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| Titel |
Isotopic Characterization of Diamond Growth in Fluids |
| VerfasserIn |
Hélène Bureau, Laurent Remusat, Imène Esteve, Daniele Pinti, Pierre Cartigny |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250154093
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| Publikation (Nr.) |
 EGU/EGU2017-19147.pdf |
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| Zusammenfassung |
| Trapping inclusions in diamonds has been used as a diagnostic to constrain diamond growth
media (e.g. Navon et al., 1994; Weiss et al., 2015) in the Earth’s upper mantle. Experimental
works now generate inclusion-bearing diamonds from seeds in mixtures of carbonates,
graphite, and silicates in the presence of excess of pure water or saline fluids (H2O-NaCl) and
investigate in more details the conditions of natural diamond growth (Bureau et al.,
2012; 2016). Experiments were carried at conditions compatible with the Earth’s
geotherm between 6-7 GPa (1300-1675°C) in multi-anvil presses at the Bayerisches
Geoinstitut, Bayreuth from a few hours two a few days. Results show that within
the timescale of the experiments diamond growth occurs on seeds if water and
alkali-bearing carbonates are present. We show that water promotes fast diamond
growth, which is favorable to the formation of inclusions. Thin sections of a few
diamond seeds containing exposed inclusions were prepared using a Focus Ion Beam
(about 2 to 5 µm thickness). These sections were deposited on silicon wafers and
gold coated for micron-scale determination of the delta 13C isotopic compositions
using the NanoSIMS 50 installed at the Muséum National d’Histoire Naturelle,
Paris.
Carbon isotope measurement with NanoSIMS were calibrated against a natural Ia and
a synthetic IIa diamond used for diamond anvil cells, whose compositions were
determined by gas-source mass spectrometry at IPGP at 3.6±0.1‰ and -20.9±0.1‰,
respectively (Pinti et al., 2016). All the starting materials used for the experiments
were also characterized for their delta 13C by the same technique at GEOTOP,
Montréal.
The isotopic composition of the new diamond grown areas were measured close to
the inclusions. They exhibit a different isotopic signature than that of the starting
seeds (starting diamond composition: -29.6 to -30.4±1.4‰). The new diamond
signatures are falling into the range of signatures of the starting carbonates used
for the experiments (- 4.8±0.1 to -16.2±0.1‰) when they are far away from the
composition of the starting graphite (-26.4±0.1‰). This shows that the carbon source for
diamond growth must be the carbonates present either as CO32− ions dissolved in
the melt or as carbon dioxide species CO2 in the aqueous fluid and that diamond
growth occurred from carbonate reduction rather that from graphite dissolved in the
melt.
We suggest that the presence of small discrete or isolated volumes of water-carbonate-rich
fluids are necessary to grow inclusion-bearing peridotitic, eclogitic, fibrous, cloudy and
coated diamonds, and may also be involved in the growth of ultrahigh pressure metamorphic
diamonds.
Bureau, H., Langenhorst, F., Auzende, A.-L., Frost, D.J., Estève, I., Siebert, J., (2012).
Geochimica et Cosmochimica Acta, 77, 202-214.
H. Bureau, D.J. Frost, N. Bolfan-Casanova, C. Leroy, I. Esteve, P. Cordier (2016) Lithos
265, 4-15.
D.L. Pinti, A. Ishida, N. Takahata, Y. Sano, H. Bureau, P. Cartigny (2016) Geochemical
Journal 50/4, E7-E12. DOI: 10.2343/geochemj.2.0427
Navon, O., Hutcheon, I.D., Rossman, G.R., Wasserburg, G.J., 1988. Nature 335,
784-789.
Weiss, Y.; McNeill, J.; Pearson, D. Graham; et al. 2015 Nature 524 : 339-344. |
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