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Abstract:
Oxygen isotope ratios (δ 18 O) of biogenic apatite have been
widely used to reconstruct palaeotemperatures in the distant past. The development of secondary ion mass spectrometry (SIMS) enables in situ δ 18 O analyses on single microfossil elements, but requires a calibration employing reference materials (RMs) with a matrix that is “similar” to the samples. We investigated several gem-
quality apatites to evaluate their potential as RMs by using both gas source isotope ratio mass spectrometry and SIMS. Compared to the oxygen isotope composition of bio-apatite ranging from 9-25‰ VSMOW, all gem-like apatites investigated here have lower δ 18 O values of
between 6 - 11‰ VSMOW due to their magmatic origin. We
have found large variations in δ 18 O for gem-like apatites both between crystals from a single deposit as well as within individual crystals. Durango apatite has an inter-crystal δ 18 O range of 4.9‰ (N=9). Madagascar 1st Mine apatite and Ipria apatite have inter-crystal
δ 18O ranges of 3.5‰ (N=9) and 2.9‰ (N=8), respectively. Madagascar Green apatite and South Africa Blue apatite have the smaller inter-crystal δ 18 O ranges of 1‰ (N=8) and 0.9 (N=6). Durango apatite, which has been previously used as a RM for SIMS analyses, is the most heterogeneous in δ 18 O with an intra-crystal variation in δ 18 O up to 2‰. Due to the large intra-crystal variation observed in δ 18 O, Durango apatite cannot be considered as a suitable reference material, and data calibrated to Durango apatite should be treated with caution.
