Chemical and oxygen isotope composition of gem-quality apatites: Implications for oxygen isotope reference materials for secondary ion mass spectrometry (SIMS)

Oxygen isotope ratios (δ18O) of biogenic apatite have become a widely used tool for reconstructing palaeoenvironmental conditions in the past. Ongoing improvements in secondary ion mass spectrometry (SIMS) technology have made in situ δ18O analyses on sub-nanogram domains within single microfossil samples possible; however this method requires calibration with reference materials (RMs) with a matrix “similar” to that of the samples. Here we evaluated five sources of gem-quality, abiogenic apatites to assess their potentials as SIMS RMs. Our results show that all these gem-quality apatites are low-REEs calcium fluorapatites with δ18OVSMOW values between 6.6 and 11.4‰. Large variations in δ18O have been found for between crystals from a single deposit as well as within individual crystals. Durango apatite has an inter-crystal δ18O range of 4.4‰ (6.6-11.0‰, N = 9 crystals). Madagascar Green apatite, Madagascar 1st Mine apatite and Ipira apatite have inter-crystal variations in δ18O of 4.1‰ (7.3-11.4‰, N = 15 crystals), 3.5‰ (7.6-11.1‰, N = 9 crystals) and 3.1‰ (7.1-10.2‰, N = 11 crystals), respectively. South Africa Blue apatite has a smaller inter-crystal δ18O range of only 0.9‰ (8.7-9.6‰, N = 6 crystals), though this might be an artefact due to the restricted number of samples studied. Intra-crystal δ18O variations of studied crystals generally range from 0.8 to 1.8‰. However, several gem apatite crystals from Madagascar have minor δ18O variation within 0.5‰ and represent most promising candidate RM.Gem-quality abiogenic apatites can be used as RMs for calibrating SIMS δ18O analyses, however these require homogeneity testing at sub-millimetre scale in advance. Durango crystals, commonly used as RMs in earlier SIMS-based studies, display a conspicuous heterogeneity in δ18O, with intra-crystal variations ranging from 0.7 to 2‰ as detected by both gas source isotope ratio mass spectrometry and large geometry SIMS. Thus, Durango apatite cannot be considered as suitable for SIMS calibration and alternative reference materials need to be sought.