HR: 11:20h
AN: V42A-05    [Abstracts]
TI: Evidence for methane-derived diamond from a single lherzolite xenolith (Premier, South Africa)
AU: * Thomassot, E
EM: thomasso@ipgp.jussieu.fr
AF: Lab. Geochimie des isotopes stables, IPG-Paris tour 54-64 1ier etage 4 place Jussieu , Paris, 75251 France
AU: Cartigny, P
EM: cartigny@ipgp.jussieu.fr
AF: Lab. Geochimie des isotopes stables, IPG-Paris tour 54-64 1ier etage 4 place Jussieu , Paris, 75251 France
AU: viljoen, F
EM: fanus.viljoen@debeersgroup.com
AF: GeoScience Centre, De Beers Consolidated Mines Ltd, P.O. Box 82232, Southdale, 2135 South Africa
AU: javoy, m
EM: mja@ipgp.jussieu.fr
AF: Lab. Geochimie des isotopes stables, IPG-Paris tour 54-64 1ier etage 4 place Jussieu , Paris, 75251 France
AB: We report the first concentrations and isotopic compositions of carbon and nitrogen of 24 diamonds released from a single lherzolite xenolith from the Premier kimberlite in South Africa. This nodule of about 40 cm$^{3}$ contained 59 colourless sharp-edged octahedral diamonds, ranging from 0.05 to 0.17 carats. Their nitrogen concentration range from 47 to 1221 ppm (n = 59) and are clearly correlated with nitrogen speciation (10 to 85 % of IaB defects, the most stable state for nitrogen in a diamond cristalographic network). Nitrogen aggregation is a diffusive process depending on residence time, and temperature in the mantle, together with total nitrogen concentration. For Premier xenolith diamonds, the correlation between N-content and N-aggregation state confirms the synchronised formation at T = 1180 $\pm15\deg$C for a mantle residence time of 750 Ma (age of 1.2 Ga). $\delta^{13}$C values vary in a small range (from -4.26 to -0.84 $\permil$) and are positively correlated to both $\delta^{15}$N (from -1.12 to +7.18 $\permil$), and nitrogen contents (between 47 and 1221 ppm). $\delta^{13}$C and N-contents reported in the present work are within the ranges previously measured in diamonds from Premier (e.g. Deines, 1984). Positive correlations among the parameters cannot result from any mixing between two realistic end-members (e.g. mantle-derived and metasedimentary carbonates). In contrast, they could be explained by an isotope fractionation related to diamonds crystallisation. Most authors suggest that diamonds formed from oxidized carbon. However, the present carbon isotopic compositions are incompatible with a diamond crystallisation from mantle-derived carbonate-bearing fluids/melt. Parameters required by such a model are indeed not supported by previously determined isotope fractionation factors. Alternatively, we show that diamonds $\delta^{13}$C-$\delta^{15}$N-N-content covariations are compatible with an open-system crystallisation from reduced carbon bearing fluids. We believe to have therefore a first strong evidence for methane-related diamond formation.
DE: 3620 Crystal chemistry
DE: 1025 Composition of the mantle
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting