194-4 Reducing Pit-Volume Measurement Bias in Laser-Ablation (U-Th)/He Thermochronology and In-Situ (U-Th)/He Analysis of Complex Iron Oxides

Session: Broad Applications of Thermochronology to Understanding Geologic Rates and Processes Through the Sedimentary Record (Posters)

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Abstract:

Laser-ablation (LA)-(U-Th)/He microanalysis offers several advantages over conventional whole-crystal analysis for planetary thermochronology including less destructive analysis of high-value material and in-situ analysis of texturally complex minerals such as iron oxides. LA-(U-Th)/He dating involves ablation of a pit for He analysis by noble gas mass spectrometry, measurement of the ablated pit to determine the He-degassed volume using a profilometer, and ablation of a second pit for U-Th analysis by ICP-MS. LA-(U-Th)/He dates are sometimes corrected for systematic bias using a standard-date calibration factor, but it is unclear if this method is appropriate because the underlying cause of the date bias could stem from the He, degassed-volume, or U-Th measurement. Accurate determination of the He-degassed volume is especially challenging, and incorrect measurements may lead to unreliable and/or biased LA-(U-Th)/He dates.

Here, we present research on improving the accuracy and precision of degassed-volume measurements and then apply the LA-(U-Th)/He method to texturally complex Neoproterozoic hematite veins in the Colorado Front Range. First, we analysed two low U-Th, minimally zoned standards—Durango apatite and Juina zircon—at a variety of spot sizes, fluences, and lasing durations and determined pit volumes using both volume extraction from 3D models of the ablated pit and application of idealized pit geometries. Volume extraction methods yielded accurate apatite and zircon dates for large He spot sizes (~80 μm) but show an increasing too-old date bias with decreasing spot size, due to occlusion of the degassed volume by accumulation of degassed condensate within the pit. Dates using idealized pit geometries avoid this bias but are highly sensitive to accurate pit-depth, top-diameter, and pit-wall-slope determinations, especially when the pit walls are curved, stepped, or terraced. Next, we acquired LA-(U-Th)/He for a hematite vein characterized with rock and reflected light images. Due to the accumulation of melt globules within the ablated pits, idealized pit geometries were used. The in-situ dates range from ~800–100 Ma, match published bulk hematite (U-Th)/He and in-situ U-Pb dates, and reproduce the published He-date correlation with crystallite size. These results demonstrate that LA-(U-Th)/He dates are particularly sensitive to uncertainties and biases in degassed-volume determination. Improving degassed-volume measurement methods can improve the accuracy and reproducibility of LA-(U-Th)/He dates on materials from Earth and beyond.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-11096

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