High precision, high accuracy measurement of oxygen isotopes in a large lunar zircon by SIMS

Ninety-seven high precision, high spatial-resolution ion microprobe analyses of oxygen isotopes in a single, large zircon crystal from a lunar soil sample yield a surprising and unexpectedly large range in δ18OSMOW from 4.5 to 9.2‰. Detailed consideration of sample mounting, polishing, and sample-standard orientation suggests that this variation was caused by analytical artefact(s) degrading the accuracy of δ18O values. Careful control of these parameters effectively eliminated the variation and the two most reliable sets of analyses, obtained when the sample and standard were mounted within 2 mm in the centre of the mount, yield a combined weighted average δ18OSMOW value of 5.70 ± 0.07‰ (95% confidence). This value is indistinguishable from bulk rock analyses of low-Ti lunar basalts and suggests that, at least under anhydrous conditions, there is minimal fractionation accompanying zircon crystallisation.