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| Titel |
Is there really carbon in the detrital zircons from Jack Hills, Western Australia? |
| VerfasserIn |
Martina Menneken, Thorsten Geisler, Alexander A. Nemchin, Kilian Pollok, Martin Whitehouse, Robert Pidgeon, Simon Wilde |
| 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 |
250097869
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| Publikation (Nr.) |
 EGU/EGU2014-13489.pdf |
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| Zusammenfassung |
| We have previously reported the discovery of graphite and diamond inclusions in detrital
zircon grains from the Jack Hills in Western Australia, with the oldest inclusion-bearing
zircon being 4,252 Myrs (Menneken et al. 2007). When the diamond inclusions were first
identified by Raman spectroscopy, several characteristics were taken as evidence against
possible contamination from diamond polishing powder used to prepare the samples. Some
diamond inclusions appeared larger than the grain-size of the diamond polishing powder,
while most of them were associated with graphite. Raman spectra of the diamond inclusions
were clearly different to those from diamond particles in the polishing powder. Also, the
extremely large spread of carbon isotope compositions of the diamond-graphite
composite inclusions with δ13C values extending from -58 to -5 per mil argued
against contamination, as both synthetic and natural diamonds have a narrow range of
carbon isotope composition (Nemchin et al. 2008). In this study, we have applied
transmission electron microscopy (TEM) and Raman spectroscopy on focussed ion beam
sections cut from a fully embedded diamond inclusion and a fully embedded carbon
inclusion in two zircon grains from Jack Hills. Results show that the graphitic material
is not a solid inclusion, but covers the walls of a void as a 10-20 nm thick layer.
Since we identified numerous CO2 inclusions in the same host zircon by Raman
spectroscopy, it is likely that this inclusion was once filled with CO2 On the other
hand, similar to a study by Dobrzhinetskaya et al. (2014), performed on surface
diamond-graphite inclusions from one of our samples, we found that the embedded diamond
inclusion consists of several hundred nano-meter large angular diamond fragments
mixed with some gold particles. This is strong evidence that the embedded diamond
particles originated from the diamond polishing powder. The diamond particles
could enter the cavity through an opening 2 – 3 μm wide. Previously, we considered
it impossible to remove and insert apparently larger solid minerals through such
narrow openings during the polishing process. However, newly discovered CO2
inclusions along with graphite in Jack Hill zircons indicate that diamond grains from the
polishing paste likely entered into cavities that were originally filled with CO2 and
coated with graphite-like carbon, generating artificial graphite-diamond composite
inclusions
Dobrzhinetskaya et al. (2014) Diamonds in Earth’s oldest zircons from Jack Hills
conglomerate, Australia, are contamination. Earth Planet. Sci. Lett., 387, 212.
Menneken et al. (2007) Hadean diamonds in zircon from Jack Hills, Western Australia.
Nature, 448, 917.
Nemchin et al. (2008) A light carbon reservoir recorded in zircon-hosted diamond from
the Jack Hills. Nature, 454, 92. |
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