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Abstract:
Since december 2013 the Helmholtz Zentrum Potsdam has operated a 1280-HR instrument: the latest generation of Cameca large geometry ion microprobe. Here we report the observed performance of this tool, with particular emphasis on the determination of isotopic ratios on geomaterials. We have already acquired extensive experience in the determination of
δ 18 o values in silicates, for which a single analysis requires circa 80 seconds of actual data acquisition. Running in full automatic mode and with the reference material(s) embedded within the grain mount, we are thus able to complete some 250 analyses in a 24 hour cycle. For such analyses we
typically achieve an analytical repeatability of ± 0.15
‰ (1sd), though we have recorded repeatabilities down to ± 0.092 ‰ (1sd, n = 100) in one suite of zircon analyses. a main consideration in δ 18 o analyses with the 1280-HR is the need for careful sample preparation: on samples with as little as 5
μm topographic relief between silicate grains and epoxy embedding media we have observed charging problems despite using both a 35 nm gold coating and > 1 μA of low energy electron flooding. Thanks to the availability of a well characterized suite of tourmaline rMs, boron isotope analyses are already a well established application in the potsdam
SIMS facility. Here a single analysis involves 80 s of data acquisition, from which we can obtain a typical repeatability of ± 0.25 ‰ (1sd) and an overall analytical uncertainty of ± 0.9 ‰ based on a suite of four distinct tourmaline rMs. We have also been able to analyze the boron isotopic compositions on
areas as small as 3 μm with only a modest reduction in overall precision. Large demand exists for U-Th-Pb age determinations,
for which we currently have rMs only for zircon. For such work a single analysis commonly requires 12 to 15 minutes of data acquisition, meaning that a complete project typically involves
circa 5 days of laboratory usage. Our zircon U-Pb protocol calls for the analysis of a quality control material during all
analytical sessions. Here we often observe a ~1 % bias in the Pb/U results from the QCM as calibrated by the primary reference material. We conclude that under normal circumstances our inter-element results are reliable at the ± 2 ‰ level. A key feature of the 1280-HR tool is the ability to conduct oxygen flooding during the analyses, which results in a 2x
improvement in instrument sensitivity for pb data acquisition. mThe technology behind the 1280-HR is sufficiently mature such that the main factor limiting analytical quality has, at least in some isotopic systems, become the interlaboratory bias observed during the “bulk” characterization of the reference materials. A key limiting factor in the performance of the instrument itself is the constraint that only a single, 1-inch
diameter sample may be in the secondary ion source at a time. This limitation often compels frequent sample exchanges in order to access calibration materials, thereby prohibiting extended automated runs.
