High-Precision, High-Accuracy Oxygen Isotope Measurements of Zircon Reference Materials with the SHRIMP-SI

摘要

Analytical protocols for SHRIMP-SI oxygen isotope analysis (delta O-18) of a suite of zircon reference materials (RMs) are presented. Data reduction involved a robust estimate of uncertainties associated with the individual spot as well as for groups where the spot data are combined. The repeatability of delta O-18 measurements is dependent on both the analytical conditions and the choice of the primary reference material. Under optimised conditions, repeatability was often better than 0.4 parts per thousand (2s) allowing sample uncertainties to be obtained to better than 0.2 parts per thousand (at 95% confidence limit). Single spot uncertainty combined the within-spot precision with the scatter associated with repeated measurements of the primary zircon reference material during a measurement session. The uncertainty for individual spots measured under optimised conditions was between 0.3 and 0.4 parts per thousand (at 95% confidence). The analytical protocols described were used to assess a variety of zircon RMs that have been used for geochronology and for which laser fluorination oxygen isotope data are available (Temora 2, FC1, R33, QGNG and Plesovice), as well as zircons that have been used as RMs for trace element or other types of determination (Mud Tank, Monastery, 91500, AS57, AS3, KIM-5, OG1, SL13, CZ3 and several other Sri Lankan zircons). Repeated analyses over nine sessions and seven different mounts show agreement within analytical uncertainty for Temora 2, FC1, R33, QGNG, Plesovice and 91500, when normalised to Mud Tank. For existing ion microprobe mounts with these materials, an appropriate delta O-18 can be determined. However, care should be taken when using zircons from the Duluth Complex (i.e., FC1, AS57 and AS3) as reference materials as our data indicated an excess scatter on delta O-18 values associated with low-U zircon grains.