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Stable isotopes: Carbon (LG-SIMS)

Carbon isotopes in diamond mapped with LG SIMS

The CAMECA large-geometry SIMS has been designed in order to achieve excellent precision and reproducibility on stable isotope ratio measurements:

  • High transmission at high mass resolving power (D/H2, 17O/16OH, 33S/32SH...)
  • Simultaneous detection of different isotopes using the multicollector system with the possibility to combine EM & FC dectectors
  • Availability of both Cs and O primary sources
  • Efficient charge compensation on insulating samples
  • Precise control of the instrumental mass fractionation variability...

Carbon isotope composition of natural diamonds is widely used as an indicator for their conditions of formation and the source of fluid/melt from which they precipitate in the Earth’s mantle.

The selected diamond (Venezuela) is notable for its complex Catholuminescence (CL) pattern which has led to it being referred to as the “Picasso” diamond. A 624 point mapping across the whole crystal was obtained using multicollection FC/FC mode: 40s per point in chain analysis mode, internal reproducibility for each point < 0.2 ‰. The CL image is matched exceptionally closely by the carbon isotope map. This result strongly supports the view that the δ13C values measured on natural diamonds are directly related to the δ13C composition of the mantle fluid reservoir from which the diamond grew.

The CAMECA large-geometry ion microprobe enables rapid, high precision analyses performed automatically over long periods of time. It is possible to map minerals for minor variations in isotopic ratios.