New matrix-matched reference material for SIMS analysis of oxygen isotopes in gem corundum

The aluminum oxide (Al2O3) corundum is of high economic interest, not least because of its rare gemstone varieties sapphire and ruby. Geochemical analyses are helpful to gain a better understanding of the geographical origin and distribution of corundum. In addition to trace elements, O-isotopes (δ18O) are a powerful tool to identify provenance of gem-quality corundum. High spatial resolution analysis by secondary ionization mass spectrometry (SIMS) plays an essential role for this because it can identify intracrystalline heterogeneity and thus more reliably identify genetic processes. However, accurate SIMS analysis requires suitable matrix-matched reference materials. Some in-house reference materials for corundum are currently in use by different SIMS labs, but these materials generally lack comprehensive chemical and isotopic characterization. Moreover, crystal-orientation effects can create bias in SIMS analysis. Therefore, it is timely to develop appropriate matrix-matched reference materials for SIMS analysis of corundum.

We have investigated O-isotopic homogeneity and composition of a synthetic laser ruby single-crystal. A rod of it was cut symmetrically from the interior to the rim and analyzed in bulk by laser-fluorination technique in two different laboratories. A preliminary average value of δ18O= +18.3‰ is reported, with no systematic variability across the rod. An adjacent slice of the rod was sectioned in four prominent crystallographic orientations confirmed by EBSD. SIMS results show homogeneous O-isotopic values independent of crystallographic orientation (2SD = 0.18‰ in δ18O, n = 4, six measurements on each orientation). Besides O-isotopes, abundances and homogeneity of key trace elements will be characterized by electron microprobe and SIMS.