady et al
[6].
References
[1] Frost D et al, Nature 428, 409-412 (2004)
[2] Jacob D E et al, Contrib. Mineral. Petrol., 146, 566-576 (2004)
[3] Lin et al, Science, 295, 313-315 (2002)
[4] Dubrovinsky L. et al, Nature 422, 58-61 (2003)
[5] Jones A P et al, 9th Int Kimb Conf 9IKC-A-00360 (2008)
[6] Grady et al, Int. J. Astrobiol. 3, 117–124 (2004) |
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