3:00 PM - 3:15 PM
[SCG16-05] Carbon isotopes in a Juina diamond with carbonate inclusions
Keywords:Carbon cycle, Carbon isotopes, Diamonds, Carbonate, Juina, Transition zone
A total of 14 raster analyses (1 x 1 μm) were carried out on the three foils, using a Cs+ beam. We used 2 detectors on different 3 magnetic fields, so we could estimate and compare three 13C/12C ratios. N amount was also determined. Instrumental mass fractionation (IMF effect) was calculated using the data of GM02. Quasi-simultaneous arrival (QSA) effect was taken into account and data corrected consequently, because of the large amount of secondary ions observed (above 100,000 cps). Counting was between 35 and 80 cycles, depending to the variable thickness of the foils (ca. 150 nm) to avoid that the Cs+ beam pierces the foil and measures C from the TEM carbon grid.
Results showed δ13C values in the Juina diamond ranging from -9.1±3.8 ‰ to -0.43±2.82 ‰ with an average δ13C value of - 4.2 ‰ and a median value of -4.0 ‰. The diamond sample contains also a small amount of nitrogen with an average concentration of 62 ppm, similar to previously reported concentrations measured by SIMS. These results suggest a peridotitic origin for the diamond, possibly Type IaB, which is the most abundant class among the Juina diamonds. The origin of this diamond calls for alternative processes to explain the genesis of carbonate inclusions.