SIZE MATTERS: HOW SMALL IS TOO SMALL FOR ISOTOPE RATIO ANALYSIS? (Invited Presentation)

Smaller is always better for isotope analysis, until it isn’t. Sample volumes by Secondary Ion Mass Spectrometry (SIMS) are often a million times smaller than by bulk analysis. Sample volumes by Atom Probe Tomography (APT) can be more than a million times smaller than SIMS. As the technical problems limiting sample size are overcome, counting statistics impose a physical limit for the minimum useful sample volume. This limit varies according to sample, isotope system, desired precision, and analysis technique.

For oxygen isotope analysis of d¹⁸O in silicates (¹⁸O/¹⁶O ~2 x 10^-3), precision of ±0.3‰ (2 SD) is attained for 10 micron SIMS spots and ±2‰ for sub-micron spots (IMS-1280, Page et al. 2007) that are within an order of magnitude of counting statistics. The precision limit for d¹⁸O is approximated by N^0.5 for number of atoms counted of the minor isotope by SIMS (10⁶ atoms of ¹⁸O ~ 1‰). For sulfur, analysis of d³⁴S in pyrite (³⁴S/³²S ~ 4.4 x 10^-2) will attain similar precision with samples 20 times smaller for a similar ion yield. Thus for materials at terrestrial abundance, useful analytical volumes approach physical limits and smaller spots should be limited to extraterrestrial or experimental samples where extreme isotope ratios can be studied and high precision is not required. Likewise for lead, analysis of ²⁰⁷Pb/²⁰⁶Pb can been extended to far smaller volumes because radioactive decay of U causes greater variability, ratios are closer to unity, and precisions over 10% can be geologically informative. The ability by APT to locate and identify single atoms by mass requires decisions on how small a population must be analyzed to be meaningful. For APT ²⁰⁷Pb/²⁰⁶Pb ratios in zircon, the precision of both isotopes and their backgrounds must be considered and results are most significant for older Precambrian zircons, or if Pb is concentrated in clusters or other features. For example, in the 2.5 Ga ARG zircon, ²⁰⁷Pb/²⁰⁶Pb = 0.17 ± 0.065 (2 SD) by APT (vs. 0.1648 by SIMS) based on counting 1326 atoms of clustered Pb (Valley et al. 2015). The high spatial resolution afforded by in situ analysis provides fundamental new information for the study of small, precious and zoned materials from Earth and elsewhere.