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

Abstract

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 δ 18O SMOW 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 δ 18O 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 δ 18O SMOW 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.