33 Mineralogy/Crystallography, Petrology, and Geochemistry

Added due to amount of discipline submissions.

Tuesday, 10 March 2026
4:00 PM - 5:30 PM
RCC, 104

Optimizing Iridium Concentration Analyses in Geological Samples From the Acraman Impact

Paper number: 33-3
Program: Technical Sessions
Time: 4:45 PM – 5:00 PM

Authors

Abstract

Previous work in the Johnnie Formation in California and Nevada has shown heterogenously distributed grains of iridium dispersed within the matrix directly below the Johnnie Oolite. To determine the bulk concentration, microanalysis is performed by first identifying iron-rich regions via SEM imaging of thin sections, with subsequent quantitative analysis using laser ablation LA-ICP-MS. An open question is whether a finite number of grain analyses accurately represent the bulk concentration of iridium within the thin sections. Our work focuses on two objectives: a better quantification of the dilute iridium within the thin sections per ablation, and accurately extrapolating the sparsely distributed iridium-rich grain detections into a measurement of the bulk concentration.

This project confirms the high percent error inherent in quantifying such dilute, heterogeneous elements. Analysis of NIST 610 and 612 standards reveals that percent error is driven by the standard deviation of the measurements, particularly in light of the fact that a ‘hit’ versus a ‘non-hit’ only further exacerbates the statistical variance. For NIST 610 ([Ir] = 64 ppb), the standard error for Ir 193 decreases predictably. However, NIST 612 ([Ir] = 6ppb) shows higher and unpredictable error, with Ir 193 error fluctuating over the same range of trials. This indicates that low concentration directly leads to high standard deviation and unreliable measurements. 

Thus, simply increasing the number of trials to reduce the error would be burdensome in time and cost and eliminating the high throughput advantages of using LA-ICP-MS. Instead, this work explores reducing the standard deviation as the most effective strategy by utilizing a higher [Ir] standard. For example, based on the hypothetical calculations for Ir 193, a 25% reduction of standard deviation would only require about 33 laser spots to produce the same sample error, whereas reducing the standard deviation  by 50% would only require 15 spots. As above, trying to reduce error by trials alone would require 107 and 240 ablation spots, respectively, or up to 30% of the area of a standard thin section. Therefore, future work should prioritize lowering variability both with higher [Ir] standards, such as MPI-DING, and devising methods of extrapolation to bulk concentration through strategic spatial coverage, rather than attempting to ascertain a statistically representative area of the thin section through ablative coverage.

Geological Society of America Abstracts with Programs. Vol. 58, No. 1

doi: 10.1130/abs/2026SE-411504

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